INFLUENCE OF ENVIRONMENT 283 
and no doubt this extreme sensitiveness is an advantage 
to the plant. 
We have already 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 Leguminose 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 leayes, 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. 
