288 



The following, based on Schwendener's work, may serve 

 student in better understanding the theory. 



the 



Leaving out of consideration, for a moment, the force w4:iich 



we 



does the work, and turning to the structure of stomata, 

 find that, at least for a great part of the ordinary flowering 

 plants, the stoma of Tradescantia discolor may be regarded as a 

 fair representative. In this species each guard-cell, In cross-sec- 

 tion, is seen to have a somewhat triangular cell-lumen (see fig. I a), 

 one of the apices pointing towards the other guard-cell Its 

 walls along the slit of the stoma are much thickened, the thick- 

 enings consisting of tw^o parallel bars, one above and one below, 

 leaving between them a narrow line of thin cell-wall correspond- 

 ing, in position, to the apex of the triangle formed by the cell- 

 lumen. The posterior wall, or that away from the slit, is thin 



and uniform, while the upper and lower walls are triangular in 

 cross-section, thickest near the slit, and becoming thinner towards 

 the side away from this. Looking down on such a cell it Is seen 

 that these bar-like thickenings are uniform for the whole length 

 of the slit, but are thinner at each end where the two guard-cells 

 meet. Moreover, it will be noticed, in the stoma of Priimis, for 

 example (see fig, i b), that the outer wall of the epidermal cells, 

 elsewhere quite thick, has a thin place close to its junction with 

 the guard-cell. This thin place is seen as a line from above, and 

 permits a free movement of the guard-cell, like a hinge, which 

 otherwise, owing to the rigidity of the outer epidermal wall, 

 could not take place. Since the other connections of the guard- 

 cell with the cells of the epidermis are formed by thin, easily 

 bent walls, we may regard the guard-cell as a tube fastened at 

 both ends to another tube like itself, but otherwise free to move. 

 Before trying to explain the manner in which the force is sup- 

 posed to act, it is well to consider some of the properties of the 

 material of which cell-walls are made. 



