How Plants Secure Change of Location 383 



It is obvious that the ways described in this section, while 

 efficient so far as they go, secure no wide spread for plants; they 

 are, indeed, supplementary, or extra, methods which happen to 

 be rendered available by some peculiarity of growth or habit in 

 the plants concerned, all of which 

 possess also the far more efficient 

 methods we are now to consider. 

 As to these latter, they depend 

 in reality upon a single principle. 

 Forbidden by their mode of life 

 to move when adult, plants have 

 taken advantage of that stage in 

 their lives when they are small 

 and therefore easily transportable, 

 the stage of the embryo. This 

 embryo, with its vitality sus- 

 pended for a time, together with 

 its store of food substance and 



protective COatS, Constitutes the FIG. 149. Pod of a Vetch, explosively 

 , i i j f propelling its seeds. 



Seed, which is severed from the 



plant and can then be transported in various ways as we shall 



now proceed to consider. 



3. Projection by Elastic Machinery. The appreciable size and 

 weight of seeds makes possible their projection to some dis- 

 tance by a sudden application of sufficient power; and this fact 

 has been readily turned to use by plants for their dissemination. 

 The propelling machinery is variously made. In some kinds of 

 seed-pods, definite bands of cells ripen in a state of stretched 

 tension, which presently becomes so great that the pod bursts 

 suddenly, hurling out the loose-lying seeds to a distance of several 

 feet. In the Vetches, the two halves of the pea-like pods twist 

 suddenly apart in opposite directions (figure 149); in the Wild 

 Geranium, the ripening styles suddenly curl up from the length- 

 ened receptacle (figure 150); the valves of the capsules burst 



