248 The Living Plant 



vary in direction all through the twenty-four hours, but often- 

 times are obscured altogether. Moreover, this principle of sub- 

 stitution stimuli is often important in connection with the de- 

 velopment of structures, for it helps to explain how an organ or 

 other feature can form in advance of perception of the stimulus 

 to which it is later to react, e. g. the formation of the eye before 

 birth in animals, and of chlorophyll in the embryos of plants. 



The way in which the gravitation stimulus is perceived by the 

 plant seems clear. Gravitation draws the heavier contents of 

 the cells, especially the starch grains, down to the bottom of the 

 cell, where their weight presses hard on the sensitive protoplasm 

 and produces a condition of strain different from anything in the 

 upper part of the cell; and this difference establishes the line of 

 direction. Then the responding mechanism is so set that main 

 roots are sent growing towards this pressure, main stems away 

 from it, and side parts across it, precisely as in other typical 

 responses. Geotropism, by the way, is a perfect illustration of 

 the fact that a stimulus acts merely as a guide, and not as a 

 physical aid, to responses; for while gravitation might be sup- 

 posed to help pull roots downward, obviously it cannot be 

 imagined to help push stems upward or to drive side parts out 

 crossways. 



Thus much for the geotropism of stems and roots; what of 

 leaves, flowers and fruits? As to leaves, their geotropism is 

 usually disguised by their stronger phototropism; but that they are 

 geotropic is shown by the vertical or horizontal positions they 

 assume when kept in dark rooms. We see another illustration 

 thereof, as I take it, in Nature, in some of the broad-leaved 

 shrubs which grow in the shade of the forest; here the diffused 

 light is so evenly distributed that it exerts no one-sided stimulus, 

 and the leaves are left free to assume their geotropic position, 

 which is strikingly horizontal. As to flowers, they also, for the 

 most part, are definitely geotropic. Thus, if one selects a long 

 terminal cluster of unopened irregular flowers, such as Larkspur 



