THE PLANT-BODY 



23 



B 



The Stem (Caulome) 



The stem is the axis of the shoot which serves primarily to sup- 

 port the leaves and raise them to the light. It is also the medium 

 of communication between the subterranean absorbent organs, the 

 roots, and the aerial assimilating organs, the leaves. We find, there- 

 fore, that in the stem, the highly specialized conductive tissiies form- 

 ing the vascular bundles, are best developed, and besides this, the 

 mechanical tis- 

 sues, like wood \V> A 

 and fibrous tissue, 

 are present. 



Modified Stems. 

 While the 

 stem is primarily 

 a structure for 

 support and con- 

 duction of food, 

 it may become 

 much changed 

 and thus serve 

 other purposes. 

 It may be buried 

 in the earth, and 

 replace to some 

 extent the roots, 

 which are absent 

 (Psilotum, Coral- 

 lorhiza); but 

 more commonly FlQ 

 the subterranean 

 stems mainly 

 serve as reser- 

 voirs of food, 

 where starch and other reserve stuffs accumulate for future use. 

 Such underground stems are especially common in plants of cold 

 or dry regions where the growing season is a short one. Many 

 of the early flowers of our northeastern States, like the Spring- 

 beauty (Claytonia), Bloodroot (Sanguinaria), Spring-cress (Carda- 

 mine and Dentaria), Trillium, etc., develop thickened underground 

 stems (Tubers, Rhizomes) (Fig. 14), in which are stored up, during the 

 short period of growth, the necessary nourishment for the rapid 

 growth of the flowering shoot in the spring. Resembling the tubers, 

 but of more regular forms, are the Bulbs and Conns, which are espe- 

 cially common in the Lily family. The wild Tiger-lilies and Dog- 



^ 7 bulb of Narci$sus jonquilla ; II, longitudinal 

 section of /, showing the short stem . st, and the thick scale- 

 leaves; two young bulbs are forming as buds within the 

 old one. B, rhizome of Sanguinaria Canadensis; two 

 aerial shoots with scale-leaves, sc, at the base ; r, roots. 



