148 CISTOMES. [BOOK i. 



advantage of breaking up the epidermoidal cells with the 

 production of a blue colour, or of entirely dissolving them ; 

 after which the cuticle can be readily distinguished and 

 separated. By this process Mohl ascertained that there is 

 a direct continuation of the cuticle into the stomates, and 

 thence down between the porous cells, in the form of a tube 

 very much compressed laterally. It is certain that this tube 

 is not closed either at the entrance into the stomates or lower 

 down between the porous cells. At the bottom of the 

 stomatic aperture this tube dilates into a funnel-shaped 

 expansion, which clothes the under surface of the epidermis. 

 The cistome differs in various plants. Generally its expanded 

 part extends only as far as true cells of the epidermis form 

 the outer wall of the spiracles ; and it is cut off abruptly at 

 their termination. Mohl failed to discover processes pene- 

 trating from the edge of the cistome into the intercellular 

 passages running beneath the epidermis, and connected with 

 the air-holes in the stem of Euphorbia officinarum, Cacalia 

 Kleinia, Lepismium Myosurus, in the leaves of Agapanthus 

 umbellatus, Narcissus Jonquilla, Pothos lanceolata, and in 

 the leaf-like branches of Ruscus aculeatus. In other cases, 

 however, he found processes evidently proceeding from the 

 margin of the cistome through the intercellular passages 

 on the inferior surface of the epidermis to neighbouring 

 cistomes, thus forming communications : as on the under side 

 of the leaves of Helleborus niger and viridis, and in the leaves 

 of Euphorbia Caput Medusae. In other cases, as in the leaves 

 of Betula alba and Asphodelus luteus, that these processes 

 penetrate into all the intercellular passages situate beneath 

 the epidermis, and extend in the form of a net-work over 

 the whole under surface of the epidermis, so that the epi- 

 dermoidal cells are clothed on both sides by a true cuticle, 

 which however in the inside does not form a continuous 

 membrane. 



When, says Mohl, the epidermis consists of several layers 

 of cells, packed one over the other, as in Cereus peruvianus 

 and in Opuntia, the continuation of the cuticle clothes the 

 sides of that portion of the air-hole which is situated in this 

 thickened epidermis; it appears not so much as a wide- 



