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 especially attended to this plant, has never seen it in fruit. The _Carex rigida_ often fails to perfect its seed in Scotland, Lapland, Greenland, Germany, and New Hampshire in the United States.[432] The periwinkle (_Vinca minor_), which spreads largely by runners, is said scarcely ever to produce fruit in England;[433] but this plant requires insect-aid for its fertilisation, and the proper insects may be absent or rare. The _Jussiæa grandiflora_ has become naturalised in Southern France, and has spread by its rhizomas so extensively as to impede the navigation of the waters, but never produces fertile seed.[434] The horse-radish (_Cochlearia armoracia_) spreads pertinaciously and is naturalised in various parts of Europe; though it bears flowers, these rarely produce capsules: Professor Caspary also informs me that he has watched this plant since 1851, but has never seen its fruit; nor is this surprising, as he finds scarcely a grain of good pollen. The common little _Ranunculus ficaria_ rarely, and some say never, bears seed in England, France, or Switzerland; but in 1863 I observed seeds on several plants growing near my house. According to M. Chatin, there are two forms of this Ranunculus; and it is the bulbiferous form which does not yield seed from producing no pollen.[435] Other cases { 171} analogous with the foregoing could be given; for instance, some kinds of mosses and lichens have never been seen to fructify in France.