200 ELEMENTS OP BOTANY. 



9,000,000 plants the third year, 

 27,000,000,000 plants the fourth year, 

 81,000,000,000,000 plants the fifth year, 

 243,000,000,000,000,000 plants the sixth year, 

 729,000,000,000,000,000,000 plants the seventh year. 



It is not difficult to see that the offspring of a single 

 morning-glory plant would, at this rate, soon actually cover 

 the entire surface of the earth. The fact that morning-glories 

 do not occupy any larger amount of territory than they do 

 must therefore depend upon the fact that the immense 

 majority of their seeds are not allowed to grow into mature 

 plants. 



242. Importanoe of dispersal of Seeds. — It is clear that 

 any means of securing the wide distribution of seeds is of 

 vital importance in continuing and increasing the numbers 

 of any kind of plant, since in this way destruction by over- 

 crowding and starvation will be lessened. 



A few of the means of transportation of seeds have been 

 hinted at in §§ 221-224, but the cases are so numerous and 

 varied that a special treatise might well be devoted to this 

 subject alone. 



Seeds are transported by the wind, by the water, by men 

 and other animals, and (to short distances) by the explosive 

 action of the capsules in which they mature, or by similar 

 contrivances. A most valuable topic for study in late 

 summer and autumn is that of the various devices for seed- 

 carrying found in common plants. 



Not only are small seeds and fruits, like those of the 

 willow and thistle respectively, borne for long distances by 

 the wind, but an entire flower-cluster may ripen into a light, 

 buoyant object, which can be blown along for many miles. 

 Some of the so-called " tumble-weeds " of the prairies are of 

 this description, like the tickle-grass, Fig. 183. Other 

 tumble-weeds break off at the root, and the whole plant is 



