414 CONSERVATIVE ORGANS. [LECT. VIII. 



under the second, B. those which are hollow; so 

 that by again subdividing these, I shall be able to 

 convey to you some general ideas of the prominent 

 characters which constitute the diversities. 



A. The entire stem has no conspicuous internal 

 cavity, but the whole of that space which is filled 

 with the medulla in solid ligneous dicotyledons, 

 is made up of cellular matter turgid with fluid. 

 The stem, indeed, consists chiefly of similar cel- 

 lular texture, with detached bundles of vessels re- 

 gularly arranged through it, and the whole en- 

 closed by a bark. As the difference in point of 

 structure in this class depends on the arrangement 

 of the vascular bundles, it may be divided into 

 two genera ; ihejirst, a. comprehending all those 

 herbaceous dicotyledonous stems in which the dispo- 

 sition of the vascular bundles approaches to that 

 characterizing monocotyledonous stems ; and the 

 second b. those in which it resembles, in some de- 

 gree, the concentric circles oj ligneous dicotyle- 

 dons: each of these genera necessarily including 

 many species and varieties. 



a. To illustrate the first of the genera of stems 

 just defined, I have selected the stem of the 

 White Bryony, Bryonia alba, because it is a plant 

 very readily procured. 



If we examine with the naked eye a thin 

 transverse slice of the stem of Bryony, placed on 

 a dark surface, we perceive that it consists of a 



