viii] OF STOMATA 629 



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. 



As I have said, the mechanical cause of the split which constitutes 

 the orifice of the stoma is not quite clear. It may be directly due 

 to the subepidermal air-space which the stoma communicates with, 

 for an air-surface on both sides of the dehcate epidermis might 

 well cause such an alteration of tensions that the two halves of 

 the dividing cell would tend to part company. 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 spht 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*. It is noteworthy, 

 and Nageli took note of it wellnigh a hundred years ago, that the 

 stomatal mother-cells remain small while the others grow, and also 

 that they only divide once for all, while their neighbours divide 

 and divide again, 4» produce the lateral or accessory guard-cells. 



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 

 of a boundary-surface, which, whether it become at once a. soHd 

 or semi-sohd partition or whether it remain semi-fluid, exercises 

 in all cases an effect on the position and the form of the boundary 

 which comes into being with the next act of division. In contrast 

 to this general process stands the phenomenon known as "free 

 cell-formation," in which, out of a common mass of protoplasm, 

 a number of separate cells are simultaneously, or all but simul- 

 taneously, differentiated; and the case is all the more interesting 



* Botanische Beitrage, Linnaea, xvi, p. 238, 1842. Cf. Garreau, Mem. sur les 

 stomates, Ann. Sc. Nat., Bot. (4), i, p. 213, 1854. 



