responds in the direction from which the stimulus or disturbing factor 

 comes to it, and sometimes in the opposite direction. We distinguish tro- 

 pisms accordingly. The turning of leaves and stems toward the light we call 

 positive tropisms, whereas the bending of the stem away from the direction 

 of gravity we call a negative tropism. 



These are convenient terms, but we must not let them mean "for" or 

 "against" in the sense in which we speak of our own likes and dislikes or 

 our attitudes in a debate. Nor must we suppose that these terms in any way 

 explain what happens. They are convenient for summing up the facts that 

 nearly everybody can observe for himself. We are still in the dark as to 

 how these delicate and adaptive changes are brought about, even when we 

 call them changes in growth. 



Growth Substances' In 1927 Frits Warmolt Went (1903- ), a 

 young Dutchman who came to the California Institute of Technology, suc- 

 ceeded in answering partially the question, What besides food or tempera- 

 ture affects the rate of a plant's growth ? He cut off the tips of young oat 



If we keep some sprouting potatoes 

 or seeds in the dark and others in 

 the light, we find that those in the 

 dark grow faster. Farmers remove 

 seed-potatoes from dark cellars 

 and spread them out in a lighted 

 shed to prevent the growth of long 

 sprouts 



L. P. Flory, from Boyce Thnmpson Institute 



Plant loves light and bends toward 



it. Light attracts plant. Shaded 



side grows faster, bending stem 



toward light. 



Do these statements all mean the 



same thing? 



Do they equally describe what we 



can see? 



Which most agree with the facts? 



I.. I'. Flory, from Hoyre Tlionipsoii Institute 



LIGHT AND GROWTH 



^See No. 3, p. 271. 

 257 



