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 _Jussiaea 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. Some of these endemic and naturalised plants are probably rendered sterile from excessive multiplication by buds, and their consequent incapacity to produce and nourish seed. But the sterility of others more probably depends on the peculiar conditions under which they live, as in the case of the ivy in the northern parts of Europe, and of the trees in the swamps of the United States; yet these plants must be in some respects eminently well adapted for the stations which they occupy, for they hold their places against a host of competitors.