20 



BIOLOGY AND HUMAN LIFE 



These movements show us, at the same time, that plants are 

 really very sensitive to what is going on around them, although, 

 compared to animals, they respond rather slowly (see Fig. 9). 



Thus we find that plants and animals have in common certain 

 processes or functions— food-taking and growth, irritability (or 

 sensitiveness) and movement, and reproduction. There are, to 

 be sure, many differences also ; plants and animals differ very 



Fig. 9. Sensitive plant (Mimosa pudica) 



a, leaves in normal position; b, leaves folded after disturbance. It is not necessary 

 for us to assume that this movement is of any real value to the plant. It is true 

 that in the new position the leaf exposes less surface and sheds the water better. But 

 hundreds of plants with similar leaves have no difficulty in shedding rain without 

 being so sensitive. Many plants (clover, oxalis, and others) droop their leaves in 

 the dark in a few minutes. It is possible that in the clover and others the drooping 

 of the leaf is the direct result of reduced transpiration. But that does not give the 

 plant any advantage. It is very likely that the sensitive plant is simply more sen- 

 sitive than any of its relatives (the bean family), many of which are sensitive in 



the same way but not in the same degree 



much as to the materials which they take in from the out- 

 side, for example, and as to the way in which they make use 

 of this material; but we are now considering their common 

 characteristics. 



16. Comparison with non-living things — growth. The fact 

 of growth is universal for living things. Yet the crystals of 



