SECT. I. 



ORGANOGRAPHY AND GLOSSOLOGY. 



(45.) Aerial Stems. The stem is said to be "herb- 

 aceous," when it continues soft, and lasts only for 

 a short time ; dying soon after the flower has ex- 

 panded, and the seeds ri- 

 pened. It is called "woody," 

 when it continues to increase 

 for several years. Herba- 

 ceous stems belong to " an- 

 nuals/' " biennials," and 

 " perennials," which are 

 thus named, according to 

 the several periods which 

 their roots continue to live. 

 Woody stems are confined to 

 shrubs and trees; the former 

 having many stems rising 

 from the surface of the 



ground, and the latter possessing one main trunk, which 

 branches or not, according to the nature of the species 

 to which it belongs. An '' undershrub," is where the 

 branches are partly woody and partly herbaceous, so 

 that a portion only dies back every year. Besides these, 

 there are the " succulent" stems, so called from the 

 highly developed state of their cellular tissue, which 

 often remains replete with juices for many years, without 

 hardening into wood. 



(46.) Internal Structure of Stems and Roots. In 

 arts. 34, 35. we have given an account of the leading 

 differences, observable in the internal composition of the 

 stems of dicotyledonous and monocotyledonous plants ; 

 and we have now to explain a few more particulars 

 respecting them. 



(47.) Dicotyledonous Stems. In some stems of 

 dicotyledonous trees it is difficult, and in others im- 

 possible, to distinguish any separation of the wood into 

 concentric layers. This is especially the case with 

 trees of tropical climates, where vegetation is not liable 

 to the periodic checks which it receives in colder regions. 

 In a few examples, also, the medullary rays are not 



