6o Professor J?. //. Vaj^p on 



this region grows, it bends in response to the stimulus, until the 

 root tip, with its starch grains, is once more in the accustomed 

 position. The means of communication between the sensory and 

 motor regions may well be the living protoplasm itself. Even the 

 dead cell-walls are traversed by excessively delicate threads of 

 living matter, so that there is a real continuity of protoplasm from 

 cell to cell. Other theories have been suggested, but so far the 

 falling starch grain theory certainly fits the known facts better 

 than any other. An interesting analogy from the animal kingdom 

 is the so-called auditory organ of certain Crustacea. jThis is now 

 known to be really a balancing organ, in which small grains of 

 sand are acted on by gravity, and are utilised in almost precisely 

 the same way as is suggested for the starch grains of the root. 

 In the root-cap, then, we are apparently dealing with a delicate 

 sense-organ, as effective for its particular duties as the balancing 

 organs of Crustacea, or the semicircular canals of the ear of a man. 

 Plant organs are sensitive also to other external stimuli besides 

 that of gravity. Thus the tendrils of climbing plants respond to 

 the stimulus of contact with any rough object. This form of 

 sensitiveness, which may perhaps be compared with the human 

 sense of touch, enables the tendril to clasp any suitable support. 

 Here again it is possible in many cases to distinguish simple sense 

 organs. They are minute pits, filled with protoplasm, in the walls 

 of the outer cells of the tendril. The protoplasm is thus brought 

 so near to the surface as to readily perceive any slight friction 

 between the tendril and an external object. 



Light is another stimulus which has a directive effect on the 

 growth movements ot plants. As with gravity, plant organs do 

 not all respond in the same way to light. Thus in general, stems 

 grow towards and roots away from light ; while leaves take up a 

 position at right angles to the incident rays of the strongest 

 diffuse light. 



It is much easier to experiment with light than with gravity : 

 hence, by the simple expedient of excluding light from each part 

 of an organ in turn, it is possible in many cases to locate a region 

 specially sensitive to light. In grass seedlings it is the tip of the 



