PLANT ANATOMY

The sc ience of the structure of the organized plant body learned by d issect ion is ca l led

P lant Anatomy (anatomy-dissect ion) .   In general , P lant Anatomy refers to study of internal

morphology, perta in ing to d i f ferent t issues.   The subject of th is chapter is structure of

Angiosperms, with emphasis on pr imary t issues.

 

T ISSUE SYSTEM

Plant body in Angiosperms is d i f ferent iated into root stem, leaf and f lower.   A l l

these parts are made up of d i f ferent types of t issues contain ing d i f ferent cel l

types.   A t issue is a mass of s imi lar or d iss imi lar ce l ls performing a common

funct ion.

The body of a vascular p lant is composed of dermal t issue, Ground t issue and Vascular

t issue.

 

Dermal T issue (Skin)

Dermal T issue is protect ive in funct ion.   Bas ing on i ts or ig in, i t is c lass i f ied into two types –

Epidermis and Per iderm.

 

Epidermis:    This is the pr imary surface t issue of the ent i re p lant .   Epidermal cel ls are

compact ly and cont inuously arranged; the cont inui ty is lost by the presence of Stomatal

pores or breaks in the t issue. Cover ing the aer ia l epidermis, cut in ( fatty substance) is

present as an impregnat ion on cel l wal l .   The cut ic le can be separated from epidermis. The

epidermis may produce unicel lu lar or mult ice l lu lar hairy outgrowths and other appendages.  

Epidermis provides mechanical protect ion, a l lows gaseous exchange through stomata,

restr icts t ranspirat ion with cut ic le , and is a lso involved in storage, photosynthesis ,

secret ion, absorpt ion and percept ion to st imul i .   Stomata are Pores, each guarded by two

guard cel ls , which contro l the s ize of the pore.   Cel ls surrounding guard cel ls , but d i f fer ing

f rom other epidermal cel ls , are ca l led subsid iary cel ls .     Guard cel ls are k idney shaped;

their ce l l wal ls are th ick on the inner surfaces.   Guard cel ls contain many chloroplasts .

 

Per iderm:    This is formed dur ing secondary growth replac ing pr imary epidermis.

 

Ground Tissue

This is inner to dermal t issue and is composed of s imple t issues l ike parenchyma.

 

Vascular t issue

Vascular t issue consists of conduct ing e lements – xy lem and phloem.   Vascular t issue may

be scattered in ground t issue or regular ly arranged forming a r ing.   In the latter

arrangement, ground t issue is d i f ferent iated into cortex (outer to vascular t issue) and pi th

( inner to vascular t issue) .   The ground t issue of leaves is ca l led mesophyl l , bound by upper

and lower epidermis.

 

T ISSUE TYPES

In broad sense, t issues are c lass i f ied as – mer istemat ic and permanent t issues.

 

 

Mer istem (Mer istos – d iv is ib le)

In i t ia l ly a l l embryonic cel ls of an embryo have the capaci ty to d iv ide and mult ip ly but as the

embryo develops into a p lant body, th is capaci ty for d iv is ion is restr icted to certa in parts of

the p lant body cal led mer istems which are act ive throughout the l i fe of the p lant body

(unl ike that of an animal body) .   When mer istemat ic cel ls d iv ide, a group of the daughter

cel ls remain mer istemat ic ; the other daughter cel ls ca l led der ivat ives d i f ferent iate into

var ious t issue e lements.   Before the occurrence of any cel l d iv is ion, usual ly cel ls become

enlarged accompanied with addit ion of protoplasmic and cel l wal l mater ia l .  

 

Mer istemat ic cel ls are isodiametr ic , compact ly arranged with dense cytoplasm, large

nucleus, and smal l vacuoles or without vacuoles. Cel l wal ls are th in.

 

Mer istems which occur at the apices of stem, root and other branches are cal led apical

mer istems, which br ing about pr imary growth of the p lants , hence a lso cal led as pr imary

mer istems.   In many plants in addit ion to apica l mer istems, latera l mer istems l ike vascular

cambium, cork cambium, intercalary mer istems are found.   Latera l mer istems are arranged

paral le l to the s ides of organs in which they occur .

 

Intercalary Mer istem:   This is a lso a pr imary mer istem, found inserted between permanent

t issues, in the bases of internodes and leaf sheaths of grasses.   Sometimes, as in gynophore

of groundnut, xy lem may be present in intercalary mer istem.   Wherever stem is jo inted,

elongat ion of internodes is due to intercalary mer istem.   Example: Bamboos. Even pro longed

growth of leaves, f lowers and fru i ts may be regarded as an intercalary growth.

 

Vascular cambium and cork cambium:   These are referred to as secondary mer istems

because they produce secondary t issues, and increase the th ickness of the p lant body.   This

process is ca l led secondary growth, seen in d icoty ledons and gymnosperms.

Permanent or Mature Tissues

Cel ls der ived from meristems gradual ly change in their structure, metabol ism and chemistry

and acquire specia l ized characters by their var ious modes of d i f ferent iat ion. Not a l l the

cel ls tota l ly d i f fer f rom the mer istems.   Some cel ls reta in the power of d iv is ion and others

cannot d iv ide.   In a str ict sense only cel ls which have lost the power for d iv is ion must be

regarded as permanent t issues, but in a broad sense, ce l ls der ived from meristem that have

acquired a specia l funct ion l ike photosynthesis , secret ion, storage are treated as part of

matured t issue.   There are d i f ferent types of mature t issues.   Example: Parenchyma,

Col lenchyma, Sc lerenchyma, Xylem and Phloem.

