THE ANATOMY OF STEMS. 259 



incipient stage of organization. But the vertical di- 

 rection of the cells, while those of the cuticle are hor- 

 izontal, is sufficient to overturn this opinion. Mirbel 

 regards the whole of the cellular integument as a glan- 

 dular body serving to separate the transpirable matter 

 from the other fluids. The cells vary considerably 

 in form, according to the species of plant on which 

 they are found. The cellular integument is filled both 

 with colored and colorless secreted juices ; and it is 

 very probable that this part performs some changes 

 on the sap thrown into its cells, similar to those effect- 

 ed in the leaf. 



The cellular integument is partially destroyed and 

 reproduced, a great part of the old portion being push- 

 ed outwards with the cuticle which is annually detach- 

 ed ; while new cells are added to that which remains 

 at the time the new cuticle is produced. 



3. Vascular layer. Imbedded in the cellular integ- 

 ument and impinging on the internal surface of the 

 bark, are distinct bundles of entire vessels, each of 

 which is so arranged as to present, in the transverse 

 section of the stem under consideration, a semilunar 

 aspect ; and, in the longitudinal section, that of a fas- 

 cis of flexible cords, readily separable from each other, 

 and from the surrounding cellular substance ; which 

 is condensed where it comes in contact with these 

 bundles. These vessels are supposed to convey down- 

 wards the proper juice of the plant, elaborated from 

 the sap, by the action of the light and air in the leaf; 

 and this opinion is supported by the fact, that it is 

 from them the milky juice of the Fig tree and the col- 

 ored juices of other plants exude, when the stem is 

 transversely divided. In some stems, as, for exam- 

 ple, that of Laburnum, the vascular bundles coalesce, 

 and form nearly one continuous layer or circle around 



