stems

1. stems 2011

2. STEM • 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),

3. 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.

4. Characteristics of the Stem • 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

5. Characteristics condt…… • 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.

6. TYPES OF STEMS • There is a variety of stem structures adapted to perform diverse functions. • aerial or • 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.

7. 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.

8. 1. Erect or strong stem • 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.

9. Weak stem • Three kinds: • 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.

10. when the plants are much branched • the branches spread out on the ground in all directions, they are said to be diffused • eg. Boerhavia.

11. 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.

12. 3. Climbers: • 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.

13. Internal structure • 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.

14. Group Discussion

15. What are the differences in cross section view between dicot and monocot stem?

16. Activity • What are the underground modification of stems? • What are the aerial modification of stem? • What are the functions of stems?

17. END