 

Types of Mature Tissues

Parenchyma (Para-beside, enchyma- In poured)

Parenchyma is the fundamental t issue of the p lant body.   I t is found in every part of the

p lant body l ike p i th and cortex of stem and root , mesophyl l of leaves, f lesh of f ru i ts , f lora l

parts and even in xylem and Phloem.   Cel ls have th in pr imary wal ls and polyhedral shapes.  

The average number of faces of a polyhedron is 14 and is ca l led as "tetra decahedron".  

Cel ls are compact ly arranged or more commonly spaciously arranged with intercel lu lar

spaces as in cortex and pi th.   Cel ls possess dense cytoplasm and are act ive metabol i tes.

 

 

General ly cel ls of parenchyma are involved in storage of starch, sucrose, prote in, water ,

phenol der ivat ives, many mineral substances, etc .   Other metabol isms l ike respirat ion,

prote in synthesis etc . , are act ive.   Parenchymatous cel ls may a lso perform specia l ized

funct ions and are structura l ly modi f ied.   The fo l lowing are the d i f ferent types of

parenchyma.

 

Aerenchyma:   Intercel lu lar spaces f i l led with a i r , are large in s ize and many in number.  

Cel ls occupy a smal ler area.   Though the cel ls are smal ler , they provide the required

strength to the aquat ic p lants .   In these p lants a i r spaces are common, help ing in aerat ion

and buoyancy.   A i r spaces are a lso seen in roots of grasses, pet io les of canna, aro ids etc .

 

Chlorenchyma:   Cel ls of photosynthet ic parenchyma contain numerous chloroplasts .   These

cel ls are commonly seen in leaves, some t imes in young shoots.   Cel ls of Chlorenchyma are

of two types –

 

1)          Pa l isade cel ls that is e longated and compact ly arranged.

2)        Spongy cel ls that are spaciously arranged and i r regular ly shaped.

 

Prosenchyma (Pros-to) :  Cel ls are e longated, have th ick wal ls and serve as a support ing

t issue.   Example: Endosperm cel ls of seeds.

 

Armed Parenchyma:   Cel ls are ste l late in shape with inward pro ject ions of ce l l wal l as arms.  

Example: P inus leaf Mesophyl l .

 

Col lenchyma (Kol la – Glue)

Col lenchyma is a s imple, l iv ing t issue.   Cel l wal ls are th ickened due to deposi t ion of pect in.

Col lenchyma is the pr imary support ing t issue in stems, leaves and f lora l parts of d icots ,

where as in stems and leaves of monocots co l lenchyma is usual ly absent, ( instead,

sc lerenchyma is present in monocots) .  

 

Col lenchyma is usual ly hypodermal in pos i t ion.   Cel ls are more e longated and narrower than

parenchyma.   L ike Parenchyma, co l lenchyma may a lso contain chloroplasts or may regain

the th ickening. Intercel lu lar spaces may or may not be present.   Three forms of

co l lecnchyma are recognized based on the types of th ickenings –

 

1.          Thickening is on the tangent ia l wal l – lamel lar co l lenchyma.

2.        Deposi t ion of pect in is in the corners where several ce l ls meet-angular

co l lenchyma.

3.        Thickenings are around the intercel lu lar spaces – lacunar co l lenchyma.

Being p last ic , co l lenchyma di f fers f rom elast ic f ibers of sc lerenchyma.

 

Sc lerenchyma (Scleros-hard)

Cel ls of sc lerenchyma are th ick wal led and are usual ly l igni f ied.   The th ickness is due to

format ion of secondary wal l .   At least in i t ia l the secondary wal l is f ree f rom pr imary wal l .  

At matur i ty , usual ly the cel ls are devoid of protoplast .   The cel l wal l encloses a cavi ty

lumen.   On the cel l wal l , p i ts are usual ly present (s imple type) .   Commonly sc lerenchyma

cel ls are c lass i f ied into f ibers and sc lere ids.   The pr imary funct ion is mechanical .   (P i ts - th in

areas without secondary wal l mater ia l deposi t ion) .

 

F ibers:   These are usual ly long, spindle shaped structures, with taper ing or b lunt ends.  

Longest f iber is seen in Boehmeria n ivea (55 cms).   They are arranged in groups.  

Secondary th ickening may account for 90% of the area of the cel l ; the lumen is narrow.  

F ibers are grouped into xylary f ibers and extraxylary f ibers . Xylary f ibers , a lso ca l led wood

f ibers are parts of xy lem and are longest among xylem elements.   Extraxylary f ibers are

c lass i f ied as – Bast or phloem f ibers , cort ica l f ibers and per ivascular f ibers (per ipheral to

the vascular bundles) .   These are l igni f ied or non l igni f ied.

 

On account of their e last ic i ty , f ibers enable the p lant body to withstand var ious stra ins.  

Commercia l f ibers l ike jute, f lax, ramie are extraxylary f ibers .

