656 



STEMS BEARING FOLIAGE-LEAVES. 



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

 Contrasts, like that lietween filamentous leai'y stems, barely a centimetre long, and 

 the giant trees of North America ami Australia, have not their like in t\w 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 {Gapsella pauciflora) 

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

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



Fig. 152.— Cotton Trees (Cavanillesia luberadata) of the Braziliau catingas. (After Martius.) 



in the Castor-oil plant {Ricinus ccmimunis), many of the stems attaining to a 

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

 comis and glandidlgera) which sometimes have a diameter of 4 centimetres. In 

 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 

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

 call " herbaceous" {stiiys herbacea). In the leafless stems of perennial plants, now 

 transformed into columns, a dried crust or liark replaces the succulent gi'een cortex 



