Sense-Life and Sensibility 



a way that it grows horizontally at right angles to the 

 force of gravity. 9 So guided by light and gravity, the 

 Solomon's seal discovers the precise depth at which it 

 is adapted to grow. It must be remembered that such 

 stimuli as these are not necessarily motive forces. 

 They may be used like the mariner's compass which 

 guides a ship across the ocean, and need not be com- 

 pared to the steam which drives its engines. 



It has been found by careful and minute investigation 

 that in those parts of plants which show themselves 

 sensitive to the action of gravity, there are almost always 

 numbers of small starch grains called statoliths. Suppose 

 that in the cells of a root the protoplasm is only 

 " happy" when these starch grains are resting on the 

 lowest wall or floor of the cell. If the root is sup- 

 posed to be tilted sideways, then the grains will drop 

 into a corner of the cell. The protoplasm is supposed 

 to react to this uncomfortable condition, and by the 

 growth of the cell or movements of the cell contents 

 bring the cell back into the right position. To do this 

 the root turns or grows downwards. 



The similarity of this reaction to that of the otoliths 

 in the ears of the higher animals is very remarkable, 

 and in spite of the complex nature of the whole pro- 

 ceeding most recent authorities seem to believe that it 

 is by the tumbling about of these starch-grains on a 

 sensitive protoplasmic layer that both stems and roots 

 react to gravity. 



Some of the evidence is very strong. Thus in the 

 roots of parasites, and in those of certain arums, willows, 

 and other water plants which do not show the usual 

 downward growth, there are no such statoliths. 10 In 

 some grasses (Festuca sp. and Poa sp.) the seedlings 

 show at first no geotropy, but when they are mature 

 there is a marked reaction. In those roots the starch 



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