p. W. Zimmerman and A. E. Hitchcock, from Boyce Thompson Institute 



NEGATIVE GEOTROPISM IN PLANTS 



Stems of most plants tend to grow upward, unless forced or disturbed by some 

 outside agency. Is the tomato's turning away from the earth connected with the 

 formation and distribution of growth substances? If it is possible to remove auxins 

 from the tips of oat seedlings, is it possible to remove auxins — if any — from the 

 lower or upper halves of the horizontal tomato stem? 



Further experiments showed that this "artificial auxin" counteracts plant 

 auxin in the normal negative geotropism of stems (see illustration opposite). 

 From these experiments we may conclude that auxin and indole-acetic acid 

 have essentially the same effects in stimulating growth-responses to light 

 and to gravity. Chemists have found certain similarities between the chemi- 

 cal make-up of natural plant auxin and that of several synthetic compounds 

 which all have the same effects on plant growth. 



Do Animals Respond to Stimuli Automatically? 



Animal Tropisms^ Fruit flies and common houseflies turn toward the 

 light. Earthworms turn away from strong light, but toward a very weak 

 light. Such turnings usually involve the whole organism rather than merely 

 a single organ or portion. On summer evenings we can see swarms of in- 

 sects, usually several different species, around any street lamp or other 

 exposed light. Insects in large numbers often get stuck in the radiators of 

 motorcars driving through the country at night. Lighthouse-keepers report 

 that hundreds of birds dash themselves against the windows and get killed, 

 especially during the migration periods. 



These tropisms of animals are unlike the growth-movements of plants, 

 for they are brought about by the contractions of special portions — the 



iSee No. 4, p. 271. 

 260 



