14 BULLETIN OF THE NATURAL HISTORY SOCIETY. 



///. Locomotion by Movements of Bij^ening Tissues. 



Although the skeleton built up by plants out of 

 cellulose and its derivatives is not contractile, it never- 

 theless is capable of producing locomotive movement if 

 ripenened in special ways. By this means either slow, 

 creeping movements or very rapid projection may result, 

 though the resulting locomotion can never be great. It 

 is clear that for projection, seeds, not buds, nor even very 

 light spores are adapted, and the most efficient seeds for 

 projection are nearly or quite spherical, smooth, hard and 

 rather heavy. The various modes are as follows : 



1. Cell walls may become strongly hygroscopic, swelling 

 and shrinking and altering shape as moisture is absorbed 



.and given up, just as a board warps, and in small pieces 

 the movement may l^e rapid. Sometimes slender hyroscopic 

 arms are attached to spores and move them from the 



• capsule or even farther, as in the spores of Equisetum. In 

 Vanda teirs, a tropical orchid, hygroscopic hairs force out the 



: seeds to be carried off by the wind. There are even a few seeds 

 and fruits which are pushed along the ground by the hygroscopic 

 twistings of some of their hairs, certain hooks which point 

 backward allowing only of a forward motion. Such are some 

 STpecies of Trifolium. In Avena sterilis, the "living oat," this 

 movement is very perfect and may be somewhat sudden. Related 

 movements assist in burying seeds as will presently be considered, 

 and in closing fruits when weather is unfavorable, and opening 

 them when favorable. 



2. Certain bands of tissue may ripen under restraint in a 

 state of tension so that finally, and more or less connected with 



-drying-up or hygroscopicity, when the restraint is released or 



• overcome, the parts spring suddenly to a new position and hurl 

 ■ out the seeds, sometimes to a distance of many feet. This may 



come about by the formation of special elastic unicellular 

 '' elaters " which force out the spores from the capsule in many 

 mosses. Or the sides of the carpels may come to press harder and 



