Presentation on theme: "2011. stem may be defined as something that bears leaves. But this definition is too simplistic; it needs to be expanded From where does a stem."— Presentation transcript:
stem may be defined as something that bears leaves. But this definition is too simplistic; it needs to be expanded From where does a stem come? What part of the seed produces it? When one dissects a seed, it is found to bear a seed coat or two and, within the seed coat, an embryo. The embryo consists of cotyledons (the seed leaves, which are food sources),
a radicle (which grows downward upon germination to produce the root), and a plumule (which grows upward to produce the stem and leaves). The cotyledons are attached to the plumule. The seedling thus has two parts: the epicotyl (that part above the cotyledons) and the hypocotyl (that part below the cotyledons). Thus, a stem can be defined as that part of a plant above the hypocotyl.
The stem is the ascending portion of the axis of the plant, develop directly from the plumule, and bears leaves, branches and flower. When young its is normally green in color. The growing apex is covered over and protected by a number of tiny leaves
It branches exogenously and is provided with nodes and internodes which may not be distinct in all cases. Leaves and branches normally develop from the nodes. When the stem or branches end in a vegetative bud it continuous to grow upwards or side ways. If however, it ends in a floral bud, the growth ceases.
There is a variety of stem structures adapted to perform diverse functions. 1. aerial or 2. underground. Aerial: may be erect, rigid and strong, holding themselves in an upright position, while there are some too weak to support themselves in such position. They either trail on the ground or climb neighboring plants and other objects.
Underground Stem: Some stems remain permanently underground and from there, periodically give off aerial shoots under favorable conditions. Such stems are meant for food storage and perennation.
The unbranched and stout stem, marked with scars of fallen leaves is called caudex as in palms. Jointed stem with solid nodes and hollow internode is called culm, Eg. Bamboo some herbaceous plants, particularly monocotyledons have no aerial stem. The underground stem in them produce as erect unbranched aerial shoot bearing either a single flower or cluster of flowers such a flowering shoot is called scape. Eg. Tuberose, onion, aroids banana etc.
Three kinds: 1. Trailers: those plants whose thin and long or short branches trail on the ground, with or without rooting at node. Such plants like prostrate on the ground they are said to be prostrate or procumbent. When the branches of such plants, after trailing for some distance, tend to rise at their apex they are said to be decumbent Eg. Tridax.
when the plants are much branched the branches spread out on the ground in all directions, they are said to be diffused eg. Boerhavia.
2: Creepers Week-stem plants with their long or short branching creeping along the ground rooting at the node are said to be creepers, a creeping stems may be runnner, stolon, offset or sucker according to its varied nature.
Plants that attach themselves to any neighboring object, mean of some special devices, and climb it to a long or short distance. Eg pea, vine, passion flower, gourd etc etc.
Gymnosperms are entirely woody, while both woody and herbaceous forms occur in angiosperms. Angiosperms include both monocot and dicot. These differences create a need for several descriptions of stem anatomy.