THE PERCEPTION OF THE FORCE OF GRAVITY. 533 



ionable, need not trouble us. We are not concerned with why the plant 

 grows up into the air or down into the ground; we are only concerned 

 with the question of how the plant perceives the existence of gravita- 

 tion. Or, in other words, taking the reaction for granted, what is the 

 nature of the stimulus? If a plant is beaten down by wind or by other 

 causes into a horizontal position, what stimulative change is wrought 

 in the body of the plant by this new posture? 



It is conceivable in the case of a stem supported by one end and 

 projecting freely in the air that the unaccustomed state of strain might 

 act as a signal. The tissues on one side (the upper) are stretched, and 

 they are compressed below; this might guide the plant; it might, in 

 fact, have evolved the habit of rapid growth in the compressed side. 

 This is only given as an illustration, for we know that the stimulus 

 does not arise in this way, since such a plant, supported throughout its 

 length, and, therefore, suffering no strain, is geotropically stimulated. 

 The illustration is so far valuable, as it postulates a stimulus produced 

 by weight, and we know from Knight's centrifugal experiment that 

 weight is the governing factor in the conditions. Since we can not 

 believe that the stimulus arises from the strain as affecting the geo- 

 tropic organ as a whole, we must seek for weight-effects in the indi- 

 vidual cell of which the plant is built. We must, in fact, seek for 

 weight-effects on the ectoplasm of those cells which are sensitive to the 

 stimulus of gravity. 



If we imagine a plant consisting of a single apogeotropic cell we 

 shall see that the hydrostatic pressure of the cell-contents might serve 

 as a signal. 



As long as the cell is vertical the hydrostatic pressure of the cell- 

 sap upon the ectoplasm at C (Fig. 1) is equal to that at D. But the 

 pressure on the basal wall, B, differs from that at A (the apical wall) 



c- 



A 



B 



D 



B 



D 



Fig. 1. 



by the weight of the column AB. If the plant be forced into the hori- 

 zontal, the pressure at A and B becomes the same, while the pressure 

 at C no longer equals that at D, but differs by the weight of the column 

 CD. Here undoubtedly is a possible means by which the plant could 

 perceive that it was no longer vertical, and would have the means of 



