48 THE INTERNAL 



may be to the Botanist the search for the laws on which 

 depend the formation of these innumerable varieties, they 

 are at present of no importance to us, since the life of the 

 whole plant is the question before us, and here, passing by 

 all those distinctions, we must endeavour to establish other, 

 quite different divisions of the tissues of plants, which, as 

 we shall see, in some cases have no agreement, and in 

 others but a very moderate one, with the definite forms of 

 the cells. 



Every plant in its course of formation, and every unde- 

 veloped part of a plant, consists exclusively of small, deli- 

 cate, roundish cells. Differently as the individual cells of 

 this tissue may be modified afterwards, there are yet only 

 two portions which in their subsequent development and 

 their importance to the life of the whole plant, distinguish 

 themselves essentially from that fundamental mass, which 

 forms the chief tissue of the plant in the subsequent full- 

 grown condition. One is the whole external layer of the 

 plant, which developes in contact with water or earth, but 

 which is more especially exposed to the air. The cells of 

 this layer are so firmly united together, that it may gene- 

 rally be stripped off the plant as a continuous membrane. 

 It becomes clothed, sooner or later, with a layer of varying 

 thickness, of a homogeneous substance, which receives 

 besides, a thin coating of wax or resin ; thus the envelop- 

 ing membrane becomes impenetrable by fluids, and even 

 repels them, since water runs off it as from a greasy 

 substance. In certain places, however, little orifices are 

 left between the cells, leading into the interior of the plant. 

 In these orifices usually lie two crescent-shaped cells, having 

 their concave sides applied together, so as to leave a slit 

 open between them, but otherwise closing up the orifice. 

 These slits, through which the plant communicates with 



