e 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 _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 s