"The trial was repeated on another variety with white petals tipped with red. Many stigmas on two corymbs were covered with pollen from the foregoing purple variety, and these produced eleven and twenty-two seeds, which germinated well. A large number of the stigmas on several of the other corymbs were repeatedly smeared with pollen from their own corymb; but they yielded only five very poor seeds, which were incapable of germination. Therefore the above three plants, belonging to two varieties, though growing vigorously and fertile with pollen from either of the other two plants, were utterly sterile with pollen from other flowers on the same plant."
| Fig. 73.—Tropœolum Majus. (Young bloom.) | Fig. 74.—Tropœolum Majus. (Old bloom.) |
A condition similar to that described in the cineraria is found in the nasturtium (Tropœolum majus). In the young blossom may be observed the five stamens fully developed, while the stigma remains quite out of their reach. Later, when all the pollen is discharged from the anthers, the pistil throws up its style and stigma, now ready for fertilisation, which must be effected by the transfer of pollen from some other blossom of the same or another plant of the species. This transfer is effected, usually, by the bees.
Another example of this non-coincidence in the times of development of the stamens and pistil is found in the foxglove (Digitalis purpurea), but the fertilisation is effected by the larger humble-bees. The two upper and longer stamens shed their pollen before the two lower and shorter ones. This arrangement partly avoids the risk of self-fertilisation, while their position, which changes just when the anthers are ripe, enables them to smear the under side of any entering bee; while they also shed their pollen abundantly on the thickly-set hairs lining the mouth of the corolla. A second use is served by these hairs, viz., that of obstructing the entrance of the smaller kinds of bees, which could not so effectually fertilise the ovules. The larger sorts, in their raids upon the nectar, carry pollen from flower to flower, thus in the best manner bringing about the most desirable result of cross-breeding.
Passing now to plants in which the pistil develops earlier than the stamens, we may note the knotted figwort (Scrophularia nodosa). On making a section of a recently opened flower, the style, with its stigma, may be observed protruding just beyond the lip of the corolla, while the stamens are hiding away, as it were, in a little pouch below the entrance of the blossom. When fertilisation by pollen from another flower has taken place, the pistil droops and withers; while the anthers grow upwards to the mouth of the corolla, and present their nectar to the honey-seekers, for conveyance to other flowers of the species.
Fig. 75.—Section of Scrophularia Nodosa. | |
Fig. 76.—Scrophularia Nodosa. |
Fig. 77.—Scrophularia Nodosa. |
In the common sage (Salvia officinalis) we find a very remarkable contrivance, by means of which the anthers, through a sort of hinge-like connective, are brought down on the back of a bee entering the flower. The pollen thus discharged is carried by the insect to other blossoms, in which the place of the withered stamens has been occupied by the stigma now ready to receive the grains. Thus again inter-breeding is secured.
Fig. 78.—Salvia Officinalis. |
Fig. 79.—Salvia Officinalis. |
Fig. 80.—a. Erica Tetralix. b. Anther of Tetralix. | |
The next figure represents the very curious arrangement of the stamens in the Erica tetralix, or common heath. These, eight in number, are seen standing round the style, at about half its length. Each is held by a long filament (only one of which is shown in the figure), and is armed with a horn-like process. When the anthers are ripe, the pollen would escape from openings in their sides, were it not that the little slits abut on each other. When, however, a bee visits the flower, and thrusts up her proboscis, this strikes against one of the little horns, and pulls apart its anther from the rest. Pollen then drops on the bee's head, to be carried by it to other blossoms, which, while receiving a portion brought to them, in their turn give the bee a supply to carry to other flowers. It is a very salutary thing for the propagation of the heath that bees have such a strong liking for its nectar, and are thus induced to perform, albeit unconsciously, the process of fertilisation.
