I 



291 



r 



the pressure on the large tube or bulb, and the small tube will be 

 seen to bend quite readily. The more pressure is applied, the 

 more it bends. 



Returning to our guard-cell we can at once apply all that we 

 have found to hold in the case of the rubber tube. Furthermore, 

 smce both cells share in the movement, a very slight amount of 

 bending suffices to produce the desired opening. When a stoma 

 opens or closes scarcely any change of form, other than a sliglit 

 bending, is noticed in the guard-cells. We might expect this, as 

 the force will tend to bend the tube just as soon as the stretching 

 begins, and since this begins — theoretically, at least — in the v^ry 

 beginning of the application of power, the bending also begins 

 at that time. 



Besides the peculiar thickening of the walls of one side of 

 the guard-cell, it has been noticed also by the same author that 



many guard-cells are suspended in such 

 a way that the thin posterior walls stand 



i 



do- 



jt^ 



%^ 



obliquely to the surface (see 

 On being distended this wall is crowded 

 out and becomes uniformly curved^ 

 causing the base of the cell, movable on 

 account of the thin cell- wall between the 



^Pper and lower thickening, to recede from the other guard-celL 



There is still something in the structure 

 of these 



but 



cells which all authors have observed, 

 none have laid as much stress on as itA( 



tJeserves. It is the above mentioned fact that 

 the thickenings extend only along the opening 

 rapidly, often suddenly, giving place to a thin 

 cell- wall at the ends where the two guard- 

 cells are united, and also that the thickenings 

 of each cell are independent and do not 

 unite with those of the other cell. Let figure 

 4 represent a tube, and A any point along the 

 thin end, B a point on the thick part of the 

 ^vall. Then if a pressure causes B to move to 



B 



A 



A 



B, 



<^istance, say to Ai, for if the wall is assumed 



d 



! 



<;. 



^h 



