VEGETATIVE MORPHOLOGY OF FLOWERING PLANTS

Stems, roots, and leaves are the vegetative parts of vascular plants. Stems are thebasic organs, as they give rise to other plant organs, such as roots, leaves, and flowers.Both aerial and subterranean stems are divided into nodes (the places where leavesarise) and internodes (the regions between adjacent nodes). Stems may be simple orvariously branched, either at the base or along the length of the stem, and eitherherbaceous (leaflike in texture; non-woody) or woody. The soft tissues found in stemsof herbaceous plants result from primary, not secondary, growth. Woody plants,however, exhibit primary growth in their first year followed in subsequent years bysecondary stem growth, which involves the addition of vascular tissue each year aroundthe previous year's vascular tissue. The yearly increment of vascular tissue can berecognized in many plants by annual rings. When evident, the annual rings can becounted 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 orhabit, which may be described as herbaceous, shrubby, or arborescent. Herbaceousplants 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 cyclein one growing season, thereby avoiding extreme environmental conditions, particularlyhigh temperatures and low rainfall. Suffrutescent plants are herbaceous perennialswith herbaceous above-ground parts that die back annually to a woody base. Otherherbaceous perennials exist with persistent above-ground stems. A shrubby plant iswoody, considerably branched, and usually less than ten meters tall, and anarborescent plant is one that is treelike.

Using the following terms and illustrations, identify the stem types on plants you see oncampus. Note that the term 'rootstock' frequently has been used to include a rhizome ora caudex. A caudex is a short, woody, persistent stem at or just beneath the surface ofthe ground; its anatomy merges stem with root tissue and is often difficult to defineprecisely. Subterranean stems obviously must be dug up before they can be identified.Make sure you can interpret and recognize the following stem types and conditionssince 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 groundonly near the base, being mostlyerect

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

Erect and upright aerial stem with the leavesCaulescent arising from all or some of the nodes on

the above-ground stems

Erect and upright aerial stem (=scape) withAcaulescent the leaves arising directly from an(Scapose) underground stem 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

Stolon 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 twoto 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 (commonexample: potato)

Roots anchor plants in soil, often store carbohydrates,and serve as the principal organs for absorption ofwater and nutrients. Primary roots are those derivedfrom the primary root of a seedling. These include afibrous root system in which the branches are allabout the same size, and a taproot system in whichthe central root is larger than the branches. Whenwater-absorbing structures arise from a stem, theyform an adventitious root system. Most monocotsand seedless vascular plants (ferns and their allies)and some dicots have a fibrous adventitious rootsystem. Many dicots and most gymnosperms have aroot system that is either fibrous or taproot.

Leaves are the primary photosynthetic organs ofplants, and are borne only at the nodes of a stem.Leaves usually consist of a blade (lamina) and apetiole (except when sessile), and may be eithersimple or compound. If compound, the individualunits of the leaf are called leaflets, which are eitherpinnately or palmately arranged. On either side of thepetiole or leaf base, there are often st ipulessubtending the leaf; these may be leaflike, or modifiedas bristles or sheaths. In the axil, or angle betweeneach simple or compound leaf and stem, there is anaxillary bud that will develop into a shoot system(branch) if not inhibited by plant hormones (auxins).Pruning or removing the terminal parts of branches ofshrubs or trees usually is a good way to release thehormonal inhibition of axillary buds located lower onthe branches, thus permitting new branches todevelop and resulting in a diffuse branching system.

Leaves that persist on a plant for several years orgrowing seasons are called evergreen, while thosethat fall after one growing season are deciduous.Leaves vary in shape, margin, apex, base, andvestiture (refers to covering of glands or hairs; whensurface is smooth, without hairs or glands, thecondition is called glabrous). Leaves also vary bytheir arrangement on a stem (phyllotaxy). The common leaf arrangements arealternate, opposite, whorled, and spiral. On the following pages, you will find examplesof the common features of leaves. Become acquainted with these terms. Use plants inyour own surroundings to study and identify leaf characteristics: duration (evergreen ordeciduous), 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 theunknown plants on display in the lab.

Unknown LeafArrange-ment

Venationpattern

Leaf type Shape Apex/base Margin

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