The vegetative body of the sporophyte of the vascular plant consists of three organs: stem, leaf and root. This division U fundamentally theoretical, for the plant is a unit on the basis of iti development, evolution and structure.
The embryonic axis in the seed consists of a hypocotyl aflfl radicle. At the tip of the hypocotyle one or more cotyledons and the bud of the shoot (ie) the plumule are found. At the tip of the radiclfl is the root cap.
The bud of the shoot usually consists of an axis, the epii containing a few internodes, which have not elongated and some primordia. With the germination of the seed the embryo enlarges aaM starts to grow, the apical meisterm of the young shoot adds further leaf primordia, and the internodes between the lower primordH Buds develop in the axis of the developing leaves giving rise to « branched shoot.
That part of the stem from which a leaf or leaves develop it called the node and that portion of the stem between two such nodes, the internode. At each node one, two or more leaves may be found, The arrangement of the leaves on the stem between is tennol phyllotaxy.
The primary structure of the stem may be conveniently described into three tissue systems: dermal, fundamental or ground tissue and the vascular. In stems having the vascular system in the form of a solid cylinder the ground tissue located between the epidermis and the vascular system constitutes the cortex. If the vascular system has the shape of a hollow cylinder, it encloses pari ol the fundamental tissue, the pith, If this cylinder is divided into strandi called the vascular bundles or fascicles, the spaces among tho strands, the interfascicular regions are occupied by parenchymatic ground tissue. These plates of tissue are sometimes designated at pith rays or mudullary rays.
Types of Stem
The stems can be distinguished into woody, herbaceous, vi type, monocot and anomalous types. These groupings however not necessarily based on sharp distinctions. In some instances t
differences are mainly quantitative, in others stern types placed into I different groups integrade through transitional types.
Of particular interest is the separation into woody and rbaceous stem types. The prevalent view is that in angiosperms the Woody type of plant is more ancient than the herbaceous (Bailey, l'M4; Takhtajan, 1959). The more primitive angiosperms are 'imposed of woody plants. More than half of the families of licotyledons have no herbaceous species. The evolution of lerbaceous dicotyledons from the woody involved a decrease in the ii livity of the vascular cambium after supplemented by a widening of he interfacicular regions.
