Plant Circulation and Transport

Chapter 25

I. Xylem A. primary xylem

1. apical meristems procambium primary xylem2. formed early in development3. herbaceous and woody plants4. transport of water

B. secondary xylem1. formed later in development from vascular cambium2. woody plants only (annual rings in wood)3. structural support; fair amount of transport

C. xylem is a complex tissue1. tracheids and vessel elements

a. transport of H2O b. hollow and dead at maturity; only cell walls remainc. stacked on top of each other, forming pipelines

2. parenchyma cells• storage of H2O and minerals

3. xylem fibers• provide structural support

Fig. 24.6 Xylem structure

D. how is H2O moved upwards in a plant?1. many trees are very tall and H2O moves against gravity2. root pressure

a. H2O moves into plant’s roots from soil (osmosis)b. H2O pressure in roots increases and pushes H2O upc. not strong enough to account for rise of H2O in tall treesd. strongest influence is in small plants in spring

3. cohesion-tension modela. often begins with root pressureb. plants only use a fraction of the H2O they absorb

• most evaporates into the air and is lostc. water moves up plants through transpiration

i. evaporation of water from plant parts exposed to airii. H2O is pulled up by the drying power of air

• top H2O molecules are evaporated off by transpirationo this E pulls up the rest of the water column

• evaporation occurs through the stomata in leaves• water moves up a plant (roots stems leaves)

iii. transpiration creates a negative pressure in xylem• extends downward from the leaves to roots• similar to sucking liquid up a straw

The process of transpiration

Fig. 25.12 Cohesion-tension model of xylem transport

d. rate of transpirationi. regulated by stomata

and their guard cellsii. affected by env. factors

• wind, heat, drynesse. only works if H2O column

is unbroken and narrow• cohesion and adhesion

II. Phloem A. transport in phloem is much slower than in xylemB. primary phloem

1. apical meristems procambium primary phloem2. formed early in development3. herbaceous and woody plants4. transport of nutrients (sugars)

C. secondary phloem1. formed later in development from vascular cambium2. found only in woody plants (part of the bark)3. structural support; fair amount of transport

D. phloem is a complex tissue 1. sieve-tube members

a. transport nutrientsb. function as living cells, lack a nucleus and most organellesc. stacked on top of each other, forming pipelines

2. companion cellsa. lie adjacent to sieve-tube membersb. continually nourish sieve-tube members keeps them alivec. one companion cell per sieve-tube member

3. parenchyma cells – storage of nutrients4. phloem fibers – provide structural support

Fig. 24.7 Phloem structure

E. materials flowing through phloem 1. mainly sugars (sucrose)2. small amounts of other materials

• proteins, hormones, chemical defenses, lipids, wastes, etc. 3. phloem sap

F. how nutrients are moved within a plant1. phloem sap moves from sources to sinks

a. sourcesi. areas where sap is produced or storedii. leaves and roots (winter only)

b. sinksi. areas that require sap ii. all other areas of the plantiii. phloem sap moves primarily down from the leaves

2. phloem sap moves through translocationa. accomplished through a fluid pressure-flow mechanismb. process

i. involves buildup of sap at sources transported into phloem• phloem loading

ii. involves release of sap at sinks nutrients to tissues• phloem unloading

iii. sap moves from high pressure at sources to low pressure at sinksiv. inflow of H2O from neighboring xylem cells assists the process

c. ATP needed for this is provided by companion cellsd. sieve-tube members, companion cells, and xylem all work together

The process of translocation

Translocation

Fig. 25.15 Pressure-flow model of phloem transport (translocation)