CAUSES OF STERILITY.

241

produce its impregnating tubes in too low a temperature, or when the air is charged with moisture; neither, in the absence of wind or insects, have some plants the power of conveying the pollen to the stigma, their anthers having no special irritability, and only opening for the discharge of the pollen, not ejecting it with force, unless the filaments are irritable enough to knock the anther violently against the pistil; or, unless the stigmatic apparatus possesses special irritability, as is the case with certain Orchids. If we watch the Hazel, or any of the Coniferous order, in which the enormous quantity of pollen employed to secure the impregnation of the seed renders it easy to see what happens, it will be found that no pollen is scattered in damp cold weather; but, in a sunny, warm, dry morning, the atmosphere surrounding such plants is, in the impregnating season, filled with grains of pollen discharged by the anthers. In wet springs the crops of fruit fail, because the anthers are not sufficiently dried to shrivel and discharge their contents, which remain locked up in the anther cells till the power of impregnation is lost; or perhaps because, as a critic has suggested, the wet operates injuriously upon the very constitution of the pollen, and of the stigmatic surface. In vineries and forcing-houses generally, into which no air is admitted to disturb the foliage, nor any artificial means employed for the same end, and when the season is too early for the presence of bees, flies, and other insects, the grapes will not set: and in the frames of Melons and Cucumbers, from which insects are excluded, no seed is formed unless the pollen is conveyed by hand, from those flowers in which it is formed, to others in which the young fruit alone is generated. In all cases of this kind, the remedy for sterility, where plants exist in an artificial condition, is evidently to set, or fertilise them by hand; but, when they occur in the orchard or the flower-garden, science suggests no assistance. It is by hand-setting alone that in hot-houses, and in tropical Asia, the Vanilla, a native of tropical America, can be made to bear fruit; because, as is believed, the insects which haunt the Vanilla flowers in America, and set them, are unknown in Europe and Asia.

R

242

MONSTERS ARE STERILE.

It sometimes happens that particular parts of plants, distant from the fruit, are so constructed as to attract to themselves the food intended for the fruit, and thus to prevent the formation of seed. For example:-The early varieties of Potato do not readily produce seed, owing to the abstraction by their tubers of the nutritive matter required for the support of the seed. Mr. Knight found that by destroying the tubers in part, as they formed, seeds were readily procured from such varieties.

But perhaps the most frequent cause of sterility is the monstrous condition of the flowers of many cultivated plants. It was explained in Book I. that the floral organs of plants are nothing more than leaves, so modified as to be capable of performing special acts, for particular purposes; but they are not capable of performing those acts any longer than they retain their modified condition: and therefore the stamens cannot secrete pollen, when, by accidental circumstances, they are changed into leaves, as happens in double flowers; in such cases, there is nothing to fertilise the stigma, and, of course, no seed is produced. Or the carpels themselves may be converted into leaves, and have lost their seed-bearing property. Double flowers in the latter case cannot possibly bear seed; but in the condition first mentioned they may, and often do. To bring this about, the cultivator plants in the vicinity of his sterile flowers others of the same species, in which a part at least of the stamens are perfect, and they furnish a sufficiency of pollen for the impregnation of the other flowers in which there are no stamens.

In some cases, principally in those of Composite flowers, the seed is formed and advanced towards perfection, and then decays; this is owing to the flower heads of such plants being composed, in a great measure, of soft scales, absorbent and retentive of moisture, to which, in their own country, they are not exposed in the fruiting season, but by which they are affected under the hands of the cultivator. When the heads of such flowers are soaked with moisture, which they cannot get rid of, the scales rot, decay spreads to all the other parts, and thus the production of seed is prevented. The Chinese

SEEDS MUST BE RIPE.

243

Chrysanthemum is a familiar instance of this. Such plants seed readily if the flower heads are kept warm and dry; and it is thus that the sterile Chrysanthemum has been made seedful; that is to say, by growing it in a dry warm winter border, protected from showers by a roof of glass; or by using some similar means of guarding it; or by rearing it in a warm dry climate.

