VEGETATIVE MORPHOLOGY OF FLOWERING PLANTS Stems, roots, and leaves are the vegetative parts of vascular plants. Stems are the basic 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 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 tissue each year around the previous year's vascular tissue. 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 habit, 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 rhizome or a caudex. A caudex is a short, woody, persistent stem at or just beneath the surface of the ground; its anatomy merges stem with root 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 Caulescent upright aerial stem with the leaves arising from all or some of the nodes on the above-ground stems Erect and Acaulescent (Scapose) upright aerial stem (=scape) with the leaves arising directly from an 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 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: potato)
Roots anchor plants in soil, often store carbohydrates, and serve as the principal organs for absorption of water and nutrients. Primary roots are those derived from the primary root of a seedling. These include a fibrous root system in which the branches are all about the same size, and a taproot 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 (ferns and their allies) and some dicots have a fibrous adventitious root system. Many dicots and most gymnosperms 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 petiole (except when sessile), and may be either simple or compound. If compound, the individual units of the leaf 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 bud that will develop into a shoot system (branch) if not inhibited by plant hormones (auxins). Pruning or removing the terminal parts of branches of shrubs or trees usually is a good way to release the hormonal inhibition of axillary buds 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.
Worksheet exercise: describe the vegetative morphology of the five unknowns set out as demos in lab. 1 Leaf type Leaf arrangement Leaf veination Leaf shape Leaf base/apex Leaf margin 2 3 4 5