 

Sc lere ids:   These are shorter than f ibers .   Sc lere ids occur s ingly or in groups.   Sc lere ids are

commonly found in f ru i t wal l , seed coat , epidermal scales, and occasional ly found in cortex,

p i th, mesophyl l and pet io le of submerged aquat ics .

 

There are many types of sc lere ids –

1.          Asterosc lere ids are star shaped

2.        Macrosc lere ids are s imi lar to pal isade cel ls

3.        Osteosclere ids are bone l ike that are enlarged at their ends

4.        Brachysclere ids are isodiametr ic l ike parenchyma

 

Vascular Tissue

This t issue is a lso ca l led complex t issue.   I t is heterogeneous in nature with d i f ferent types

of ce l ls .   The chief e lements are xylem, phloem and per icyc le.

 

Xylem (Wood)

Xylem is the water conduct ing t issue.   I t consists of l iv ing cel ls l ike parenchyma and dead

cel ls l ike tracheary e lements. F ibers are a lso present.   Xylem elements d i f ferent iat ing f rom

an apical mer istem const i tute pr imary xylem where as secondary xylem is d i f ferent iated

f rom vascular cambium.   Xylem is the most preserved t issue in foss i ls , due to the

development of l igni f ied secondary wal ls .

 

Tracheary e lements ( t rachea – rough):  These are of two types – 1) .   Trachea or vessels 2)

Tracheids, Vessels are l imited in their growth, are jo ined end to end to form cont inuous

tubular structures with perforat ions in their cross wal ls .   Water and minerals are ef f ic ient ly

conducted through these perforat ions.   Vessels are present in most of the Angiosperms and

a lso in certa in lower p lants l ike Gnetum, Mars i lea and Selaginel la .

 

Tracheids are general ly e longated and non-perforated.   They have only p i t pairs , at the

regions of union with other t racheids.   Conduct ion of water and minerals is not as ef f ic ient

as in vessels .   Tracheids are found in Pter idophytes, most of the Gymnosperms and a few

Angiosperms.

 

Secondary th ickenings in t racheary e lements are deposi ted in var ious       forms-1) as r ings,

ca l led annular .   2) as cont inuous hel ices, ca l led hel ica l or sp ira l , 3) as net work cal led

ret iculate, 4) as transverse nets ca l led scalar i form,   5) as extensive th ickenings except in

the region of p i ts , ca l led p i t ted.    

 

Pr imary Xylem:   This is d i f ferent iated into Protoxylem and Metaxylem, denot ing the xylem

that appears f i rst .   In stems, protoxylem is found nearest to centra l ax is (endarch xylem)

and in roots away from centre (exarch xylem).   The protoxylem elements have annular ,

sp i ra l and sometimes ret iculate secondary th ickenings. F ibers are absent in protoxylem.  

Metaxylem appears after protoxylem.   Metaxylem has only p i t ted secondary wal ls .

Metaxylem is more complex than protoxylem and i ts e lements are wider .

 

Secondary Xylem:   The vascular cambium ( intrafasc icular cambium interfasc icular cambium)

producing secondary xylem consists of fus i form and ray in i t ia ls .   Here a lso the secondary

xylem is a lso of two-system-1) Axia l or vert ica l system with l iv ing and nonl iv ing cel ls and 2)

ray or hor izonta l system with l iv ing cel ls .   When compared with pr imary xylem, secondary

xylem is more complex and shows order ly development.   P i t ted and scalar i form secondary

th ickenings are developed.

 

Phloem (Phloos Bark)

Phloem is the food conduct ing t issue.   I t is composed of s ieve e lements, parenchyma cel ls ,

f ibers and sc lere ids. Phloem di f ferent iated f rom procambium is ca l led pr imary phloem;

secondary phloem is in i t iated f rom vascular cambium.

 

When xylem and phloem elements are found in the same l ine the arrangement is ca l led

conjo int ; i f phloem is only external to xylem i t is ca l led col latera l as in many stems; i f

phloem is both external and internal to xylem i t is ca l led b ico l latera l as in leaves and a few

stems; i f xy lem completely surrounds phloem i t is ca l led amphivasal ; i f phloem surrounds

xylem i t is ca l led amphicr ibal ; i f xy lem and phloem al ternate with each other as in roots , i t

is ca l led radia l .  

 

S ieve E lements:   These are c lass i f ied into s ieve cel ls and s ieve tube members. S ieve cel ls

are commonly long, s lender, and taper at both ends.   These cel ls over lapping, many s ieve

areas are seen.   S ieve areas have c luster of pores through which adjacent s ieve cel ls are

interconnected by protoplasmic strands.   Surrounding the pores, carbohydrate Cal lose

mater ia l is found in a sect ional v iew.   S ieve cel ls are found in Pter idophytes and

Gymnosperms. S ieve tube members are tubular , p laced in long ser ies and have specia l ized

s ieve areas cal led s ieve p lates occurr ing at end wal ls .   These s ieve p lates are often

inc l ined.   In addit ion to s ieve p lates, there may be a lso s ieve areas on latera l wal ls of s ieve

tube members.   In s ieve p late, s ieve areas, having large pores are seen.   In i t ia l ly s ieve tube

members contain uninucleated protoplast .   Gradual ly the nucleus and i ts associated

endoplasmic ret iculum become disorganized.   Mitochondr ia may lose their inner

membranes.   Cytoplasm is usual ly par ieta l .   In the centre of the cel l a mixture of vascular

sap and disorganized cytoplasmic matter is seen.   Tonoplast is not found surrounding the

vacuole.   Studies s ieve contents show that they are under posi t ive pressure of 30 atm and

veloc i ty of conduct ion is 100 cm/ hour.   The transported mater ia l is usual ly sucrose which is

more than 20% in concentrat ion.

