PROTECTIVE MOVEMENTS 



361 



Here are three results of this simple reaction that may 

 presumably be of use to the animal under various conditions. 



The sea anemone shows a remarkable amount of contrac- 

 tion when disturbed. In fact, all the animals of this branch 

 (coelenterates) are extremely contractile (see Fig. 177). 



Fig. 179. Sensitive plant (.Mimosa pudica) 



a, leaves in normal position ; b, leaves reduced 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 sensitive than any of its relatives (the bean family), 

 many of which are sensitive in the same way but not in the same degree 



In clams and oysters, contraction of special muscles results 

 in closing the shell. In snails, contractions withdraw the body 

 into the shell. The turtle withdraws head and legs into his 

 "shields," and the box turtle closes the shell up even more 

 completely. 



We do not usually think of plants as moving, either to get 

 food or to escape danger. Some plants, however, can do a 

 great deal of moving in connection with the capture of insects 



