656 



STEMS BEARING FOLIAGE-LEAVES. 



already-described scaly stems; here only are the differences in size more marked. 

 Contrasts, like that between filamentous leafy stems, barely a centimetre long, and 

 the giant trees of North America and Australia, have not their like in the whole 

 vegetable kingdom. In those plants which germinate, grow, blossom, and fruit and, 

 after the distribution of their seeds, perish, all in a single year in these short- 

 lived annuals the foliage-stem seldom attains to a considerable diameter. In many 



small Cruciferse, e.g. in the small-flowered Shepherd's Purse (Capsella pauciflora) 

 and in the tiny Chaffweed (Centunculus minimum), the diameter of the stem often 

 scarcely amounts to half a millimetre. The largest dimensions in annuals are found 



****?&^- 



^^v? 



Fig. 152. Cotton Trees (Cavanillesia tuberculata) of the Brazilian catingas. (After Martius.) 



in the Castor-oil plant (Ricinus communis), many of the stems attaining to a 

 diameter of 7 centimetres, and in the balsams of the Himalayas (Impatiens tri- 

 coniis and glanduligera) which sometimes have a diameter of 4 centimetres. ID 

 these annual plants the stem which bears the leaves perishes with them every year. 

 It is otherwise with plants whose stem remains alive for more than one period of 

 vegetation, and which have been called perennial. When these throw off their 

 foliage, they do not die, but fashioning themselves into supports for the leafy shoots 

 which arise from their buds, attain a circumference in just proportion to the new 

 burden to be borne. The structure of such foliage-stems then becomes altered. 

 The stems of annuals and those of the young new shoots of perennial plants have 

 a green succulent cortex with a peculiarly-developed epidermis; such a shoot we 

 call " herbaceous" (stirps herbacea). In the leafless stems of perennial plants, now 

 transformed into columns, a dried crust or bark replaces the succulent green cortex 



