VII] OF EPIDERMAL TISSUES 493 



or c6, where cell-surface is in contact with the surrounding medium, 

 these latter surfaces will tend to increase and the surface of cell- 

 contact to diminish. In short there will be the usual balance of 

 forces between the tension along the surface <x\ and the two 

 opposing tensions along ca and ch. If the former be greater than 

 either of the other two, the outside angle will be less than 120°; 

 and if the tension along the surface cc be as much or more than 

 the sum of the other two, then the drops will merely touch one 

 another, save for the. possible effect of external pressure. This is 

 the explanation, in general terms, of the peculiar conditions ob- 

 taining in Nostoc and its allies (p. 477), and it also leads us to a 

 consideration of the general properties and characters of a super- 

 ficial or "epidermal" layer*. 



While the inner cells of the honeycomb are symmetrically 

 situated, sharing with their neighbours in equally distributed 

 pressures or tensions, and therefore all tending closely to identity 

 of form, the case is obviously different with the cells at the borders 

 of the system. So it is with our froth of soap-bubbles f. The 

 bubbles, or cells, in the interior of the mass are all ahke in general 

 character, and if they be equal in size are alike in every respect: 

 as we see them in projection 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 (just as 

 in the cells of a honeycomb), and these 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 surface-layer always tends to have, ipso facto, a character of its own : a "skin " 

 has such and such characteristics just because it is a skin. The "Beilby layer" 

 on a metallic surface is, in its own special way;- a consequence of its own externality. 



f A froth is a collocation of bubbles containing air; or in the language of colloid 

 chemistry, an emulsion with air for its disperse phase. The power of forming 

 a froth is not the same as that of forming isolated bubbles; for some liquids, such 

 as a solution of saponin, of gum arable, of albumin itself, give a copious and lasting 

 froth, but we find it hard to blow even a single tiny bubble with any of them. 

 Something more than surface-tension seems necessary for the production and main- 

 tenance of a film : perhaps a certain amount of viscosity, to resist the tendency of 

 surface-tension to tear the film asunder. 



