MORPHOLOGY OF ANGIOSPERMS. 105 



Furthermore the stems of a number of plants grow under- 

 ground and these are known as rhizomes or root-stocks; from 

 the upper portion of the nodes overground branches arise which 

 bear leaves (so that the work of assimilation may be carried on) 

 as well as flowers, and from the lower surface, roots (Fig. 68). 



While most rhizomes are perceptibly thickened, and more or 

 less fleshy when fresh, as Saiiguinaria, in other instances they are 

 of the ordinary thickness of the overground stem. 1^ 



There are some rhizomes that are excessively thickened, as 

 in the common white potato, and these are called tubers. The 

 so-called " eyes " are small buds covered with small scale-like 

 leaves which develop into shoots. Tubers should not be con- 

 founded with tuberous roots, as those of the sweet potato and 

 jalap, for these latter have the morphological characters of roots. 



Instead of the node, or internode, or both, becoming excess- 

 ively thickened, they may be reduced in size and crowded upon 

 each other, the leaves at the same time becoming thickened and 

 filled with nutriment. Such a modified stem and leaves, as in the 

 onion, is called a bulb. Bulbs are sometimes produced in the 

 axils of the leaves of overground stems, as in some lilies, and 

 are then called bulbils or bublets. They are also found in Allium, 

 forming what are commonly known as "onion sets." P>ulbs and 

 tubers serve not only as storage-organs and carry the life of the 

 plant over from one season to another but may form, as in bulb- 

 lets, an important means of distributing the plants. The thick- 

 ened fleshy stems of Cactacese are also regarded as storage organs. 



A coRM is intermediate between a true tuber and a bulb ; 

 it is more in tlie nature of a thickened internode, being sur- 

 rounded in some cases by thin membranous scales, as in Crocus 

 and Colchicum. 



The function of the vegetative shoot is to absorb nutrition 

 from the earth as well as from the air. The shoot may be aerial 

 or subterranean. Some plants possess only aerial shoots or 

 LIGHT-SHOOTS, as for instance trees, shrubs and herbs that flower 

 but once. Other plants possess both aerial and subterranean 

 shoots and of these the subterranean shoot may exhibit some of 

 the peculiarities of roots, in that they do not develop chlorophyl 

 and produce secondary roots for the purpose of obtaining nutri- 