When seeds are freely produced, it is not altogether a subject of indifference in what way they are saved, if it is desired that their progeny should be the most perfect that can be obtained. Weak seeds produce weak plants, and therefore recourse should be had, in all delicate cases, to artificial means for gaining seminal vigour. In general, the cultivator trusts to his eye for separating the plumpest and most completely formed seeds; or to floating them in water, selecting only the heavy grains that sink, and rejecting all those which are buoyant enough to float. But the energy of the vital principle in a seed may be, undoubtedly, increased by abstracting neighbouring fruits, by improving the general health of the parent plant, by a full exposure of it to light, and by prolonging the period of maturation as much as is consistent with the health of the fruit.

It is a general rule that seedlings take after their parents, an unhealthy mother producing a diseased offspring, and a vigorous parent yielding a healthy progeny in all their minute gradations and modifications; and this is so true, that, as florists very well know, semi-double Ranunculuses, Anemones, and similar flowers, will rarely yield double varieties, while the seed of the latter as unfrequently give birth to semi-double degenerations.

Independently of these things, it is indispensable that the seed of a plant, when saved, should be perfectly ripe, if it is intended to be laid by for future sowing. The effect of ripening is to load the seed with carbon in the form of starch, or some other substance of a similar kind, and to deprive it of water, conditions necessary for its preservation: but, if a seed is gathered before being ripe, these conditions are not secured; and, in proportion to the deficiency of the requisite elements

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SEEDS MUST BE KEPT DRY.

which maintain vitality, and superabundance of water, is the seed liable to perish.

The complete maturation of the seed is, however, a disadvantage, when it has to be sown immediately after being gathered; for the embryo is formed, and capable of germinating, long before the period of greatest maturity. There are two periods in the latter part of the organization of a seed which, although separated by no limits, require to be distinguished. The first is that when the embryo is completed; and the second is when nature has, in addition, furnished it with the means of maintaining its vitality for a long period. It is just as capable of growing at the expiration of the first period as of the second; it will do so immediately if committed to the ground; and we see it actually happening to Peas, Beans, Corn, and other field crops, in wet summers; but at the end of the second period, it cannot germinate till it has relieved itself of matters not required for vegetation, which, during that period, were deposited in its tissue.

If seeds are to be preserved for a length of time, a state of complete dryness is so necessary to them that it has been recommended to increase it by artificial means; not, however, by the application of heat, or by any process like that of kilndrying, which would destroy their vitality; but by some of those chemical processes that dry the atmosphere without raising its temperature. It occurred to Mr. Livingstone, that air made dry by means of sulphuric acid might be advantageously employed for this purpose, and he says that the success of his experiments was complete. He placed the seeds to be dried in the pans of Leslie's ice machine, and carefully replaced the receiver without exhausting the air; small seeds were sufficiently dried in one or two days, and the largest seeds in less than a week. (Hort. Trans., iii. 184.) Other contrivances might easily be adopted. Muriate of lime, for instance, which has the property of absorbing the moisture of the atmosphere, might, perhaps, be employed with advantage in drying the air in which seeds are placed after being gathered. But such devices have little practical value, the sun being the great power to which the gardener very properly trusts.

CHAPTER VII.

OF SEED-PACKING, AND PLANT-PACKING.

IT seldom happens that seeds are sown as soon as they are ripe; it is sometimes desirable that they should be preserved for long periods of time; the power of conveying them for great distances, through various climates, is one of those upon which man most depends for the improvement of the horticultural resources of all countries; and for this purpose large sums are annually expended, both by governments and individuals. It is, therefore, an object of the first importance to ascertain what is not well understood, as it would seem, namely, the causes by which the destruction of the germinating power of seeds is effected; for it is only by doing this, that their preservation can be secured.

Seeds are probably possessed of different powers of life, some preserving their vital principle through centuries of time, while others have but an ephemeral existence under any circumstances. The reasons for this difference are unknown to us, and apparently depend upon specific vitality, over which we have no control; but the fact of great longevity in some seeds is certain, and it is highly desirable that the conditions which enable them to preserve their germinating power for long periods of time should be discovered.

It is, however, extremely difficult to reconcile with all known facts any theory which physiology may suggest. What applies to one class of cases fails to explain others. The instances already mentioned at p. 103, are sufficiently conflicting; for they include examples both of dryness and wetness, and of exclusion of air and its admission; dryness and exclusion of air