50 PLANT MIGRATION 
needs to reach fresh ground if it in turn is to attain 
healthy development. And it is undoubtedly of ad- 
vantage to plants, if they are to continue to exist on 
the Earth, to be able to jump barriers and to colonize 
fresh suitable habitats which may arise in the course 
of natural changes, which sooner or later may render 
old habitats untenable. Thus the very existence of 
plants upon the Earth depends on the adequacy of 
seed-dispersal. This being so, the imaginative mind, 
viewing the marvellous and infinitely varied con- 
trivances of Nature, will possibly be struck more by 
the want of special provision for dispersal shown by 
the majority of the higher plants—their helplessness 
in this respect—than by the beautiful devices ex- 
hibited by the few. In the first place, seeds are inert, 
devoid of any power of locomotion—though in some 
instances the last act of the parent is to discharge 
them with an explosive action into the air. They are 
dependent on the movements of external media—air, 
or water, or wandering animals—for transportation 
of any magnitude, and while many possess very 
beautiful devices for enabling them to take advantage 
of opportunities in this regard, the majority are 
devoid of any special structures. They are as inert 
as pebbles or grains of sand: but they possess two 
attributes which form important assets— namely, 
numbers and vitality. The amount of seed produced 
annually is hundreds, or more usually thousands, 
sometimes hundreds of thousands, for each parent. 
What matter if myriads perish? If one in so many 
thousands takes root and grows, the species will not 
diminish in numbers. Vitality also largely affects the 
problem. The seed can endure extremes of heat and 
