118 ON THE PLANT-CELL. 



62. Of the filamentous tissue of Fungi and Lichens we know 

 at present next to nothing. The cells usually contain a clear co- 

 lourless juice ; in the Lichens, occasionally, air. 



63. The epidermal cells contain a clear aqueous or coloured 

 fluid, rarely here and there peculiar substances, as resin (in Aloe 

 nigricans). Externally the true epidermis affords peculiar secre- 

 tions, at first a waxy material, usually in the form of a delicate 

 layer, which renders the surface smooth or shining, more rarely 

 in that of minute granules (the so-called bloom, pruina), in 

 either case protecting the epidermis against being wetted or pene- 

 trated by water, thus rendering all interchange of gases and va- 

 pours impossible excepting only through the stomates. A second 

 layer (cuticula) is subsequently formed beneath this first secretion, 

 which is composed of an assimilated material not yet precisely 

 investigated. This layer is in many cases of great thickness, and 

 constitutes tubercles, warts, and such-like productions, especially 

 in the neighbourhood of the stomates. In their vital properties 

 these epidermoidal appendages exhibit numerous varieties, and in 

 them again we meet with very various contents and peculiar 

 secretions. With respect to cork, we only know that it soon dies 

 and decays bit by bit. 



The epithelium differs from the parencliymatous cells only in its clear 

 aqueous juice : the epiblema has not as yet been sufficiently investigated. 

 But as soon as the epithelium is converted in the air into epidermis, it 

 becomes covered with a delicate layer of a material which can be re- 

 moved by absolute alcohol or ether, and which always gives the epi- 

 dermis a certain brilliancy, and affords a perfect protection against its 

 being wetted by water : the latter is the most important point. We 

 well know that a membrane penetrated by moisture offers no impediment 

 to the evaporation of the water enclosed by it, and to the absorption and 

 transmission of gases, but that the contrary is the case with a dry mem- 

 brane. In this way the epidermis isolates the cells of the parenchyma 

 from all action of the atmosphere, as, owing to the intervention of the 

 epidermis, they cannot receive anything from it nor give out anything 

 to it. The whole reaction, therefore, is limited to the stomates, through 

 which alone is evaporation or interchange of gases possible. This pe- 

 culiar investment of the epidermis has been hitherto wholly unnoticed, 

 and has been recognised only in those cases where it is presented in 

 greater quantity, in the form of minute granules, as the " bloom : " 

 it exists, however, on every epidermis, and may be removed by ether, 

 when the cells of that membrane, like all others, become permeable to 

 water. 



In a section perpendicular to the surface this waxy secretion can be 

 demonstrated only in those cases in which, as in Elymus arenarius, 

 Strelitzia farinosa, &c., it attains a considerable thickness : conse- 

 quently it is not shown in all the woodcuts appended to this Section. 



The object of preventing, by this layer, all evaporation, &c. on the 

 surface of plants is probably still further promoted by the secondary 

 secretion. 



Upon examining a fine transverse section of the epidermis of Aloe 



