94 EXPERIMENTS WITS 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 se^ 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 



