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.
According to D. Miiller("Bot.Zeit," 1857) the pollen of the small flowers of Viola elatoir and V. lancifolia is minute and round. Herr von Mohl, however, found no difference between the pollen of the large and small flowers in V. mirabilis ("Bot. Zeit.," 1863). The number of grains in these flowers is very small. So also in the cleistogamous flowers of Oxalis acetosella, there are not above two dozen pollen-grains in each of the five larger anthers, and one dozen in each of the five smaller ones. The ovules are about twenty in number.
It is interesting to notice that the contrivances by which cross-fertilisation is favoured, or ensured, are probably of a very different geological antiquity. Thus as Muller has pointed out, the special peculiarities of the Umbelliferse and Compositae have been inherited respectively from the ancestral forms of those orders; those of Delphinium, Aquilegia, Linaria, and Pedicu-laris, from the ancestral forms of the respective genera; those of Polygonum Fagopyrum, P. Bistorta, Lonicera Caprifolium, etc, from the ancestors of those species; while in Lysimachia vulgaris, Rhinanthus Cristagalli, Veronica spicata, Euphrasia Odontites, and E. officinalis, we find that differences have arisen even within the limits of one and the same species.
In some species, for instance, we find two varieties, one with larger flowers, which are fertilised by insects, and others with smaller flowers, which are self-fertile.
In other cases the differences between the two kinds of flowers are so marked, and have become so fixed, that the two kinds are usually considered to be distinct species; while in yet other cases the differentiation is still more complete.
Fig. 43. - Malva sylvestris.
Fig. 44. - ` rotundifolia
Fig. 45. - Stamens and stigmas of Malva sylvestris.
Fig. 46. - Ditto of Malva rotundifolia.
Among other obvious evidences that the beauty of flowers is useful to them, in consequence of its attracting insects, we may adduce those cases in which the transference of the pollen is effected in different manners in nearly allied plants, sometimes even in different species belonging to the same genus.
Thus, as Muller has pointed out, Malva sylvestris (Fig. 43) and Malva rotundifolia (Fig. 44), which grow in the same localities, and therefore must come into competition, are nevertheless nearly equally common. In both species the young flower contains a pyramidal group of stamens which surround the stigma, and produce a large quantity of pollen, which cannot fail to dust any insect visiting the flower for the sake of its honey. In Malva sylvestris (Fig. 43), where the branches of the stigma are so arranged (Fig. 45), that the plant cannot fertilise itself, the petals are large and conspicuous, so that the plant is visited by numerous insects; while in Malva rotundifolia (Fig. 44), the flowers of which are comparatively small and rarely visited by insects, the branches of the stigma are elongated and twine themselves (Fig. 46) among the stamens, so that the flower can hardly fail to fertilise itself.
Another remarkable instance occurs in the genus Epilobium, which is, moreover, specially interesting, because in E. angustifolium, as I have already mentioned, the curious fact was first noticed that the pistil did not mature until the stamens had shed their pollen. E. angiistifolium (Fig. 47) has conspicuous purplish-red flowers, in long terminal bunches or racemes, and is much frequented by insects; E. parviflorum (Fig. 48), on the contrary, has small solitary flowers, and is seldom visited by insects. Now, to the former species the visits of insects are' necessary, since the stamens ripen before the pistil, and the flower has consequently lost the power of self-fertilisation. In the latter, on the contrary, the stamens and pistil come to maturity at the same time, and the flower can therefore fertilise itself. It is, however, no doubt sometimes crossed by the agency of insects; and indeed I am disposed to believe that this is true of all the flowers which are either coloured or sweet scented.
Fig. 47. - Epilobium angustifolium.
Fig. 48. - Epilobium parviflorum.
The genus Geranium also affords us an instructive example. There are a number of species which, as will be seen in Fig. 49, differ much in the size of the flowers. Thus those (Fig. 49 a) of Geranium pratense (Fig. 40) are nearly twice as large as those of G.pyre-naicum (Fig. 49 b), which again are much larger than those of G. molle (Fig. 49 c), while those of G.pusilhun (Fig. 49 d) are still smaller. These differences of size appear to be connected with other remarkable differences between these species. Fig. 41, as already mentioned, represents a flower of G. pratence when first opened. Five of the stamens have raised themselves and stand upright, and surround the still immature pistil. When they have shed their pollen they sink back and shrivel up, when the other five raise themselves. At a later stage these in their turn fall back and shrivel up, but the stigma does not become mature (Fig. 42), until all the stamens have shed their pollen. Under these circumstances G. pratense has lost the power of self-fertilisation, and is absolutely dependent on the visits of insects.
G. pyrenaicum (Fig. 49 b) is also pro-terandrous; but while in G. pratense the pistil is not mature until the stamens have shed all their pollen and fallen back, in G. pyrenaicum the second series of stamens are still upright when the stigmatic lobes unfurl; the flower is consequently less absolutely dependent on insects, and we see that the corolla is much smaller.
In the third species, G. molle (Fig. 49 c), the pistil matures before the second series of stamens, and the corolla is still smaller; while in G.pusillum (Fig. 49 d) the pistil matures before any of the stamens. Thus then these four species may be arranged in a table as below: -
Fig. 49. - Corolla of - a, Geraniun pratense; b, G. pyremiicum; c, G. molle; d, G. pu-sillum.
First exclusively male, then exclusively female.
First exclusively male, then hermaphrodite.
First exclusively male, then hermaphrodite.
First exclusively female, soon becoming hermaphrodite.
Generally fertilised by insects.
Incapable of self-fertilization.
Indeed, though further observations on the point are no doubt required, it would seem that, as a general rule, where we find within the limits of one genus some species which are much more conspicuous than others, we may suspect that they are also more dependent on the visits of insects.
Sprengel also suggests, and, as it would appear, with reason, that the lines and bands by which so many flowers are ornamented have reference to the position of the honey;1 and it may be observed that these honey-guides are absent in night-flowers, where of course they would not be visible, and would therefore be useless, as, for instance, in Lychnis vespertina (Fig. 50), or Silene nutans. Night-flowers, moreover, are generally pale; for instance, Lychnis vespertina is white, while Lychnis diurna, which flowers by day, is red. Brown flowers, such as Scrophularia, some species of Epipactis, of Lonicera, etc, perhaps owe their hue to the selective influence of wasps. Fly flowers also are often livid or flesh-coloured.
1 I did not realise the importance of these guiding marks until, by experiments on bees, I saw how much time they lose if honey, which is put out for them, is moved even slightly from its usual place.
I have been good-humouredly accused of attacking the Bee, because I have ventured to suggest that she does not possess all the high qualities which have been popularly and poetically ascribed to her. But if scientific observations do not altogether support the moral and intellectual eminence which has been ascribed to Bees, they have made known to us in the economy of the hive many curious peculiarities which no poet had dreamt of, and have shown that bees and other insects have an importance as regards flowers which had been previously unsuspected. To them we owe the beauty of our gardens, the sweetness of our fields. To them flowers are indebted for their scent and colour; nay, for their very existence in its present form. Not only have the present shape and outlines, the brilliant colours the sweet scent and the honey of flowers, been gradually developed through the unconscious selection exercised by insects; but the very arrangement of the colours, the circular bands and radiating lines, the form, size, and position of the petals, the relative situations of the stamens and pistil, are all arranged with reference to the visits of insects, and in such a manner as to ensure the grand object which these visits are destined to effect.
Fig. 50. -Lychnis vespertina.