270 THE LIFE OF THE PLANT 



too great cooling, and are less accessible to morning frosts, 

 during which plants are frequently frozen (actually 

 because of irradiation) although the thermometer may 

 not have fallen below 32 F. The purpose of the 

 movements of the catchfly is obvious : the very name 

 of the plant expresses it ; for, as we shall see later on, 

 together with certain other plants, the catchfly in reality 

 feeds upon captured insects. The use of motion in the 

 irritable leaves of mimosa is less obvious. Nobody has 

 apparently even tried to explain its purpose. Only 

 more or less probable suggestions can be brought forward 

 in this connexion. Any one who has observed the 

 effects of heavy showers and hailstorms will certainly 

 have noticed how it chips the foliage of our trees. 

 Such delicate organs as the leaves of mimosa would 

 suffer still more from tropical storms, if the first drops 

 of rain did not cause them to gather their outstretched 

 leaflets together and fold them against the stem. These 

 leaves, therefore, behave like the rush in the fable : 

 they weather storms which shatter oak-trees. I repeat, 

 this is only a conjecture, the accuracy of which can be 

 verified only by observation on the spot where these 

 curious plants grow. It is much more difficult to explain 

 the object of the continuous movements of the leaflets 

 of Desmodium, unless we admit that these movements 

 serve the plant to scare away pernicious insects, attracted 

 by its sweet juicy foliage. 1 If this be true, plants 

 would seem to use their capacity for movement for two 

 different ends : for getting rid of enemies on the one 

 hand and for catching and feeding upon them on the 

 other. 



1 The same explanation may hold with regard to mimosa; at all 

 events I have observed cases when mimosas in our hot-houses 

 perished from certain white lice, which found their abode at the very 

 articulations of the leaf. This is possible only in cases where the 

 leaf has lost its irritability. The tissue of the articulations must 

 specially attract insects, on account of the abundant sugary substances 

 it contains. 