S ieve tube member is associated with one or more specia l ized parenchyma cel ls ca l led

companion cel ls are seen only in Angiosperms.   Both s ieve tube member and companion cel l

are der ived from the same meristemat ic cel l .   Often, companion cel ls of s ieve tube members

form longitudinal ser ies. Companion cel l is nucleated.   Probably these cel ls provide energy

for food conduct ion.

 

Pr imary Phloem:   This is c lass i f ied into Protophloem and Metaphloem.   In protophloem, s ieve

tube members are without companion cel ls .   S ieve tubes funct ion for a br ief per iod and soon

they get crushed by the surrounding pressure.   The crushed cel ls may disappear.  

Metaphloem t issue surv ives for a longer per iod.   I ts e lements are longer and wider .   Usual ly

f ibers are absent in d icoty ledons, where as in monocotyledons and herbaceous d icots ,

parenchyma cel ls are absent.

 

Secondary phloem:   S imi lar to secondary xylem, there are two systems of arrangements in

secondary phloem – 1) Axia l system producing s ieve e lements, phloem parenchyma and

phloem f ibers . 2) Transverse system producing ray parenchyma cel ls .

 

Per icyc le (A Circ le Al l Around)

Per icyc le is the region, consist ing of one – few layers of ce l ls , found external to centra l

cy l inder (Ste le) .   Example: Roots and Dicot stems.

 

Per icyc le may be composed of parenchyma cel ls or sc lerenchyma cel ls or both.   Latera l

branches and phel logen may ar ise f rom per icyc le.

 

Summary

Tissue:  a group of ce l ls performing a part icular funct ion is ca l led t issue.   Bas ing on the

types of ce l l s tructure and funct ion, t issues are c lass i f ied into Mer istemat ic , S imple and

Complex t issues.

 

Mer istemat ic T issues:   These are act ively d iv id ing cel ls , which are isodiametr ic in shape,

r ich in cytoplasm, with smal l vacuoles or no vacuoles and cel ls are act ively metabol ic .  

Depending upon the posi t ion they are c lass i f ied into apical mer istem (at the t ips of the

stem and roots) , latera l mer istem (vascular and cork cambium), intercalary mer istem (at the

basal port ion of internodes in Graminae members) .   Apica l mer istem is responsib le for the

pr imary growth, ex. , growth of the p lant in length; latera l mer istem is responsib le for

secondary growth and intercalary mer istem is responsib le for the e longat ion of the

internodes in Bamboo plants etc .

 

S imple T issues:   T issues contain cel ls of s imi lar structure, funct ion and have common or ig in.

 

Method of study:   Stem cutt ings are put in a macerat ing f lu id and a l lowed for some t ime;

meanwhi le some th ink T.S. are taken and sta ined with saf f ranin and mounted on a s l ide.  

The treated stem pieces are taken on the s l ide, washed, teased and mounted on the s l ide

for microscopic observat ion.

 

Parenchyma:     Important l iv ing t issue found abundant ly in cortex, p i th of the stem, root and

a lso in the mesophyl l t issue of leaves.   Depending upon the structure, content and funct ion

they are d i f ferent ly named l ike storage t issue, chlorenchyma, spongy parenchyma, ground

t issue, aerenchyma etc.   Cel ls are th in wal led, r ich in cytoplasm, with smal l or a large

centra l vacuole, act ively metabol ic and loosely arranged with smal l or large intercel lu lar

spaces.

 

Col lenchyma:       Important L iv ing mechanical t issue, found most ly just below the epidermis of

stem, s imi lar to that of parenchyma but with d i f ference in the cel l th ickenings.   Here cel ls

are larger , compact ly arranged and cel l wal l at the angles of the meet ing of ce l ls is

character ist ica l ly th ickened by pect in deposi t ion.

 

Sc lerenchymatous F ibers:  These are true mechanical t issues e i ther dead or l iv ing.   Two

types of ce l ls are found a) Sc lerenchyma f ibers b) Sc lere ids.

 

Sc lerenchymatous F ibers:   They are dead cel ls , long with taper ing ends, with uni form

thickening, leaving a narrow lumen.   They are found below the epidermis, cortex, around

vascular bundles and so on.

 

Sc lere ids:    They are found in f ru i t or seed wal ls and a lso in the f lesh of apples and

pineapple.   The cel ls may be l iv ing or dead, but short and var ious ly shaped; cel l wal l is

heavi ly th ickened with many pi t l ines unevenly d istr ibuted.

 

Complex T issues:   The cel ls that are grouped here are of var ious types in their structure,

shape and funct ion, and have di f ferent or ig in, but together they perform a common

funct ion  Example:  vascular t issues, secretory t issues etc.

