200 ELEMENTS OF 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 thereforé depend upon the fact that the immense 
majority of their seeds are not allowed to grow into mature 
plants. 
242. Importance 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 
