146 VEGETABLE CELLS. 



Schleiden's theory. If this were correct, we ought con- 

 stantly to find the epidermal cell next the air flat ; but 

 there exist such as are not merely not flat, but of which 

 the radial diameter is considerably greater than the taii- 

 gental. Moreover, there ought to.be a thorough distinc- 

 tion of form between epidermal cells in contact with water 

 and those exposed to air, which is not the case. Again, 

 the epidermal cells next the air should have a flattened 

 outer surface, while in many cases they are elevated like 

 papillae. The cells of cellular plates (simple layers of 

 cells) which exist in air, for instance the leaves of most 

 Mosses, should be plane and flattened on both sides, which 

 they often are not ; while, on the contrary, cellular plates 

 lying in water, e. g. Algae and Moridese not unfrequently 

 have tolerably plane and flattened cells. 



The positive reasons are more important than these 

 negative objections. Assuming that a cell with equal 

 diameters and cubical form becomes tabular, as may be 

 the case in many cells of the epidermis and of the septa 

 in the air-canals, the cell expands in two directions, which 

 I will call the tangental, while it expands little or not at 

 all in the radial direction ; but the two faces to which 

 the radial diameter is perpendicular, and which I will 

 call end-faces, then expand, although in contact with air, 

 not merely as much, but more than the other or lateral 

 faces. If we assume that all three of the diameters of 

 the cell were originally = 1, and such an increase to 

 occur that both the tangental diameters become = 3, the 

 radial diameter remaining = 1, we have the six faces, 

 each of which originally possesses a square content = 1 , 

 now increased in such a way that the square content of 

 the four lateral faces = 3, while the square content of 

 each end-face has become = 9. In this case, therefore, 

 the superficial square expansion of a lateral face standing 

 in contact with other cells is three times, while that of an 

 end-face in contact with air is nine times the original 

 content. The latter is thus three times greater than the 

 former. It may indeed be objected here, that the 



