374 



THE MAINTENANCE OF THE INDIVIDUAL 



of the boulevard. The Carolina ])oplar has lost its vogue as a tree for 

 city planting largely because of this habit of clogging drain })ipes by 

 the response of the roots to water. The movements seen in the wilting 

 of leaves, or the changes in the position of leaves in bright sunlight 



and in slight illumination, are 

 familiar to all. There may 

 even be a quite rapid opening 

 and closing of flower petals, 

 and there are also definite 

 noticeable changes in the posi- 

 tion of the leaflets of clover, 

 alfalfa, oxalis, and other 

 plants in the morning and at 

 night. The relatively rapid 

 responses of the leaves of 

 the sensitive plant, Mimosa 

 pudica, are all brought about 

 by the functioning of struc- 

 tures called pulvini, cushion- 

 like enlargements of the 

 petiole of the leaf at the point 

 of its insertion in the stem. 

 When the leaflets of the large 

 compound leaves of the mi- 

 mosa are stimulated by heat, 

 pressure, or anesthetics, they 

 tend to droop, the stimulus 

 from the leaflets being trans- 

 mitted at the extraordinarily rapid rate (for plants) of from one to three 

 centimeters per second. When the stimulus reaches the pulvinus 

 where the cells are large and are rich in water, a change in turgor takes 

 place in these cells, with the result that the leaf stalk droops. In some 

 plants there is a rapid and temporary fluctuation in growth on opposite 

 sides of the leaves. This causes a comparatively rapid turning move- 

 ment, but it is evident that these forces are not in themselves sufficient 

 to explain all the changes that take place in such plants as the Mimosa. 



Leaf motility in the sensitive plant 

 (Mimosa pudica): above, an open leaf; 

 l)elow, a leaf whose leaflets (/) have been 

 closed by niechanieal impact ; note also that 

 the petiole (p) has dropped; s, stipule; 

 m, pulvinus. 



Mechanisms of Response in Animals 



The mechanism of the reflex arc has already been described in some 

 detail in the discussion of the various types of nervous systems found 



