172 



General Botany 



yuccas develop very rapidly, and in spite of the short internodes 

 the stem rises a few feet above the ground. In one species the 

 stem branches and forms a tree with ro- 

 settes at the tips of the branches (Fig. 70) . 

 Annual-stemmed type. These include 

 many herbaceous perennials with under- 

 ground rootstocks, tubers, corms, bulbs, 

 and fleshy roots. The aerial stems are 

 developed during the moist periods, and 

 after supporting leaves and flowers die 

 down to the ground. To this group be- 

 long the conspicuous grasses and peren- 

 nial flowering herbs. Many of the aerial 

 stems show the characteristic structures 

 which reduce transpiration. 



Summary of xerophytes. Stems of 

 xerophytes, then, are either (i) succulent 

 with a high water-holding capacity, or 



(2) hard, water-proofed, and woody, or 



(3) short underground supports for ever- 

 green or temporary rosettes of leaves, 

 or (4) temporary stems arising from un- 

 derground structures. The stems of the 

 leafless forms possess chlorenchyma. 

 Many of these stems have coverings of 

 hairs, spines, and thorns. 



A word of caution concerning the origin of xerophytes may not 

 be out of place in this summary. It is not to be assumed that 

 xerophytes may be formed from mesophytes by the direct action 

 of the environment, though- some plants are temporarily modified 

 in this way when grown under conditions of drought. The 

 characteristics of the more pronounced xerophytes are hereditary 

 quahties which may have arisen as variations entirely aside from 

 the influence of a desert environment. Likewise, the pecuhar 



Fig. 98. Century plant {Agave), 

 showing the rosette of fleshy 

 leaves and the flowering stalk. 

 It is a perennial, but, like an 

 annual or a biennial, it dies 

 when it flowers and fruits. The 

 century plant is an example of 

 the short-stemmed type of xero- 

 phyte. 