 

Vascular T issues:   1) XYLEM:   I t is a water conduct ing t issue, made up of t racheids, t rachea,

xy lem f ibers and xylem parenchyma of which the f i rst three are dead cel ls and the last one

is l iv ing.  

TRACHEIDS: have taper ing ends with secondary th ickenings of annular , sp i ra l , scalar i form

and ret iculate types and cel l wal ls are p i t ted.  

TRACHEA: are vessels with the taper ing ends and their t ransverse wal ls are d is integrated,

so that cel ls are p laced end to end to form a k ind of cont inuous channels for conduct ing

water and minerals .  

XYLEM FIBRES: are s imi lar to Sc lerenchymatous f ibers .   A l l the above three t issues are dead

and by having th ick secondary wal ls they act as mechanical t issues.  

XYLEM PARENCHYMA: even though i ts ce l l wal ls are th ick and pi t ted i t is l iv ing and perform

some vis i t funct ions probably act ive transportat ion etc.

 

3)        PHLOEM:   This t issue is so le ly responsib le for the transportat ion of the food

mater ia l f rom the source i .e . , leaves and storage organs to the regions of need.   I t

consists of s ieve tubes, companion cel ls , phloem parenchyma (a l l l iv ing) and phloem

f iber (dead cel ls) .   S IEVE TUBE CELLS : are long large cel ls with end wal ls perforated,

but cushioned with ca l lus mater ia l and the end plates are cal led s ieve p lates,

through which large number of cytoplasmic strands pass through.   The cel l has a

large centra l vacuole with th in per ipheral protoplasm.   The most remarkable feature

is the complete absence of nucleus.   Most of the food mater ia l is conducted through

th is s ieve tube in the form of sucrose.  

4)        COMPANION CELLS: are smal l ce l ls , l iv ing, found and pressed to s ieve tubes.   Cel ls

are r ich in cytoplasm and act ively metabol ic .   The funct ion is probably provid ing

energy for t ransportat ion of food mater ia ls by act ive process.   PHLOEM

PARENCHYMA:   is s imi lar to parenchyma, and found around s ieve tubes and

companion cel ls PHLOEM FIBRE OR BAST F IBRES: are most ly dead cel ls with

considerable and var ied type of secondary th ickenings.

 

PRIMARY GROWTH

 

The pr imary growth of a p lant body is brought about by the act iv i ty of pr imary mer istems,

part icular ly by apical mer istem.   Detai ls about pr imary growth occurr ing in stems, leaves

and roots , may be studied by observing a transverse structure is d i f ferent iated into outer

epidermis, the inner ground t issue and vascular t issue.

 

 

Dicoty ledonous stem:    Example:  Sunf lower (Hel ianthus annus) .

The transverse sect ion is a lmost c i rcular in out l ine.

 

Epidermis:   This is the outermost layer , uniser iate, covered with th in cut ic le , and bears a

few stomata and many mult ice l lu lar t r ichomes.

 

Ground T issue:   This fundamental t issue is d i f ferent iated into outer cortex and centra l p i th.

 

Cortex:   This is many layered and di f ferent iated into hypodermis and inner cortex.  

Hypodermis is found below the epidermis, and is const i tuted with angular co l lenchyma

which is 3-5 layered.  

 

 

The inner part of cortex is const i tuted with chlorenchyma and parenchyma; large numbers

of intercel lu lar spaces are present between these cel ls . The inner most cort ica l layer is

wavy in out l ine, r ich in starch gra ins, with barre l shaped cel ls and is ca l led endodermoid

layer .   (Endodermis is general ly absent in stem; endodermoid layer d i f fers f rom endodermis

in lacking caspar ian th ickenings) .

Per icyc le:   This is the outermost region of vascular t issue.   This consists of a l ternately

arranged patches of sc lerenchyma and parenchyma.   The sc lerenchyma is a lso known as

hard bast f ibers .   S ince i t is found external to vascular bundle, the patch of sc lerenchyma is

a lso ca l led bundle cap.

 

Vascular t issue:   This consists of many vascular bundles arranged regular ly in the form of

broken r ing (Euste le) .   In each vascular bundle the xylem and phloem elements are in the

same l ine; the presence of one to few str ipped intrafasc icular cambium is found in between

xylem and phloem; the cambial ce l ls are th in wal led and are a lmost rectangular phloem is

only external to xylem: the protoxylem is nearest to centre.   The vascular bundles are

conjo int , co l latera l , endarch, open and wedge shaped.

 

Pith or Medul la :   This is inner to vascular t issue and occupies the centra l region.   I t is

composed of ce l ls of parenchyma, with intercel lu lar spaces.   Extensions of these cel ls into

the areas in between vascular bundles are cal led medul lary rays.

 

Monocoty ledonous stem:    Example:  Zea mays (Maize) . The out l ine is c i rcular .

 

Epidermis:   This is the uniser iate, outermost layer and is covered with th ick cut ic le .  

Tr ichomes are absent.

 

Ground t issue:   The ground t issue is not d i f ferent iated; into cortex and pi th, s ince there is

scattered arrangement of vascular bundles.   Even endodermoid layer and per icyc le are a lso

not d i f ferent iated.   Hypodermis const i tutes the exter ior ground t issue, and is composed of 2

– 3 layers of sc lerenchyma.   The major part of the ground t issue is parenchymatous.

