The opportunity for migration thus offered to sedentary plants once at least in each cycle is of very great importance. A plant, living on one spot and drawing, from that portion of the soil which its roots can reach, certain mineral salts essential for its continued growth, tends to exhaust the available supply of these materials, and the succeeding generation needs to reach fresh ground if it in turn is to attain healthy development. And it is undoubtedly of advantage 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 contrivances 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 exhibited 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 cold which would be fatal to the parent; it can be drowned, or scorched, or dashed about, or in many cases eaten by animals without injury; it can lie buried in the soil for a long period of years, yet if turned up again and placed within reach of the requisite amount of air and heat, will spring up vigorously. As a matter of fact, investigation soon shows that absence of special devices for dispersal provides no measure of the breadth of a plant’s distribution, nor is profuse seed-production necessarily related to abundance of offspring. Many factors come into play, and conclusions of this obvious kind will generally only lead us astray. But that does not render the study of each one of the factors less interesting.