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VEGETATIVE OF FLOWERING

Stems, , and are the vegetative parts of vascular plants. Stems are the basic organs, as they give rise to other organs, such as roots, leaves, and . Both aerial and subterranean stems are divided into nodes (the places where leaves arise) and internodes (the regions between adjacent nodes). Stems may be simple or variously branched, either at the base or along the length of the stem, and either herbaceous (leaflike in texture; non-woody) or woody. The soft tissues found in stems of herbaceous plants result from primary, not secondary, growth. Woody plants, however, exhibit primary growth in their first year followed in subsequent years by secondary stem growth, which involves the addition of vascular each year around the previous year's . The yearly increment of vascular tissue can be recognized in many plants by annual rings. When evident, the annual rings can be counted to determine the age of the plant.

herbaceous stem woody stem

The stem structure of a plant is usually responsible for its overall manner of growth or , which may be described as herbaceous, shrubby, or arborescent. Herbaceous plants may live only one year (annuals) or persist for two (biennials) or more years (perennials). The annual condition is one that permits a plant to complete its life cycle in one growing season, thereby avoiding extreme environmental conditions, particularly high temperatures and low rainfall. Suffrutescent plants are herbaceous perennials with herbaceous above-ground parts that die back annually to a woody base. Other herbaceous perennials exist with persistent above-ground stems. A shrubby plant is woody, considerably branched, and usually less than ten meters tall, and an arborescent plant is one that is treelike.

Using the following terms and illustrations, identify the stem types on plants you see on campus. Note that the term 'rootstock' frequently has been used to include a or a caudex. A caudex is a short, woody, persistent stem at or just beneath the surface of the ground; its merges stem with tissue and is often difficult to define precisely. Subterranean stems obviously must be dug up before they can be identified. Make sure you can interpret and recognize the following stem types and conditions since these frequently are of great diagnostic value and are used in plant identification. ABOVE-GROUND STEM TYPES AND CONDITIONS

Ascendent aerial stem horizontal to the ground only near the base, being mostly erect

Decumbent aerial stem horizontal to the ground, ascending only near the stem apex

Erect and upright aerial stem with the leaves Caulescent arising from all or some of the nodes on the above-ground stems

Erect and upright aerial stem (=scape) with Acaulescent the leaves arising directly from an (Scapose) structure

Procumbent aerial stem horizontal to the ground but not rooting at the nodes

Repent aerial stem horizontal to the ground and rooting at the nodes elongate above-ground stem horizontal to the ground (may be procumbent or repent)

Thorn stiff, woody, modified stem with a sharp point

UNDERGROUND STEM TYPES

Bulb short and narrow underground stem surrounded by several to many thick, fleshy, nutritive, protective leaves (common example: onion)

Corm short, thick, and nutritive underground stem surrounded by two to several pairs of membranous, protective leaves (common example: gladiolus)

Rhizome common underground stem; usually long, slender, and creeping

Tuber short, thick and nutritive underground stem; usually a shortened part of a rhizome (common example: ) Roots anchor plants in , often store carbohydrates, and serve as the principal organs for absorption of and . Primary roots are those derived from the primary root of a . These include a fibrous root system in which the are all about the same size, and a system in which the central root is larger than the branches. When water-absorbing structures arise from a stem, they form an adventitious root system. Most monocots and seedless vascular plants ( and their allies) and some dicots have a fibrous adventitious root system. Many dicots and most have a root system that is either fibrous or taproot.

Leaves are the primary photosynthetic organs of plants, and are borne only at the nodes of a stem. Leaves usually consist of a blade (lamina) and a (except when sessile), and may be either simple or compound. If compound, the individual units of the are called leaflets, which are either pinnately or palmately arranged. On either side of the petiole or leaf base, there are often stipules subtending the leaf; these may be leaflike, or modified as bristles or sheaths. In the axil, or angle between each simple or compound leaf and stem, there is an axillary that will develop into a system () if not inhibited by plant hormones (). or removing the terminal parts of branches of or usually is a good way to release the hormonal inhibition of axillary located lower on the branches, thus permitting new branches to develop and resulting in a diffuse branching system.

Leaves that persist on a plant for several years or growing seasons are called evergreen, while those that fall after one growing season are deciduous. Leaves vary in shape, margin, apex, base, and vestiture (refers to covering of glands or hairs; when surface is smooth, without hairs or glands, the condition is called glabrous). Leaves also vary by their arrangement on a stem (phyllotaxy). The common leaf arrangements are alternate, opposite, whorled, and spiral. On the following pages, you will find examples of the common features of leaves. Become acquainted with these terms. Use plants in your own surroundings to study and identify leaf characteristics: duration (evergreen or deciduous), leaf arrangement, leaf type (simple or compound), pattern of leaf venation, shape, nature of apex and base, and margin. Vegetative morphology worksheet. Describe the stem and leaf morphology of the unknown plants on display in the lab.

Unknown Leaf Venation Leaf type Shape Apex/base Margin Arrange- pattern ment

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