Firstly, then, as well-known instances of pistil-bearing and stamen-bearing flowers occurring separately on the same plant, we may mention cucumbers, melons, marrows, et hoc genus omne. Now, when these vegetables are grown under glass, whether in greenhouses or in pits, to which bees and other insects have little or no access, gardeners find it necessary themselves to apply the pollen-bearing portions of the one kind of flower to the pistil of the other. If this is neglected to be done, the fruit makes no progress, turns yellow, and dies. Where, however, the plants are grown in the open air, or are not so shut up as to exclude insects, they will be fertilised without the intervention of man; for bees of various kinds will certainly visit the flowers, and carry the life-giving dust where it is needed. In fact, we believe it might be asserted with confidence, that in all plants, where this separation of, what we may call, the sexes takes place, the flowers possess special attractiveness to the tenants of our hives; and it is well that this is the case, otherwise the continued existence of such plants would be seriously endangered.
As examples of diœcious genera, or those having pollen-bearing flowers on one plant and pistiliferous flowers on another, we may note the willows, the poplars, and the firs; and it is remarkable that these all are special favourites with bees. In the early spring, when breeding has been going on in the hives, and when the demands of the advancing larvæ require considerable supplies of pollen, the catkins of the willow are abundantly visited, and the diffusion of their fertilising powder is thus greatly promoted. The same may be said about the poplar, and, in all probability, the gathering of propolis from trees of the fir-tribe makes the bee the unconscious, but useful, instrument of carrying pollen from the catkins to the cones, though, from the abundance of the powder, and the openness of the scales of the cones r the wind is a sufficiently effective agent for its conveyance in this order of trees.
| Fig. 70.—Epilobium Angustifolium. (Young bloom.) | Fig. 71.—Epilobium Angustifolium. (Old bloom.) |
Passing next to cases in which the stamens and pistils of the same flower come to maturity at different times, so as to make cross-breeding a necessity, we may mention first some plants in which the pollen ripens before the stigma is ready to receive it. We have such a condition of things in the willow-herb, or epilobium tribe. The pretty pink blossoms of a large variety of this genus are to be found, in summer, along the banks of brooks and running ditches. We will confine our remarks to the species distinguished by its narrow leaves, and hence named angustifolium. When the flower has fully opened, the eight stamens spread out, and their anthers shed the pollen. Bees visit the blossoms, and getting dusted with the grains, carry these away to other flowers of the same kind. And here, in passing, we may recall the fact of bees keeping to one species of plant during the whole of any one journey from the hive. The importance of this can be now better appreciated, when its influence on the fructification of blossoms is observed. But to return, the pistil of the willow-herb remains, till the stamens have withered, curved round out of the way, and unable to receive any of their pollen. Then, after they are dead, it comes into such a position that it can take what pollen may be brought to it from younger flowers. For the conveyance of this it is dependent chiefly on bees, who do not fail to carry enough for the required purpose. In this way each blossom, by the agency of these insects, both gives and gets what is necessary for the continued life of the species; and without these unconsciously conferred benefits from insect life, no seeds of this kind of epilobium would mature.
Another instance, equally interesting, is seen in the well-known cineraria tribe. The plants of this genus belong to the composite order, in which what is usually called the blossom, consists of many flowers, grouped together on one head. In the example before us, there are nearly 200 thus aggregated. These florets separately open at different times, those of the outer circles coming before those nearer the centre. The pollen-tube of each is formed by five anthers, fastened together at their edges, and discharging their pollen into the space between them. At the lower part of this inclosure the pistil is growing, but is not in a condition to receive usefully the fertilising powder. It, however, as it advances, sweeps out, and carries up with it, the pollen-grains, so that they may be conveyed to other florets of the same or other blossoms, to effect their vitalising work. At length the pistil, with its brush on its summit, comes into view, but, even yet, is not sufficiently developed for fertilisation. In due course, however, the upper end splits, and exposes the surface of the stigma ready for the pollen, which must be brought from some other floret, and probably from some other blossom. Thus cross-breeding is effectually secured.
Fig. 72.—Cineraria. (Magnified.)
Another point for attaining this end is worthy of remark. The outer ring of florets is distinguished by long, coloured petals, which make up, in common parlance, the flower of the cineraria. These serve to render the composite head conspicuous, and attractive to insects. Some varieties, moreover, emit fragrant odours, and thus present further inducements for visitation. It is remarkable that, in these florets of the outer edge, the gay-coloured petals are developed at the expense of the anthers. Consequently, they produce no pollen, and their pistils have no brushes, as there would be no office for them to perform. The bright rays have accomplished their own special purpose, and the florets may well depend on others for pollen.
Now, as to the great importance of cross-fertilisation in this species, we may quote the experiments made by Dr. Darwin.[11] He says: "Two purple-flowered varieties (of cineraria) were placed under a net in the greenhouse, and four corymbs (or bunches of flowers) on each were repeatedly brushed with flowers from the other plant, so that their stigmas were well covered with each other's pollen. Two of the eight corymbs thus treated produced very few seeds, but the other six produced on an average 41.3 seeds per corymb, and these germinated well. The stigmas on four other corymbs on both plants were well smeared with pollen from the flowers on their own corymbs; these eight corymbs produced altogether ten extremely poor seeds, which proved incapable of germinating. I examined many flowers on both plants, and found the stigmas spontaneously covered with pollen; but they produced not a single seed. These plants were afterwards left uncovered in the same house, where many other cinerarias were in flower; and the flowers were frequently visited by bees. They then produced plenty of seed, but one of the two plants less than the other, as this species shows some tendency to be diœcious.
[11] See Cross and Self-Fertilisation of Plants, p. 335.