STEMS

The Plant Body: Stems FUNCTION OF STEMS • Stems support leaves,

branches and flowers • Transport water and solutes

between roots and leaves. •

• In some plants, stems have specialized functions…

Stems support a display of leaves and sometimes flowers

Stems orient the leaves toward the light and minimize overlap among the leaves.

Asclepias - milkweed

External Form of A Woody Twig • Apical (Terminal) Bud at

twig tip – Growth makes twig longer. – Actively growing (at least

during the growing season) • Axillary (Lateral) Bud

– Can become branches or flowers in flowering plants

– Dormant

External Form of a Woody Twig • Deciduous trees and

shrubs lose all leaves annually

• After leaves fall, you can more easily see the axillary buds with leaf scars below

Apical Dominance Apical dominance refers to

the suppression of the growth of lateral meristems by hormones produced in the apical meristem.

The growth pattern of pines indicates strong apical dominance.

Bushy plants have weak apical dominance.

If apical meristem is removed, plants may become bushier.

Shoot Apical Meristem

Internal Stem

Anatomy (Woody Dicot)

Monocot Stem – cross section

Dicot Stem Cross Section Helianthus annuus- sun flower

Pith

Ring of vascular bundles

Epidermis

Cortex

Epidermis

Collenchyma

Fibers

Phloem

Vascular Cambium

Xylem

Pith

Early Secondary Growth

Vitis vinifera - grape

The vascular cambium secondary vascular tissue

Three years of Secondary Growth Tilia - basswood

Secondary Xylem

Secondary Phloem

Anatomy of a Woody Stem

Modified Stems

Stolons

• Horizontal above-ground stem

• Reproductive function • Classic example of a

plant that produces stolons is strawberry

Tuber

• Underground stem • The "eyes" of a potato

are the nodes (lateral buds) on a stem

• No leaves, but there is an axillary meristem

Rhizomes

• Rhizomes - horizontal stems that grow below ground produce leaves and adventitious roots

• Examples: ginger, irises, ferns, etc.

Bulb

• Large bud with a small stem, surrounded by fleshy leaves

• Adventitious roots at base

• Examples include onion, tulip, and lily

Corm • Resemble bulbs, but

composed of stem tissue w/ papery leaves

• Food storage organ with adventitious roots at the base

• Examples include crocus and gladiolus.

Tendrils

Grape Tendrils

Stems modified to help plants climb they can grow taller without investing in girth

Thorns • Stems modified for

defense (e.g. citrus)

• Be careful: – spines are modified

leaves – prickles are

extensions of the cortex and epidermis

Roses don’t have thorns

Cladophylls • Photosynthetic stems

– In some species the stem is more important for photosynthesis than leaves

Rosette • Rosette – a stem with very short internodes

(not really specialized, but worth mention)