CHAPTER IX 

 EXTERNAL FORCES AND PLANTS 



54. Gravity and growth. The force that acts most contin- 

 uously upon living things is doubtless gravity. Temperature 

 varies constantly, and the light is intermittent as well as vari- 

 able. We do not know much about the relation of electrical 

 conditions of the atmosphere to living things, and chemical 

 conditions we can consider only as they arise in connection 

 with particular kinds of substances. But gravity seems to be 

 constant without regard to hours or seasons. It is therefore 

 interesting to find how living things, and especially plants, 

 behave in relation to this force. 



The question often occurs to people who have planted seeds, 

 or who have watched others do so, Does it make any differ- 

 ence which side of the seed falls uppermost ? We know that 

 the lower end of the hypocotyl becomes root, and that roots 

 usually live in the earth. What would happen if a seed were 

 placed in the ground with its hypocotyl pointing skyward ? 



We can easily find out by means of experiments that permit 

 us to watch the development of the young plant under condi- 

 tions that make gravity act upon hypocotyls from different 

 directions. Incidentally we can discover that the shoot of a 

 plant is also sensitive to gravity, but that it responds in quite 

 the opposite way from the root. That is to say, the shoot 

 tends to grow away from the earth, whereas the root tends to 

 grow toivard the earth. 



55. Tropisms. To many of us this sensitiveness of the plant 

 will come as something unexpected, for we do not commonly 

 think of plants, as sensitive beings. The turnings that a plant 

 or an animal shows in response to the one-sided action of 



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