314 



THE JORMS OF TISSUES 



[CH. 



to identity of form, the case is obviously different with the cells 

 at the borders of the system. So it is, in like manner, with our 

 froth of soaprbubbles. The bubbles, or cells, in the interior of 

 the mass are all alike in general character, and if they be equal 

 in size are alike in every respect: their sides are uniformly 

 flattened*, and tend to meet at equal angles of 120°. But the 

 bubbles which constitute the outer layer retain their spherical 

 surfaces, which however still tend to meet the partition-walls 

 connected with them at constant angles of 120°. This outer layer 

 of bubbles, which forms the surface of our froth, constitutes after 

 a fashion what we should call in botany an "epidermal" layer. 

 But in our froth of soap-bubbles we have, as a rule, the same kind 

 of contact (that is to say, contact with air) both within and without 

 the bubbles ; while in our living cell, the outer wall of the epidermal 

 cell is exposed to air on the one side, but is in contact with the 



Fig. 119. 



protoplasm of the cell on the other : and this involves a difference 

 of tensions, so that the outer walls and their adjacent partitions 

 are no longer likely to meet at equal angles of 120°. Moreover, 

 a chemical change, due for instance to oxidation or possibly also 

 to adsorption, is very likely to affect the external wall, and may 

 tend to its consolidation ; and this process, as we have seen, is 

 tantamount to a large increase, and at the same time an 

 equalisation, of tension in that outer wall, and will lead the 

 adjacent partitions to impinge upon it at angles more and 

 more nearly approximating to 90° : the bubble-like, or spherical, 

 surfaces of the individual cells being more and more flattened 

 in consequence. Lastly, the chemical changes which affect the 

 outer walls of the superficial cells may extend, in greater or 

 less degree, to their inner walls also : with the result that these 



* Compare, however, p. 299. 



