94 EXPERIMENTS WITH PLANTS 



enough to make the wheel revolve rapidly (two or 

 three times a second). A piece of cloth must be put 

 over and around it, like a tent, to confine the flying 

 drops. Germinating seeds (preferably Peas) should 

 be pinned to the corks on the sides not struck by the 

 stream of water. In the course of a day or so, pro- 

 vided the wheel is turning rapidly enough, we shall 

 expect to see the roots all bending away from the cen- 

 ter of the wheel and growing straight out in the direc- 

 tion of the radius, while, on the other hand, the stems 

 grow straight in, pointing toward the center of the 

 wheel. 



Let us see what will happen if we place the ap- 

 paratus on its side so that the wheel revolves horizon- 

 tally. If the plants have become inconveniently large 

 we may replace them by fresh ones with caulicles 

 about an inch long. Two forces now act on the plants, 

 the centrifugal force and that of gravity. The roots 

 take up an intermediate position, growing away from 

 the center of the wheel as before, but also obliquely 

 downward; the stems grow in the opposite direction. 

 These experiments lead us to think that when root 

 and stem issue from the seed, gravity determines the 

 direction in which they grow: we can therefore under- 

 stand how the seed, whether above ground or below, 

 unerringly sends its stem and root in the right direc- 

 tion. We can readily see that this is an important 

 matter for the plant, for the quicker it gets its stem 



