GERMINATION (Lat. germinatio, from germinare, to bud, from germen, bud). The process by which a spore begins the development of a plant body. Technically, only spores germinate, but this term has been extended to include the process by which the embryo escapes from the seed. The so-called germination of the seed, however, is not true germination, since it is the escape of an embryo which has already been germinated, and true germination includes the very beginnings of the young plant. In the case of a seed, germination begun by a fertilized egg has been checked, and seed-germination is the resumption of activity and the escape of the young plant.
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| 1 and 2, germination of pollen-grain of pine; 3 and 4, young embryo of buttercup; 5 and 6, first and last stages of embryo of a fern. |
The conditions of germination are uniform. In general, they are suitable amounts of water, of heat, and of oxygon. Naturally the range in each one of these factors is very great, some spores germinating in the presence of a comparatively small amount of water, or at a relatively low temperature, while others need a large amount of water or high temperature. Between these extremes there is every possible combination of requirements. Some spores germinate almost immediately after they have been transported from the parent plant; while others may pass into a resting condition of greater or less duration. This difference in habit is generally apparent in the different character of the spore wall, those spores which are to germinate quickly having thin walls, and those which are to pass into a resting condition having thick walls. Since the spore consists of a single cell, the first evidence of germination is the activity of the cell, which usually enlarges, and then divides, resulting in a two-celled embryo. One or both of the daughter cells then grow to mature size, and division then occurs. In this process of growth and division, the spore wall is broken, and the young plant emerges, and continues its development by drawing upon the reserve foot-supply in the spore until it is able to maintain itself.
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| 1, Young prothallium of a fern; 2, three stages in the germination of a green alga, beginning with the spore (a); 3, young plants of a liverwort developing from spores within the sporangium. |
The early stages of germination have attracted a great deal of attention, under the impression that they furnish proofs of the relationships of groups. Accordingly, the order of succession and the direction of cell-walls have been carefully noted. In case the plant is a complex one, after a certain number of cells have been developed, the different regions of the body begin to appear. For example, in an embryonic seed-plant it would be impossible for a time to tell what kind of plant is to develop, but after a homogeneous cell-mass of greater or less extent is formed, the organs begin to appear which determine the character of the plant.
In the case of alternation of generations (q.v.) the germination of the sexual spore (fertilized egg) results in a sexless plant (sporophyte); while the germination of the asexual spore results in a sexual plant (gametophyte). Among the heterosporous plants (those producing two kinds of asexual spores), that is, in certain fern-plants and all the flowering plants, the sexual plants (gametophytes) do not escape from the spores which germinate them. For example, the pollen-grain is a spore which by its germination produces a male gametophyte, but this gametophyte is so much reduced that it is represented only by a few cells or nuclei within the pollen-grain. The same is true of the germination of the megaspore in seed-plants, which is retained within the ovule, and which in its germination develops the so-called endosperm, which is the female gametophyte. With the exception of heterosporous plants, however, the germinating plantlet soon escapes from the spore.
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| 1, Development of Botrydium (alga) from the egg; 2, segmentation of egg of a brown alga; 3, a young fungus coming from the egg; 4, young prothallium of Equisetum. |
In the so-called germination of the seed there are numerous events which may be observed. Attention has been called already to the fact that this process is not technically germination, but merely the renewal of activity, and the escape of the young plantlet. Just how long different seeds may retain their vitality in a state of suspended animation is not definitely known. Some seeds have renewed activity after having remained in a dried-up condition for many years, but such stories as that the wheat taken from the wrappings of Egyptian mummies has been made to germinate are myths. Seed-germination results in freeing the embryo from the seed-coats, and in enabling it to establish itself for independent living. The first conspicuous change noted in the seed after the absorption of water is the softening of the contents, the solid or insoluble starch, if that be the form of the food storage, being converted by a process of digestion into soluble sugar ready for transfer. Accompanying this change there is a marked evolution of heat, so that if a large mass of seeds is set to germinating, as in the process of malting, the heat may become very evident. The first part to protrude from the seed is the hypocotyl (q.v.), the tip of which is thrust out by the rapid elongation of its upper part. This protruding and rapidly elongated tip, which is to develop the root, now rapidly increases in length, and is very sensitive to the influence of gravity and of moisture, responding by developing any curvature necessary to reach the soil. Penetrating the soil and beginning to put out lateral branches, it secures the grip necessary for the extrication of the other regions of the embryo. After some anchorage has thus been obtained, the upper part of the hypocotyl again begins a period of rapid elongation, which results in the development of a curvature known as the hypocotyl arch. In the case of the germinating bean this arch is the first structure to appear above ground, and its pull upon the seed is very apt to bring it to the surface. Finally, the arch in its effort to straighten pulls the cotyledons out of the seed-coats, and with them the stem-tip, the axis of the plant straightens up, the seed-leaves and sometimes other leaves expand, and germination is over; for with roots in the soil, and green leaves expanded to the air and sunlight, the plantlet has become independent. These details are not the same for all seeds, for there are certain notable variations. For example, in the pea and acorn the cotyledons are so gorged with food as to have lost all power of acting as leaves, and are never extricated from the seed-coats. In the cereals, as corn, wheat, etc., the embryo lies close against one side of the seed, so that it is completely exposed by the splitting of the thin skin which covers it. In such a case the cotyledon is never unfolded, but remains as an absorbing organ, while the root extends in one direction and the stem with its succession of ensheathing leaves develops in the other.