601. Its nature and use. After the embryo has reached its wonted growth in the ripened seed, it becomes suddenly inactive and torpid, yet still alive. In this condition it is, in fact, a living plant, safely packed and sealed up for transportation. This is the distinctive and wonderful nature of the seed.
602. Longevity of the seed. This suspended vitality of the seed may endure for years, or even, in some species, for ages. The seeds of maize and rye have been known to grow when 30 to 40 years old; kidney-beans when 100; the raspberry after 1700 years (Lindley), and kernels of wheat found in a mummy-case, and therefore 3000 years old, were a few years ago successfully cultivated in Germany and England (Schleiden). Seeds of Mountain Potentilla (P. tridentata) were known to us to germinate at Meriden, N. H., after a slumber of 60 years. On the other hand the seeds of some species are short-lived, retaining vitality hardly a year (Coffee, Magnolia).
603. In order that seeds may long retain their vitality they must be kept dry. But an even temperature is by no means necessary, as they are generally able to resist all the changes of our climate from many degrees below zero to 110° above, provided no moisture is present.
604. The dispersion of seeds over wide, and often to distant regions is offected by special agencies, in which the highest intelligence and wisdom are clearly seen. Some seeds made buoyant by means of the coma, or pappus, already mentioned, are wafted afar by the winds, beyond rivers, lakes and seas; as the thistle, dandelion, silkgrass.
605. Seeds are also furnished with wings for the same purpose. Others are provided with hooks or barbs, by which they lay hold of men and animals, and are thus, by unwilling agents, scattered far and wide (burr-seed, tick-seed).
606. Other seeds, destitute of all such appendages, are thrown to a distance by the sudden coiling of the elastic carpels (touch-me-not). The squirting cucumber becomes distended with water by absorption, and at length, when ripe, bursts an aperture at base and projects the mingled seeds and water with amazing force.
607. Transportation. Rivers, streams, and ocean currents are all means of transporting seeds from country to country. Thus the coco3 and the cashew-nut and the seeds of mahogany have been known to perform long voyages without injury to their vitality Squirrels laying up their winter stores in the earth, birds migrating from clime to clime, and from island to island, in like manner conspire to effect the same important end.