VIII] THE DEVELOPMENT OF STOMATA 395 



between the new-formed cells splits, and again we have the 

 phenomenon of a "stoma" with its attendant guard-cells. In 

 Fig. 179 are shewn the successive stages of division, and the 

 changing curvatures of the various walls which ensue as each 

 subsequent partition appears, introducing a new tension into the 

 system. 



It is obvious that in the case of the oblong cells of the epidermis 

 in the hyacinth the stomata will be found arranged in regular rows, 

 while they will be irregularly distributed over the surface of the 

 leaf in such a case as we have depicted in Sedum. 



While, as I have said, the mechanical cause of the split which 

 constitutes the orifice of the stoma is not quite clear, yet there 

 can be little or no doubt that it, like the rest of the phenomenon, 

 is related to surface tension. It might well be that it is directly 

 due to the presence underneath this portion of epidermis of the 

 hollow air-space which the stoma is apparently developed "for 

 the purpose" of communicating with; this air-surface on both- 

 sides of the delicate epidermis might well cause such an alteration 

 of tensions that the two halves of the dividing cell would tend to 

 part company. In short, if the surface-energy in a cell-air contact 

 were half or less than half that in a contact between cell and cell, 

 then it is obvious that our partition would tend to split, and give 

 us a two-fold surface in contact with air, instead of the original 

 boundary or interface between one cell and the other. In Professor 

 •Macallum's experiments, which we have briefly discussed in our 

 short chapter on Adsorption, it was found that large quantities 

 of potassium gathered together along the outer walls of the guard- 

 cells of the stoma, thereby indicating a low surface-tension along 

 these outer walls. The tendency of the guard-cells to bulge 

 outwards is so far explained, and it is possible that, under the 

 existing conditions of restraint, we may have here a force tending, 

 or helping, to split the two cells asunder. It is clear enough, 

 however, that the last stage in the development of a stoma, is, 

 from the physical point of view, not yet properly understood. 



In all our foregoing examples of the development of a "tissue'" 

 we have seen that the process consists in the successive division 

 of cells, each act of division being accompanied by the formation 