 

Vascular t issues:  This consists of numerous, oval shaped vascular bundles scattered

throughout the ground t issue, hence the arrangement of vascular t issue is ca l led

atactoste le.   Per ipheral vascular bundles are smal l -s ized but many in numbers.   The centra l

vascular bundles are large s ized but a few in numbers in a l l the vascular bundles the xylem

and phloem are in the same l ine; cambium is absent; phloem is only external to xylem;

protoxylem faces the centre.   Hence the vascular bundles are descr ibed as conjo int ,

co l latera l , endarch and c losed.   Each vascular bundle is surrounded by a bundle sheath of

sc lerenchyma, which is prominent at the base and t ip of vascular bundle.   Xylem comprises

usual ly 2 opposi te ly arranged Metaxylem elements (p i t ted, fac ing per iphery) and a few

protoxylem elements ( fac ing the centre of stem) in a l inear row.   I f an imaginary l ine is

drawn connect ing the two Metaxylem elements with that of protoxylem, xylem appears ‘Y ’

shaped.   Usual ly the lower protoxylem elements break down and form a lysogenous cavi ty .  

Above the metaphloem, a smal l band of obl i terated protophloem is seen.   Metaphloem

consists of s ieve tubes and companion cel ls but not phloem parenchyma.

 

Compar ison between the stems of Dicot and Monocot:

Dicot stem

Ex: Sunf lower

Monocot stem

Ex : Maize

1.   Epidermal t r ichomes Present mult ice l lu lar Absent

2.   Hypodermis Col lenchymatous Sclerenchymatous

3.   Ground t issue Dif ferent iated into

cortex and P i th

Undi f ferent iated

4.   Endodermoid   layer and

        Per i -cyc le

Present Absent

5.   Ste le Eustele Atactoste le

6.   Vascular bundles Many open, wedge-

shaped

Numerous, c losed,

oval -shaped

7.   Lys igenous cavi ty Absent Present

8.   Bundle cap Present Absent

9.   Bundle sheath Absent Present

10. Secondary Growth Occurs later Does not occur

 

Dicoty ledonous root:    Example: Lablab purpureus (Beans)

Epiblema:   The external protect ive layer is ca l led Epib lema or P i l i ferous layer ( the term

epidermis is general ly not appl ied to roots) .   I t is uniser iate.   Cel ls

 

are th in wal led.   Some of these cel ls extend into unicel lu lar root hairs .

 

Cortex:   The region inner to epib lema is ca l led cortex and i t is homogenous.   The cortex is

composed of th in wal led, spaciously arranged parenchyma cel ls that const i tute many

layers.   With increase in age, the exter ior cort ica l region becomes suber ized and dead.   This

exter ior cortex is known as exodermis.   The innermost cort ica l layer is ca l led endodermis.  

I ts ce l ls are barre l -shaped and are c losely packed.   Many cel ls get suber ized on their radia l

and inner wal ls .   These th ickenings are cal led caspar ian str ipes.   Such endodermal cel ls may

become tota l ly suber ized on a l l wal l surfaces and are cal led stone cel ls .   These cel ls do not

a l low conduct ion.   The non-suber ized endodermal cel ls are ca l led passage cel ls .     Inner to

endodermis, one or two layers of parenchyma are present, const i tut ing per icyc le.

 

Ste le:    Xylem and phloem patches are in equal number and a l ternate with each other .  

Xylem or phloem patches are usual ly four in number.   In Xylem, the protoxylem elements

face per icyc le; between xylem and phloem elements, parenchyma cel ls are present

const i tut ing conjunct ive t issue, which later becomes a secondary mer istem.   Hence vascular

bundles are descr ibed as radia l , exarch and open.

P i th   is very smal l , wi th c losely packed parenchyma cel ls .

Monocoty ledonous root:  Example:  Canna

Out l ine a lmost c i rcular .

 

The nature of epib lema, exodermis, cortex, endodermis and per icyc le is s imi lar to that of

Dicot root . However in cortex, large a i r spaces of schizogenous or ig in may be seen.   The

nature and arrangement of vascular bundles are a lso s imi lar to that of Dicot root .   But

vascular bundles are usual ly more than s ix in number.   The Parenchyma present in between

xylem and phlegm is ca l led conjunct ive t issue, with th in cel l wal ls .   This t issue does not

become meristemat ic and thus d i f fers f rom Dicot root .   P i th is re lat ively large and

parenchymatous.  The parenchyma of the

 

P i th and conjunct ive t issue, become l igni f ied and Sclerenchymatous.   The vascular bundles

are descr ibed as polyarch, radia l , exarch and c losed.   An endogenous latera l branch may be

seen developing f rom per icyc le and pass ing through cortex.

 

Compar ison between the roots of Dicot and Monocot:

Dicot Root

Ex: Beans

Monocot Root

Ex : Canna

1. Vascular bundles Diarch to hexarch, open Polyarch c losed.

2. P i th Smal l or absent Relat ively large

3. Cambium Conjunct ive t issue becomes

mer istemat ic and resul ts in

secondary growth

Conjunct ive t issue does

not become a mer istem

and complete absence of

secondary growth.

