stems. roots and leaves together are sufficient to take up all essential resources, so why make...

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STEMS

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Shoot growth – primary plant body 35.11/35.10 Apical Meristems make primary shoot increase in length Lateral Meristems make secondary growth increase in girth Shoot – above ground plant structure stems, branches, leaves

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Page 1: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

STEMS

Page 2: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

Roots and leaves together are sufficient to take up all essential resources, so why make stems?

Stem functions 1. Support leaves 2. Conductance (connect root and leaf vasculature) 3. Storage (some species) 4. Photosynthesis (some species)

But lots of plants don’t have any stems at all – if not required, why “waste” the resources?

While all the above functions are important, the most general benefit is improved light environment for the leaves – especially due to competition between plants.

Page 3: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

Shoot growth – primary plant body

35.11/35.10

Apical Meristems make primary shoot increase in length

Lateral Meristems make secondary growth increase in girth

Shoot – above ground plant structurestems, branches, leaves

Page 4: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

Apical Meristem makes primary shoot 1. stem growth and tissues 2. leaf primordia – become leaves 3. bud primordia – become lateral branches

35.16/35.15

Page 5: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance
Page 6: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

35.2

This process creates the primary plant body with it’s node – internode structure

Node – leaf/branch attachment

Internode – between nodes

Page 7: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

Apical dominance Apical meristem at the leading shoot tip

inhibits bud primordia (lateral buds) nearer to the tip, releasing them later (farther) resulting in “Christmas tree” like growth form.

39.9

Page 8: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

Stem sections show all three main tissue types vasculature - often in bundles or sometimes ringsepidermisground tissue – often called cortex, or “pith” if inside

35.17/35.16

Page 9: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

These sections are useful, but mask the 3-dimensional structure

The vascular bundles appear separate in cross section, but generally have multiple cross connections throughout stem, including in monocots, creating an elaborate meshwork, including connections to the leaves.

Page 10: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

Modified Stems

Tendrils and twining stemsThorns (vs. spines)Stolons – above ground runnersRhizomes – below surface runnersFood storage

Tubers – swellings of stolons & rhizomesCorms – swellings at base of stem

Page 11: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

Corypha palm

Page 12: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

Water storage (succulence)Cacti – stem modified for water storage and

photosynthesis (leaves are spines). Two other families, Spurge (Euphorbiaceae) and milkweeds (Asclepiadaceae) have also evolved this. Classic example of convergent evolution.

Page 13: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

SECONDARY GROWTH

Secondary growth – increases in girth (width) of stems and roots resulting from lateral meristems

[aside] Simple non-rigorous categories, widely used 1.Woody plants – with 2o growth – trees and shrubs 2. Herbaceous plants – little or no 2o growth, “herbs”

a. Grassesb. Forbs (herbaceous dicots)

Page 14: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

Two main Lateral Meristems control secondary growth 1. Vascular cambium – makes 2o xylem and phloem 2. Cork cambium – makes periderm

In the secondary plant body, these form concentric cylinders or sheaths (rings in cross section) of meristem

35.11/35.10

Page 15: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

Vascular Cambium Fusiform initials – key meristematic cells, vertically elongated. They produce xylem cells to the inside and phloem outside, causing increases in girth.

xylemphloem(cross-section)

Vascular cambium

35.20/35.19

Page 16: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

Wood – secondary xylem

Eventually the tree may stop using the inner xylem.1. Sapwood – outer conducting xylem2. Heartwood – inner older xylem, no longer

conducting. Often darker due to deposits into vessels to block conductance and pathogen transmission

Xylem – remains in place and continues to function for many years. Rigid cells build on one another expanding the stem.

35.22/35.20

Page 17: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

Growth rings

In seasonal climates (cold/warm, wet/dry), early season growth, “early wood”, is less dense (larger cells, thinner walls) than “late wood”, producing annual “rings” visible to the eye.

Used to create detailed climate records.

Bristlecone pine can live 4900 years. Used to reconstruct climate to 8000 years ago.

Page 18: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

RESULTSR

ing-

wid

thin

dexe

s

2

1.5

0.5

1

01600 1700 1800 1900 2000

Year

Fig 35.21 – a composite ring-width chronology for Mongolian conifers suggesting elevated temperatures in 1900s.

Page 19: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

Phloem – usually only the current year phloem functions in sugar transport. It is eventually pushed outward and collapses due to the expanding stem.

35.19/35.18

Secondary phloemVascular cambium

Secondary xylem

Bark

Early woodLate wood Cork

cambium

Cork

Periderm

0.5

mm

Vascular ray Growth ringCross section of a three-year-old Tilia (linden) stem (LM)

(b)

0.5 mm

Page 20: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

Cork Cambium – the Periderm

A second sheath of meristem develops in the secondary phloem, outside the vascular cambium, called the cork cambium. It produces the periderm, which replaces the epidermis in secondary growth.

Cork cells – main component of the periderm, produced by the cork cambium to the outside. They are lined with suberin and dead at maturity. Impermeable to water (and gases).

Lenticels – openings in the cork layer to allow the living cells of the stem to respire

Page 21: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

Bark – all tissue outside the vascular cambium-what can be removed without killing the tree

As the tree expands, this tissue is stretched and eventually split and sloughed off. Structure of periderm determines the pattern.

Page 22: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

Cork oaks in Portugal – a renewable resource

Page 23: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

This figure nicely summarizes the structure of a woody stem

Cork cambium

Page 24: STEMS. Roots and leaves together are sufficient to take up all essential resources, so why make stems? Stem functions 1. Support leaves 2. Conductance

END