21 6 The Great World's Farm 



be larger and stronger, or they may be better fitted for the 

 soil and situation. Whatever the advantage may be, 

 those possessing it will speedily overpower their less for- 

 tunate rivals, and then, having secured sufficient elbow- 

 room, will grow up strong and healthy. 



Plants of different species, when crowded together, are 

 better off in another respect, for they do not all want pre- 

 cisely the same amount of the various mineral foods, and 

 so there is more for all. For this reason it is a very 

 usual thing to sow a grass-field with seed of different 

 species; and the greater the variety, the heavier the crop 

 of hay, because the plants have had a better opportunity 

 of obtaining food. 



On this account, therefore, as well as that they may 

 have change of air, it is well for seeds that they should be 

 scattered, or otherwise dispersed. But there are other 

 reasons still. 



Some plants need shelter, and are killed by sudden 

 exposure. If they had no means of dispersing their seeds, 

 not only they, the parents, but their whole progeny, might 

 be exterminated, by the removal of trees, etc. Or again, 

 by the draining of a pond, or drying up of a brook, plants 

 needing much moisture might be killed out of a neighbor- 

 hood, if all their seeds dropped close round them, while 

 they might continue to flourish if they were able to migrate 

 the distance of only a few yards. 



In some cases, too, the parent so exhausts the soil, 

 that the children have no chance of thriving, if they grow 

 under its shadow; and then again, if cross-fertilization be 

 an advantage to the plant, even where not absolutely 

 essential, it certainly seems — from experiments made in 

 crossing Indian-corn and beans with plants grown some 



