Fortune, an extraordinary number of little bulbs are developed in the axils of the leaves of the yam, and this plant does not bear seed. Whether in these cases, as in those of double flowers and seedless fruit, sexual sterility from changed conditions of life is the primary cause which leads to the excessive development of the organs of vegetation, is doubtful; though some evidence might be advanced in favour of this view. It is perhaps a more probable view that plants which propagate themselves largely by one method, namely by buds, have not sufficient vital power or organised matter for the other method of sexual generation. Several distinguished botanists and good practical judges believe that long-continued propagation by cuttings, runners, tubers, bulbs, &c., independently of any excessive development of these parts, is the cause of many plants failing to produce flowers and of others failing to produce fertile flowers,--it is as if they had lost the habit of sexual generation.[427] That many plants when thus propagated are sterile there can be no doubt, but whether the long continuance of this form of propagation is the actual cause of their sterility, I will not venture, from the want of sufficient evidence, to express an opinion. That plants may be propagated for long periods by buds, without the aid of sexual generation, we may safely infer from this being the case with many plants which must have long survived in a state of nature. As I have had occasion before to allude to this subject, I will here give such cases as I have collected. Many alpine plants ascend mountains beyond the height at which they can produce seed.[428] Certain species of {170} Poa and Festuca, when growing on mountain-pastures, propagate themselves, as I hear from Mr. Bentham, almost exclusively by bulblets. Kalm gives a more curious instance[429] of several American trees, which grow so plentifully in marshes or in thick woods, that they are certainly well adapted for these stations, yet scarcely ever produce seeds; but when accidentally growing on the outside of the marsh or wood, are loaded with seed. The common ivy is found in Northern Sweden and Russia, but flowers and fruits only in the southern provinces. The _Acorus calamus_ extends over a large portion of the globe, but so rarely perfects its fruit that this has been seen but by few botanists.[430] The _Hypericum calycinum_, which propagates itself so freely in our shrubberies by rhizomas and is naturalised in Ireland, blossoms profusely, but sets no seed; nor did it set any when fertilised in my garden by pollen from plants growing at a distance. The _Lysimachia nummularia_, which is furnished with long runners, so seldom produces seed-capsules, that Prof. Decaisne,[431] who has