CHAPTER XV 



EFFECT OF GRAVITY UPON GROWTH 



§ 1. Effect of Gravity xtpoit the Rate of Growth 



It is a result of the sessility of the higher plants that they 

 and their parts are acted upon continuously in one direction by 

 gravity. It might consequently be suspected that the rate of 

 their growth would be affected if they were placed in an 

 abnormal position with respect to this agent. In the experi- 

 ments which have been made to test the correctness of this 

 suspicion various methods have been employed. When the 

 growth of Penicillium which is removed from gravity's action 

 by being slowly revolved on a klinostat is compared with that 

 of a plant under normal conditions, the former is found to be 

 more rapid. The plant seems to reap an advantage from not hav- 

 ing to sustain its own weight (Ray, '97). When Phycomyces 

 is inverted, its growth is slower than in the normal position 

 (Elfving, '80). These two experiments upon fungi indicate 

 a considerable sensitiveness on their part to gravity. Experi- 

 ments made by Elfving ('80) and Schwarz ('81), upon the 

 growth of inverted phanerogams and those from which the 

 action of gravity had been eliminated by the slowly rotating 

 klinostat, as well as those which had been subjected to exces- 

 sive pressure by the centrifugal machine, yielded for the most 

 part only negative results. Upon the rate of growth of seed- 

 lings gravity has little effect. 



§ 2. The Effect of Gravity upon the Direction of 

 Growth — Geotropism 



As with contact so with gravity two classes of effects may 

 be distinguished, which, while often producing similar results, 

 bring them about through very dissimilar processes. The first 

 of these is a mechanical effect due to gravity acting upon the 



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