PHYSIOLOGICAL 323 



root. How the gravitational pull makes itself felt is a problem still 

 imder discussion and experimentation, but there is much to be said 

 for attaching importance to a change in the position of minute 

 bodies, such as starch grains, floating freely in the cell-fluid, but 

 tending, because heavier, to sink to the lower side. If some change 

 occurs in the direction of the gravitational pull, the grains will come 

 to rest on a part of the cell unaccustomed to them. This may provoke 

 a new direction of growth movement, the changed curvature restor- 

 ing the old position of equilibrium. In almost all geotropic organs 

 there are these freely moving starch grains. As they seem to bear 

 a close analogy to the particles in the balancing ears of many 

 animals, e.g. lobsters, they somefimes get the same name — 

 "statoliths". 



In twining stems the growing part shows a swinging movement, 

 and also twists on its own axis. In this case the particular sensitive- 

 ness to gravity appears to be situated on one side or flank of the 

 stem. More rapid growth follows the excitation, and as the swinging 

 movement brings a new segment of the stem into the flank position, 

 the twining continues. 



But the directed obligatory movements or tropisms may be 

 responses not to gravity, but to other stimuli. Thus tendrils respond 

 to repeated contact, such as the slight rubbing of a twig, though not 

 to the fall of a drop of quicksilver, or the touch of a perfectly smooth 

 thread of gelatin. Or the stimulation may be moisture, or diffusing 

 chemical substances, or light. 



When there is a power of free locomotion, as in some simple 

 Algae, Fungi and reproductive cells, there may be an automatic 

 adjustment of the direction of movement in reference to the direc- 

 tion of the localised stimulus. The movement may be towards or 

 away from the provocation, which may be of various kinds, such 

 as light or a chemical substance. It is probably in this way that 

 motile male cells find the female cell which they fertilise. 



When there is free locomotion it is convenient to use the terms 

 chemotactic, phototactic, and so on; when the plant is fixed but 

 moves a part in any plane, determined by the direction of the 

 stimulus, it is usual to speak of tropisms; but a curvature confined 

 to a single plane is spoken of as nastic, e.g. photonastic. In some 

 cases, as in a tendril bending roimd its support, the stimulus acts 

 as a trigger-pulling or releasing cause, for beyond a minimum limit, 

 a slender twig is as effective as a much heavier one. In other cases, 

 such as a growing plant in a room illumined through a single window, 

 there is some correlation between the amount of the stimulus and 

 the amount of the curvature towards the light. Yet it is not possible 

 to explain the phenomena in any simple or direct way, e.g. by simply 

 saying that growth is quicker on the relatively more shaded convex 

 side, the rate of growth being retarded by light. For we have to 