4. Per icyc le I t may give r ise to cork

cambium and latera l branches

I t may give r ise to only

latera l branches.

 

Compar ison between the structures of stem and root :

Stem Root

1. Cut ic le Present, th ick / th in Usual ly absent

2. Tr ichomes When present, is mult i -ce l lu lar

or unicel lu lar on the Epidermis.

Epib lema hairs are

unicel lu lar

3. Stomata Present Absent

4. Hypodermis Ei ther co l lenchymatous or

sc lerenchymatous.

Not d ist inct f rom inner

cortex.

5. Cortex When present, is

heterogeneous, with

co l lenchyma, parenchyma and

sc lerenchyma.

Homogenous with cel ls of

parenchyma.

6. Endodermis Usual ly absent; endodermoid General ly present

layer may be present

7. Per icyc le Two to few layered/absent One layered

8. Vascular bundles Many, conjo int , co l latera l or

b ico l latera l , regular ly arranged

forming a broken r ing or

scattered in ground t issue,

endarch.

2-12, radia l , exarch, form

a broken r ing.

9. P i th Wel l marked and large or not

d i f ferent iated

Relat ively smal l or absent.

 

10. Branching Exogenous, or ig inat ing f rom

cortex

Endogenous, ar is ing form

per icyc le.

 

Dicoty ledonous leaf :    Example:  Mangi fera (Mango)

The leaf is hor izonta l and dors iventra l ly d i f ferent iated (upper = ventra l , lower = dorsal ) .

 

Epidermis:  The two surfaces are bounded by their respect ive epidermal layers.   The

epidermal cel ls are barre l -shaped, compact ly arranged; upper epidermis is covered with

th ick cut ic le and lacks stomata; lower epidermis is l ight green, covered with th in cut ic le and

is interrupted by stomata.

 

Mesophyl l :    This is the ground t issue.   I t is d i f ferent iated into upper pal isade and lower

spongy parenchyma.

 

Pal isade parenchyma is found immediate ly below the upper epidermis, 2 to 3 layered, with

compact ly arranged tubular cel ls , r ich in par ieta l chloroplasts .

 

Spongy parenchyma is found above the lower epidermis; these cel ls are var ied in shape and

s izes, very loosely arranged enclos ing a i r spaces some of which open into stomata.  

Chloroplasts are par ieta l in the parenchyma cel ls .

Vascular t issue:  Vascular bundles vary in s ize; each bundle is conjo int , co l latera l and

c losed.   The vascular bundle is covered by a bundle sheath of parenchyma cel ls .   An

extension of bundle sheath is seen above and below the vascular bundle, reaching the

epidermal layers.   Phloem is towards lower epidermis; xy lem is towards upper epidermis,

with metaxylem fac ing phloem.   F ibers are absent in both xylem and phloem.   The xylem

and phloem elements are conspicuous only in large vascular bundles.

 

Monocoty ledonous leaf                Example: Maize.

The leaf is Isobi latera l in nature and is vert ica l ly or iented.

 

Epidermis:    This is uniser iate, with barre l -shaped, compact ly arranged cel ls and is covered

with th ick cut ic le .

 

Stomata are found on both upper and lower epidermal layers hence i t is ca l led

amphistomatic (more on the lower epidermis) .   Though the leaf is referred to as isobi latera l ,

i t is only in upper epidermis, a few large, empty and color less bul l i form or motor cel ls are

present.   Dur ing dry weather , these motor cel ls help the leaf to ro le over , due to the

changes in turgid i ty .   This ro l l ing of leaf reduces the rate of stomatal t ranspirat ion.

 

Mesophyl l :    There is no d i f ferent iat ion of mesophyl l into spongy and pal isade parenchyma.  

A l l the cel ls of chlorenchyma are a l ike, isodiametr ic , a lmost compact ly arranged with

numerous par ieta l chloroplasts .

 

Vascular t issue:   The vascular bundles are numerous, arranged in paral le l ser ies (venat ion

is palmate-paral le l ) , conjo int , co l latera l and c losed.   Phloem is towards lower epidermis.  

Each vascular bundle is surrounded by chlorenchymatous bundle; th is sheath a lso serves

for temporary storage of starch.   A few vascular bundles are larger in s ize, with more

amount of xy lem and phloem and with large bundle sheath cel ls .   A patch of sc lerenchyma

is present above and below the large s ized vascular bundles.

 

Compar ison between the leaf structures of Dicot and Monocot .

Dicot Root

Ex: Mango

Monocot Root

Ex : Maize

1.          Or ientat ion of

leaf

Hor izontal , dors iventra l ly

d i f ferent iated

Vert ica l , isobi latera l

2. Stomata Usual ly on lower epidermis On both epidermal layers

3. Cut ic le Thick on upper epidermis Thick on both epidermal

layers

4. Motor cel ls Absent Present in upper epidermis

5. Mesophyl l Di f ferent iated into upper

pal isade parenchyma and lower

spongy parenchyma

No di f ferent iat ion of

mesophyl l

6. Vascular bundles Al l o f them are not seen in

paral le l ser ies and are

supported by bundle sheath

extension

Vascular bundles are in

paral le l ser ies and are

supported by

sc lerenchyma patches

 

Secondary Growth in Dicot Stem

Increase in latera l th ickening is ca l led secondary th ickening or secondary growth.   This

growth occurs in stem and root parts of shrubs and trees in Dicoty ledons.   This development

takes p lace in ste le ( intraste lar secondary growth) and a lso in cortex (extra ste l lar

secondary growth) .

