54 PLANT LIFE ON THE FARM. 



Movements of Leaves. The leaves of plants exhibit 

 several kinds of motion ; some periodic, as in the case of the 

 so-called sleep of leaves, some due to the stimulus of liglit 

 or its removal, some the consequence of contact, as in the 

 case of the sensitive plant ; but those to which mention is 

 here made are the result of the same causes as those before 

 alluded to in the case of stems and roots. The growth 

 movements of leaves are observable in the stalk, or in the 

 blade, or in both, and are chiefly exerted in a vertical 

 direction, so that the leaf rises or falls; but as the ascent is 

 never quite in the same line as the descent, some side to 

 side motion must also take place. It is noticed that the 

 rise occurs generally in the evening, the fall on the 

 following morning. These movements are probably due to 

 the intensity of growth being greater first on one side, then 

 on the other. 



Growth-movements of the kind indicated have now been 

 shown to exist in the roots, in the stems, and in the leaves. 

 The probability is that they occur more or less wherever 

 growth is going on actively. In accordance with this, it 

 may be mentioned that seedling plants manifest these 

 movements to a remarkable degree. Thus all the parts of 

 seedling cabbages, the radicle, the caulicle above the radicle 

 supporting the seed leaves or cotyledons, as well as these 

 latter organs, were observed by Darwin to exhibit growth 

 movements facilitating the downward passage of the root 

 and the upward progress of the caulicles. 



