286 



On the Histology op Plants. 



By R. Braitpiwaite, M,D., F.L.S., &c. 



VI. — CoMrouND OR Heterogeneous Tissues. 



(Read June 2Gth, 1874.) 



The Cambium layer forms a cylinder between the wood and bark, 

 and consists entirely of elongated cells, which, like all other tissues, 

 originate in the parenchyma of the growing point. From it the 

 vascular bundles are developed, and all new wood and bark, by 

 which grow^th in thickness is attained, and this continues active 

 during the whole life of the plant. In Monocotyledons the cambial 

 cells are central, and surrounded by vessels, and after a certain 

 period their vital activity ceases, so that these plants no longer 

 increase in thickness, but they extend in length by means of un- 

 changed cambial cells in connexion with the end of the vascular 

 bundles. The heterogeneous tissue is therefore confined to the 

 vascular bundles, and when in its most complete form, all three 

 kinds of cells enter into its composition. In the Fungi, Algge 

 and Lichens, no traces of them are to be found, and we first meet 

 with them, though in a very rudimentary condition, in Mosses and 

 Hepatica^, where they form a ring or bundle of elongated cells, 

 the simplest representatives of bast. From these, as we ascend the 

 scale, we find they become more and more elaborated, until in the 

 highest plants they acquire their greatest distinctness. This vari- 

 able condition in the different groups of plants necessitates our 

 dealing with each of them separately. 



Vascular Bundles of Hepatic.e and Mosses. 



Vascular bundles are not found in all the genera, but when present 

 appear only as a collection of elongated cells, best seen in 3Iar- 

 chantia and other frondose Hepatic*, where they form a kind of 

 central string to the middle. In the Sphagna we find no vascular 

 bundle, but the subcuticular parcnchym cells become indurated, and 

 form the so-called woody layer. 



In true mosses a higher development takes j^lace, but yet only 

 observable in certain genera ; thus in Dicramnn scopaiiwn, Clinia- 