 

Intraste lar Secondary Growth:   In each vascular bundle between xylem and phloem a str ip o

intra- fasc icular (with in vascular bundle) cambium is present.   Between vascular bundles,

medul lary rays composed of parenchyma are found.   A few parenchyma cel ls of these rays

adjacent to intra- fasc icular cambium dedi f ferent iate into mer istemat ic cel ls and const i tute

inter- fasc icular cambium ( in between vascular bundle) .     Both inter-and intrafasc icular

cambiums jo in to form a complete r ing of cambium cal led vascular cambium, which in i t iates

intraste lar secondary growth.

 

The vascular cambia l ce l ls cont inuously d iv ide and form numerous daughter cel ls .   These

der ivat ives produced inward towards p i th mature into secondary xylem elements, whi le

those produced outward toward cortex d i f ferent iate into secondary phloem elements.   The

amount of secondary xylem t issue produced is much larger than that of secondary phloem

t issue. Due to the format ion of secondary xylem and phloem, the pr imary xylem and phloem

become separated.   The pr imary phloem is pushed outward and is f ina l ly crushed due to

internal pressure. The pr imary xylem is pushed towards the centre of the axis ; however

pr imary xylem elements are not crushed because the cel l wal ls are l igni f ied.   Between

bundles of secondary t issues parenchyma cel ls const i tut ing vascular rays are found.

 

Extraste lar secondary growth:   To keep pace with the expanding centra l ste lar region and to

mainta in the area volume equi l ibr ium between the ste le and cortex, epidermis and

per icyc le, with parenchyma or co l lenchyma cel ls acquires secondary mer istemat ic property

by dedi f ferent iat ion and is ca l led phel logen or cork cambium.   This cambium is s imi lar to

vascular cambium in cutt ing of f ce l ls on i ts outer and inner faces.   The outer der ivat ives

gradual ly lose their protoplast and become dead; their ce l l wal ls become suber ized. These

cel ls const i tute cork or phel lem.   These inner der ivat ives remain l iv ing and di f ferent iate

into parenchyma of co l lenchyma.   This newly formed inner zone is ca l led secondary cortex

or phel loderm.   The phel loderm, phel logen and phel lem are the pr imary cover ing layer i .e . ,

epidermis gets ruptured.   The newly formed per iderm becomes the protect ive layer .

 

Lent ice ls : the external phel lem or cork is impermeable to conduct ion or exchange of gases;

however i t is not a cont inuous zone; a few gaps are seen in cork.   In these gaps, a mass of

parenchyma cel ls (complementary t issue) is seen.   This structure is ca l led lent ice l .   This

helps in exchange of gases, between the atmosphere and inner cort ica l ce l ls .   Through

lent ice ls a lso, t ranspirat ion occurs and is ca l led lent icular t ranspirat ion ( loss of water is

ins igni f icant) .

 

Bark:   In cortex of t rees, general ly many phel logens ar ise, each one producing outer

phel lem and inner phel loderm.   I f such a process takes p lace, whatever t issues l ie outer to

the inner most phel logen, become suber ized and dead.   This structure is ca l led bark, which

i s of two types – 1) cont inuous bark 2) scaly bark or d iscont inuous bark.   Bark acts as

insulat ion.   I t is th icker in temperate tree forms.

 

Growth r ings:         In temperate woody plants and in a few tropical t rees, the act iv i ty of

vascular cambium is seasonal , but not uni form throughout the year .   This is ev ident only in

the region of secondary xylem.   The cambium is less act ive in autumn, producing few smal l

xy lem elements that are having smal l lumen.   This is ca l led late wood or autumn wood.   In

spr ing the cambium is more act ive producing large number of large s ized xylem elements

that are less dense with large lumen.   This is ca l led spr ing wood or ear ly wood.   The

d i f ference in secondary xylem corre lated to the cycl ic seasonal changes; resul t in the

format ion of a l ternate r ings of autumn wood and spr ing wood.   Thus in each year two zones

of secondary xylem are formed and both can be easi ly demarcated.   These two zones

const i tute an annual or growth r ing.   By count ing the number of annular r ings, i t is poss ib le

to know the age of a p lant .   The avai labi l i ty of auxin may be regulat ing the act iv i ty

cambium.

 

Wood:   The e lements of secondary xylem const i tute wood.   An act ive wood, contain ing water

and stores food substances is ca l led sap wood or a lburnum. I t is per ipheral .   I t ’s l iv ing

parenchyma cel ls store and translocate food.   Sap wood is soft and l ight in co lor .  

 

With

advance in age, the centra l ax is of secondary xylem becomes inact ive, los ing water and

stored food mater ia l .   I t becomes inf i l t rated with var ious organic chemicals l ike o i l and

ergast ic substances (Pass ive byproducts of metabol ism).   This region becomes hard in

texture and dark in co lor and is ca l led Heart wood or Duraman.   This zone loses the funct ion

of conduct ion of nutr ients .   Thus an act ive wood in addit ion to conduct ion of water a lso

conducts nutr ients .