STRUCTURE.] ITS CHEMICAL CONSTITUTION. 135 



stem of Kleinia neriifolia, and in Hoya carnosa, the internal 

 unstained layer is absent, and the external, dense, iodine- 

 stained membrane is readily perceived to be composed of 

 several superimposed layers, which are deposited on the 

 external walls of the cells, within the cavity, by which means 

 the continuation of the lateral walls of the epidermis extends 

 to the outer surface, like an uninterrupted membrane. The 

 same phenomenon occurs in the leaves of Hakea pachyphylla, 

 in which this inner layer, is indeed present, but which takes 

 the stain of iodine like the proper cuticle, and proves itself 

 by its dots to be a secondary substance when the epider- 

 mis is treated with sulphuric acid, whilst at the same time 

 a distinct lamination can be traced in the matter deposited 

 in the interior of the cells. Hartig ascribes a very compli- 

 cated structure to the cuticle, separating it into three different 

 layers : 1. An external membrane or epichroa ; 2. An internal 

 membrane or endochroa ; and 3. A central mass situated 

 between these membranes, and called the mesocolla. He 

 says that the external membrane extends over the entire leaf, 

 penetrates into the area in front of the stomates, and at the 

 same time stretches over the stomate itself, while the inner 

 membrane is immersed in the form of folds between the cells 

 of the epidermis, and penetrates in various plants, more or 

 less deeply into the subjacent cellular tissue, in which case 

 it then takes its course through the intercellular passages in 

 the form of vessels. In like manner the inner membrane is 

 said to penetrate through the stomates, and extend into the 

 leaves, as in Narcissus Jonquilla. I cannot find this struc- 

 ture. (See Cistomes, p. 147). And Mohl protests against its 

 existence. 



Payen has instituted an inquiry into the chemical distinc- 

 tions between the Membrane of Plants, and the skin or shell 

 of Insects and Crustacea. Link, in his abstract, thus states 

 the result of the investigation. The chemical differences are 

 as follow : 1st. Sulphuric acid with 1-5 atoms of water dis- 

 solves instantaneously the covering of insects, but scarcely 

 attacks the cuticle of plants in several hours. Sulphuric acid 

 with three atoms of water breaks up animal tissue in a few 

 hours, whilst the epidermis of plants resists it for more than 



