ARBOR DAY ITS HISTORY AND OBSERVANCE. 41 



(Dioncva mmcipula) shows motion in a different way. It lias at the 

 end of its leaves an expansion like two leaves of a book ready to fold 

 together, or like the shells of a clam. Around the margin of these 

 leaves are bristles, with other more delicate ones in the center. When 

 an insect alights on the open leaves and touches the central bristles, 

 the leaves shut together so quickly that it is caught and held there till 

 it dies. Other plants show motion in different ways. The trees also 

 have motions independent of those which are occasioned by the wind 

 or any external force. The locust tree and some others, for instance, 

 fold up their leaves at night as though preparing to sleep, and spread 

 them open again in the morning. Some move their leaves in a different 

 manner. In all trees, also, there is in the roots a constant movement, 

 at least during the growing season of the year. At the very beginning 

 of its life the root as it sprouts from the seed insists upon going down- 

 ward into the earth. Turn the sprouting acorn so that its root or radi- 

 cle shall point upward and very soon it will turn and double upon itself, 

 if necessary, in order to take a downward course, and though you turn 

 it again and again, it will persist in its determination and die if neces- 

 sary rather than give up the struggle. So when a tree is established 

 and growing, though its stem must remain in the same place, its roots 

 are all the while pushing out in various directions, winding around 

 obstacles of one kind and another in pursuit of moisture and nourish- 

 ment and making their way steadily on, so that nothing will so well 

 describe the character of that part of the tree which is under ground 

 as to say it is in a state of motion. Darwin, the eminent naturalist, 

 goes so far as to claim that all the growing parts of plants, above as 

 well as below ground, manifest voluntary motion, describing circles or 

 circular spirals continually, a circumnutating," as he calls this move- 

 ment. " If we look," he says, u for instance, at a great acacia tree, we 

 may feel assured that every one of the innumerable growing shoots is 

 constantly describing small ellipses, as is each petiole, subpetiole, and 

 leaflet." 1 



The action of the life principle in the trees also often manifests aston- 

 ishing force. Darwin found that the transverse growth of the radicle 

 of a sprouting beau was able to displace a weight of 3 pounds 4 ounces 

 in one case and one of 8 pounds 8 ounces in another. One can hardly 

 walk where trees are growing among rocks without seeing instances of 

 the splitting asunder of great masses of them by the growth of the 

 tree roots which have gained entrance into their crevices when small, 

 and in growing have expanded with irresistible force. So, also, it is a 

 common thing to see the walls of buildings disturbed and much injured 

 by the roots of trees growing near them. Experiments made by 

 Professor Clark, at Amherst College, led him to think that the force 

 exerted by a squash in growing was equal to about 5,000 pounds. Thus 



1 Power of Movement in Plants. 



