26 PRACTICAL COURSE IN BOTANY 
anchorage and left to drift about on the ground. The spread- 
ing, bushy tops become very light after fruiting, so as to be 
easily blown about by the wind, dropping their seeds as they 
go, until they finally get stranded in ditches and fence corners, 
where they often accumulate in great numbers during the 
autumn and winter. 
24. The advantages of dispersal. — Seed cannot germinate 
unless they are placed in a suitable location as to soil, moisture, 
and temperature. In order to increase the chances of secur- 
ing these conditions, it is clearly to the advantage of a species 
that its seeds should be dispersed as widely as possible, both 
that the seedlings may have plenty of room, and that they 
may not have to draw their nourishment from soil already 
exhausted by their parents. The farmer recognizes this 
principle in the rotation of 
crops, because he knows that 
successive growths of the 
same plant will soon exhaust 
the soil of the substances re- 
quired for its nutrition, while 
they may leave it richer in 
nourishment for a different 
crop. 
25. Self-planting seeds. — 
Dispersal is not the only 
problem the seed has to meet. 
The majority of seeds cannot 
germinate well on top of the 
ground, and must depend on 
various agencies for getting 
under the soil. Some of them 
do this for themselves. The 
seeds of the stork’s-bill, popularly known as “ filarees,’”’ have 
a sharp-pointed base and an auger-shaped appendage at the 
apex, ending in a projecting arm (the “clock” of the “laree) 
by which it is blown about by the wind with a whirling motion 
Fic. 45. — Self-planting pod of peanut. 
