11. THE STEM. 27 



or more subterranean shoots are produced which carry on by innovation 

 the elongation of the rhizome. If the older portions of the rhizome persist 

 for a long time, the basal portions of the annual shoots together form a 

 sympodium (Fig. 14 B) ; if, however, they soon perish, then each annual 

 shoot appears to constitute a distinct individual (e.g. Ranunculus acris, 

 Neottia). It is by the simultaneous formation of a number of short 

 innovation-shoots that the tufts of Grasses and Sedges are produced. The 

 innovation-shoots commonly develope roots of their own, but they may 

 remain connected with the main root of the plant as in Anemone PulsatUla. 



In rare cases (Psilotum among Vascular Cryptogams) the functions of 

 roots are performed by subterranean shoots ; these shoots are more slender 

 than the subaerial shoots, and bear the merest rudiments of leaves. 



Shoots which are unable to grow erect by themselves obtain, in some 

 cases, the Advantages of that position by climbing. The structure of the 

 shoot may be modified so as to subserve climbing. Branches are in some 

 cases (Uncaria) developed in the form of hooks, and may or may not bear 

 leaves ; these hooks serve to attach the plant to others. In other cases, 

 branches bearing small scaly leaves are developed into tendrils, which 

 twine round supports. In other cases the whole shoot twines round a 

 support (Fig. 15 A, B). 



Branches are sometimes developed as thorns (Fig. 16). Thorns are hard, 

 pointed structures ; they sometimes form the extremity of an ordinarj' 

 shoot, as in Rhamnus cathartica ; or they are dwarf-shoots, as in Cratcegus 

 coccinea ; they may bear branches which spring from the axils of scaly 

 leaves, as in Gleditschia and the Sloe (Fig. 16). 



The morphology of the constituent members of the leafy shoot, 

 namely the stem and the leaf, will now be considered. 



11. The Stem. The stem of an annual plant or of an 

 annual shoot is succulent in texture, and 

 is said to be herbaceous. 



A primary stem which persists for 

 several years, though it is herbaceous at 

 first, becomes hard and woody in tex- 

 ture, and is termed a trunk. 



The stem is commonly branched ; but 

 it may be unbranched, as in Tree-Ferns, 

 Cycads, many Palms and Grasses. 



The form of the Stem varies Very, leaf -scar, from the axil of which 



widely. It may be short and much tbe thorny branch z 8prings ; on 



,,. , , . J the thorn are // leaf -scars; in 



thickened, as in the bulb, corm and the axil of the upper one is the 

 tuber, mentioned above (p. 24) and in branch z - in that of the lower - 



. ... , the bud fc. 



some Uacti ; or a portion of it may be 



much thickened into a tuber, as in certain epiphytic Orchids, 



where one or more of the basal internodes form a pseudo-bulb 



