The Shoot: Primary Structure and Development

Chapter 25

True or False1) Shoots contain stems

and leaves.2) Shoots are the above

ground portion of the plant and are anatomically the same as roots.

The Shoot• Shoot- the above ground portion of the

plant, consisting of stem and leaves.– Stem- the above ground axis of vascular

plants. – Leaf- the principle lateral appendage of

the stem. • Foliage leaf- photosynthetic organ.

• Node- the part of the stem where one or more leaves are attached.

• Internode- the region of the stem between two successive nodes.

• Bud- an embryonic shoot protected by young leaves.

Primordium- a leaf or bud in its earliest stage of differentiation.

Pith- the ground tissue occupying the center of a root or shoot.

Pith- the ground tissue occupying the center of a root or shoot.

Basswood Alfalfa Corn

Modified Stems• Tendrils- modified leaves that spirally coil and

aid in stem support. • Stolons- runners; stems that grow horizontally

on the soil surface, often giving rise to new plants at the nodes.

• Thorns- hard, woody, sharp-pointed modified stems, arise from the axils of leaves.

• Spines- hard, woody, sharp-pointed modified leaves or leaf-parts.

• Prickles- a small non-woody, sharp-pointed outgrowth of bark or the epidermis.

True or False. This is a tendril.

True or False. This is also a tendril.

Given: the following is a hard, woody, sharp-pointed modified stem that arises from the axils of leaves.

True or False. This is a spine.

Spines

Prickles

Underground Stems• Rhizome- a horizontal stem that grows at or below the

soil surface.• Tuber- an enlarged, short, fleshy underground stem,

forms at the tip of a rhizome. i.e. Potato.• Bulb- a shortened underground stem covered by

enlarged fleshy storage leaves. i.e. Onion and garlic.• Corm- a thickened underground stem with small papery

leaves. i.e. Gladiolus and Crocus.

Leaf Arrangement• Phyllotaxy- the arrangement of leaves on a stem.

– Basal- leaves are attached to an underground stem or rhizome.

– Opposite- 2 leaves attached at each node, borne on opposite sides of the stem.

– Alternate- 1 leaf attached at each node.– Whorled- 3 or more leaves at each node.

Basal

Leaf Morphology• Blade- lamina.• Petiole- stalklike portion.• Stipules- scalelike or leaflike

appendages at the base of leaves.• Simple vs. Compound Leaves

– Simple- blades undivided.– Compound- blades divided into

leaflets.• Pinnately compound- leaflets arising

from both sides of the axis.• Palmately compound- leaflets diverge

from the tip of the petiole.

Leaf Histology

• Mesophyll- the ground tissue of the leaf, specialized for photosynthesis.– Palisade parenchyma-

upper cells.– Spongy parenchyma-

lower cells.

Hydrophyte- waterlily. Xerophyte- oleander.

Eudicot- random stomata. Monocot- parallel stomata.

Leaf Abscission

• Abscission- the normal separation of the leaf from the stem.

• Structural and chemical (ethylene) changes result in the formation of an abscission zone:– Separation layer– Protective layer

• Magnesium ions, sugars, amino acids.