INFLUENCE OF ENVIRONMENT 283 



and no doubt this extreme sensitiveness is an advantage 

 to the plant. 



We have alreaclv spoken of the development of sen- 

 sitiveness resulting in the movements of various parts of 

 the flower, in connection with the subject of pollination. 



The movements of leaves in response to stimuli of 

 various kinds are especially developed in several groups 

 of plants, of which the Leguminosse are perhaps the 

 most notable. The well-known sensitive plant (Mi- 

 mosa) is the best known of these, but many common 

 leguminous plants, like the species of clover, locust, 

 beans, and many others, exhibit marked movements of 

 the leaves, being especially sensitive to changes in the 

 intensity of the light to which they are exposed. Thus 

 most of these plants have the leaves folded up at night, 

 exhibiting the so-called "sleep movements." 



Movements of a purely mechanical kind occur in 

 many plants, both among the lower ones and the 

 flowering plants. The hygroscopic movements of the 

 elaters of liverworts, or the peristome-teeth of the moss 

 capsule, the opening of the sporangia of the ferns and 

 of the anthers of flowers, are all good examples of this. 

 These movements are entirely due to the unequal ab- 

 sorption of water by the cell-walls of the motile organ, 

 or to unequal loss of water from them. Similar hygro- 

 scopic movements are exhibited by the awns of grasses 

 and those attached to the fruits of other plants, e.g. the 

 spirally twisted awn of the fruit in alfilaria (Erodium). 

 The opening of most seed-vessels, such as those of the 

 violet or balsam, are of much the same nature. All of 

 these movements are connected with the dispersal of 

 the spores or seeds. 



