38 Practical Plant Biology. 



source of light, it will probably be noticed that the majority of the 

 motile cells are congregating at that side of the drop of water 

 which is nearest the light. The movement of the cells, although 

 apparently uncertain, has resulted in bringing much the greater 

 number to one side. It will be observed, if the slide is turned round 

 so that the side which was furthest from the window is now nearest, 

 that the cells will gradually face about and make their way to the 

 illuminated side. By alternately' cutting off the direct light from 

 the window and allowing it to fall upon the preparation it may be 

 shown that it is light which determines the direction of the 

 motion. Here we have directed energy the light controlling 

 the direction of motion of the cell. Although this cell, and 

 many others which behave similarly, have been submitted to 

 long and careful study, the mechanism by which this control 

 and conversion of energy are effected has not been discovered. 

 We cannojt say that the motion of the cell is the reaction to 

 the light acting upon it, not being able to trace the connection 

 of the cause and effect, as we often can in inanimate mechanical 

 systems : but we must class the phenomenon with many others 

 among the vital (or unexplained) phenomena of cells. This is 

 often expressed by stating that protoplasm is " irritable " and, that 

 being so, the forces or " stimuli " acting upon it evoke " responses". 

 In many " responses " stored energy is liberated and the action of 

 the stimulus may be compared to the tiny expenditure of energy 

 in the spark which explodes a mine. In such cases there is no 

 relation between the amount of energy in the stimulus and that 

 exhibited in the response. In the case before us it is probable that 

 the energy received by the cell from a feeble light towards which 

 it is moving is often very much less than that which is spent on 

 driving the cell through the water. There is reason to believe that 

 the stigma is an essential link in the chain connecting the light 

 stimulus with the response. 



When it is arranged that the illumination is approximately equal 

 in all directions it will be found that the cells move hither and 

 thither, and as we would expect, there is no massing on one side. 

 Here the cells receive no visible directed stimulus but are free to 

 move in any direction. 



Their individual motions cannot be predicted and they appear 

 to decide spontaneously upon their directions. This spontaneity 

 is often assigned as an attribute to living matter. In reality there 

 is every reason to believe that it is one and the same thing as 

 " irritability ". The cell makes all its movements in response to 

 stimuli external or internal, great or small. An important factor 



