This section is from the book "On British Wild Flowers Considered In Relation To Insects", by John Lubbock. Also available from Amazon: Nature Series On British Wild Flowers Considered In Relation To Insects.
This order is easily distinguished from all others, except the Labiatae, by the four seed-like nuts; from the Labiatae by the form of the flowers, and by the leaves being alternate. It contains eleven British genera, viz., - Echium, Pulmonaria (Fig. 96), Mertensia, Lithospermum, Myosotis, Anchusa, Ly-copsis, Symphytum, Borago (Fig. 95), Asperugo, and Cynoglossum.
In consequence of its conspicuousness, and the easy accessibility of its honey, Echium vulgare is visited by a great variety of insects. The flower is tubular, contracting towards the base, so that insects are naturally conducted to the honey. The stamens are five in number; one remains in the tube of the flower, while the other four project, and form a convenient alighting stage for insects, which can thus hardly fail to dust their undersides with pollen.
Echium is proterandrous; when the flower opens the anthers are already ripe; the pistil, on the other hand, is still quite short and immature, scarcely reaching to the mouth of the tube. Gradually, however, it extends until it reaches 10 mm. beyond the tube, and divides at the end into two short branches, with terminal stigmas. In this species, therefore, cross-fertilisation is favoured; firstly, by the fact that the stamens ripen before the stigmas; and, secondly, by the relative position of the two, the stigmas, as we have seen in so many other cases, projecting somewhat beyond the stamens. Under these circumstances cross-fertilisation is so thoroughly secured, that the plant is said to have lost the power of fertilising itself. Muller observed no less than 67 species of insects on the flowers of this plant: some of which (Osmia adunca and O. caemen-tarici) seem to confine themselves to it.
Fig. 95. - Borago officinalis.
In the Borage (Borago officinalis, Fig. 95) we find an arrangement of the stamens and pistils very different to that in Echium, but, as Sprengel has pointed out, somewhat resembling that already described in the Violet. The flowers are drooping, of a beautiful blue, with a white central circle; dark stamens, combined into a tube, and a pink pistil. The pale yellow, fleshy ovary secretes honey, which lies in a short tube formed of the basis of the stamens. The anthers are long, and open gradually from the apex to the base, so that the pollen falls into the closed space between them and the pistil. This arrangement effectually protects both the pollen and the honey from all insects, excepting bees. The latter, however, force their proboscis down to the honey, between the anthers, which, however, return to their former position again, as soon as the proboscis is withdrawn. As soon as the anthers are separated, the pollen drops down on to the head of the bee, and is thus carried from one flower to another. Cross-fertilisation is also favoured by the flower being proterandrous, the stigma not becoming mature until the anthers have shed all their pollen. The Borate is much visited by bees, especially by the common hive bee.
Pnhuoiiaria officinalis (Fig. 96) is a dimorphous species; being rich in honey and much visited by insects, it has not only lost the power of self-fertilisation, but is said by Hildebrand [Bot. Zeit., 1865) to be sterile to pollen from the same form of flower; that is to say, long-styled flowers require to be fertilised by pollen from short-styled flowers, and vice versa. Darwin, however, succeeded in obtaining seeds and raising seedlings from some long-styled plants which were fertilised with pollen of the same form. (Jour. Linn. Soc, v. x. p. 430.) We have already seen that this is partially the case with other dimorphous species. The genus Myosotis (the Forget-me-not) has already been alluded to in the introductory chapter (ante, p. 35). The species, however, appear to differ among themselves in the relative positions of the stamens and pistil.
Fig. 96. - Pulmonaria officinalis.
In this beautiful and interesting family, though we have not above twenty British species, we find, as Muller has well pointed out, the widest differences in the conditions of fertilisation. Pidmonaria officinalis is dimorphous, and sterile - not only with its own pollen, but even in some cases with that of a different flower, unless it belongs to the different form. Echium vulgare has lost the power of self-fertilisation, but, so far at least as we know, is fertile with the pollen of any other flower belonging to the species. Other species are generally fertilised by insects, but in their absence perform this office for themselves; while lastly, some species, such as Lithospermum arvcnse, and Myosotis intermedia, habitually fertilise themselves. Again cross-fertilisation is secured in Pulmo-naria by dimorphism; in Echium and Borago by proterandrousness (if I may be permitted to coin the word): in Symphytum and Anchusa, by the projection of the stigma beyond the stamens; in Lithospermum and Myosotis, by the narrowness of the flower tube.