R. Colgate informs me that he has observed the same fact in New Zealand after the introduction of the hive-bee into that island. On the other hand, H. Muller ('Befruchtung' page 224) has often seen hive-bees visiting this plant in Germany, for the sake both of pollen and nectar, which latter they obtained by breaking apart the petals. It is at least certain that humble-bees are the chief fertilisers of the common red clover.
Trifolium incarnatum.--The flower-heads containing ripe seeds, on some covered and uncovered plants, appeared equally fine, but this was a false appearance; 60 heads on the latter yielded 349 grains weight of seeds, whereas 60 on the covered-up plants yielded only 63 grains, and many of the seeds in the latter lot were poor and aborted. Therefore the flowers which were visited by bees produced between five and six times as many seeds as those which were protected. The covered-up plants not having been much exhausted by seed-bearing, bore a second considerable crop of flower-stems, whilst the exposed plants did not do so.
Cytisus laburnum (Leguminosae).--Seven flower-racemes ready to expand were enclosed in a large bag made of net, and they did not seem in the least injured by this treatment. Only three of them produced any pods, each a single one; and these three pods contained one, four, and five seeds. So that only a single pod from the seven racemes included a fair complement of seeds.
Cuphea purpurea (Lythraceae).--Produced no seeds. Other flowers on the same plant artificially fertilised under the net yielded seeds.
Vinca major (Apocynaceae).--Is generally quite sterile, but sometimes sets seeds when artificially cross-fertilised: see my notice 'Gardeners' Chronicle' 1861 page 552.
Vinca rosea.--Behaves in the same manner as the last species: 'Gardeners' Chronicle' 1861 page 699, 736, 831.
Tabernaemontana echinata (Apocynaceae).--Quite sterile.
Petunia violacea (Solanaceae).--Quite sterile, as far as I have observed.
Solanum tuberosum (Solanaceae).--Tinzmann says ('Gardeners' Chronicle' 1846 page 183) that some varieties are quite sterile unless fertilised by pollen from another variety.
Primula scotica (Primulaceae).--A non-dimorphic species, which is fertile with its own pollen, but is extremely sterile if insects are excluded. J. Scott in 'Journal of the Linnean Society Botany' volume 8 1864 page 119.
Cortusa matthioli (Primulaceae).--Protected plants completely sterile; artificially self-fertilised flowers perfectly fertile. J. Scott ibid. page 84.
Cyclamen persicum (Primulaceae).--During one season several covered-up plants did not produce a single seed.
Borago officinalis (Boraginaceae).--Protected plants produced about half as many seeds as the unprotected.
Salvia tenori (Labiatae).--Quite sterile; but two or three flowers on the summits of three of the spikes, which touched the net when the wind blew, produced a few seeds. This sterility was not due to the injurious effects of the net, for I fertilised five flowers with pollen from an adjoining plant, and these all yielded fine seeds. I removed the net, whilst one little branch still bore a few not completely faded flowers, and these were visited by bees and yielded seeds.
Salvia coccinea.--Some covered-up plants produced a good many fruits, but not, I think, half as many as did the uncovered plants; twenty-eight of the fruits spontaneously produced by the protected plant contained on an average only 1.45 seeds, whilst some artificially self-fertilised fruits on the same plant contained more than twice as many, namely 3.3 seeds.
Bignonia (unnamed species) (Bignoniaceae).--Quite sterile: see my account of self-sterile plants.
Digitalis purpurea (Scrophulariaceae).--Extremely sterile, only a few poor capsules being produced.
Linaria vulgaris (Scrophulariaceae).--Extremely sterile.
Antirrhinum majus, red var. (Scrophulariaceae).--Fifty pods gathered from a large plant under a net contained 9.8 grains weight of seeds; but many (unfortunately not counted) of the fifty pods contained no seeds. Fifty pods on a plant fully exposed to the visits of humble-bees contained 23.1 grains weight of seed, that is, more than twice the weight; but in this case again, several of the fifty pods contained no seeds.