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. [Illustration: Fig. 70.--Epilobium Angustifolium. (Young bloom.)] [Illustration: Fig. 71.--Epilobium Angustifolium. (Old bloom.)]