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 limited in Europe to the single genus Viola, of which we have, according to Bentham, five English species. Besides the showy, coloured flowers with which we are all familiar, most of the species possess minute flowers, which, however, produce abundance of seed. These appear later in the year, and are not only much smaller than the others, but almost without petals. In fact, according to Bentham, the Pansy ( V. tricolor) is the only one of our English species in which the showy flowers generally produce seed. The presence of these two totally different kinds of flowers is a very interesting fact; and as the smaller, or as they are called, "cleisto-gamous" flowers are sufficient to reproduce the species, and of course have the advantage of requiring much less expenditure of material, the persistence of the showy ones can only, I think, be accounted for by the fact that the ordinary flowers are useful in securing an occasional cross, as the cleistogamous flowers habitually fertilise themselves.
Viola canina. The structure of the coloured flowers is very curious, and has been well described by Sprengel. The petals are five in number, and irregular in form; the median one being produced into a hollow spur (Fig. 57 f), the entrance to which is protected partly by the stigma, partly by two tufts of hairs, or rather of delicate lobular processes, situated on the two median petals. The stamens consist of a short filament, to which the anther is attached, and a terminal membranous expansion, while the two lower stamens also send out each a long spur (Fig. 58;n), which lies within the spur of the median petal, and secretes honey at its fleshy end. The terminal membranous expansions of the five stamens slightly overlap one another, and their points touch the pistil, so that they enclose a hollow space. The pollen differs from that of most insect-fertilised flowers, in being drier, and more easily detached from the anthers; consequently, when the latter open, the pollen drops out; and as the flower is reversed and hangs down, the pollen falls into the closed space between the pistil and the membranous terminations of the stamens. The pistil is peculiar, the base of the style not being straight as usual, but thin and bent (Fig. 57). The stigma st is the enlarged end of the pistil; and shows several small fleshy projections. It will be obvious from the above description that when a bee visits the flower, her head will come in contact with and shake the stigma, thus opening, as it were, the box containing the pollen, and allowing it to fall on the head of the bee. It is thus carried away, and some can hardly fail to be deposited on the stigma of the next violet which the bee visits.
Fig. 57. - Section of a flower of Violet (Viola canina). Fig. 58. - Stamen of ditto.
Sprengel, in his description of V. odorata, gives the following list of questions and answers as regards this species; passing over, however, the more general points, such as the secretion of honey, the colour of the corolla, the radiating lines on the petals, and the smell,
1. Why is the flower situated on a long stalk, which is upright, but curved downwards at the free end? - In order that it may hang down; which, firstly, prevents rain from obtaining access to the honey; and, secondly, places the stamens in such a position that the pollen falls into the open space between the pistil and the free ends of the stamens. If the flower were upright, the pollen would fall into the space between the base of the stamen and the base of the pistil, and would not come in contact with the bee.
2. Why does the pollen differ from that of most other insect-fertilised flowers? - In most of such flowers the insects themselves remove the pollen from the anthers; and it is therefore important that the pollen should not easily be detached and carried away by the wind. In the present case, on the contrary, it is desirable that it should be looser and drier, so that it may easily fall into the space between the stamens and the pistil. If it remained attached to the anther, it would not be touched by the bee, and the flower would remain unfertilised.
3. Why is the base of the style so thin? - In order that the bee may be more easily able to bend the style.
4. Why is the base of the style bent? - For the same reason. The result of the curvature is that the pistil is much more easily bent than would be the case if the style were straight.
5. Finally, why does the membranous termination of the upper filament overlap the corresponding portions of the two middle stamens? - Because this enables the bee to move the pistil, and thereby to set free the pollen more easily than would be the case under the reverse arrangement.
In Viola tricolor, the form of the stigma is very different from that of V. canina, but the reason of the difference has not been satisfactorily explained. Mr. Bennett considers that this species is fertilised by Thrips. Mr. Darwin, however, has satisfied himself that when bees are excluded, it is comparatively infertile, and he has favoured me with the following memorandum on the subject.
"When," he says, "I formerly covered up a fine, large, cultivated variety, it set only 18 capsules, and most of them contained very few good seeds, several from only 1 to 3; whereas an equally fine uncovered plant, growing close by, produced 105 fine capsules.
The few capsules which are produced when insects are excluded are probably due to the curling up of the petals (as Fermond and F. Muller remark) as they wither, by which process pollen-grains adhering to the papillae may be inserted into the cavity of the stigma. The moth Plusia is said to visit the flowers largely. Humble-bees are common agents in fertilising these flowers; but I have seen more than once a fly (Rhyngia rostratd) with the under side of its body, head, and legs dusted with the pollen of this plant, and having marked the flowers which they had visited, found, after a few days, that they had all been fertilised.
" It is curious in this case, as in many others, how long the flowers may be watched without seeing one visited by an insect. During one summer, I repeatedly watched some large clumps of heartsease, many times daily, for a fortnight, before I saw a humble-bee at work. During another summer I did the same, and then one day, as well as on two succeeding days, I saw a dark-coloured humble-bee visiting almost every flower in several clumps; and after a few days almost all the flowers suddenly withered, and produced fine capsules. A certain state of the atmosphere seems to be necessary for the secretion of nectar, and as soon as this occurs, it is perceived by various insects, I presume by the odour emitted by the flowers, and these are immediately visited."