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.
Here it is at once obvious that insects alighting on the younger (male) flowers would dust themselves with pollen, some of which, if they subsequently alighted on an older flower, they could not fail to deposit on the stigma.1 In some cases flowers which are first male and then female, are male on the first day of opening, female on the second. In others the period is longer. Thus Nigella, according to Sprengel, is male for six days, after which the stigma comes to maturity-and lasts for three or four. ("Das endeckte Geheimniss der Natur," p. 287.)
Fig. 30. - Pink in the first (male) condition.
Fig. 31. - "Pink in the second condition, with mature stigmas.
Fig. 34 represents a flower of Myosotis versicolor (a species often known as the Forget-me-not) when just opened. It will be observed that the pistil projects above the corolla and stamens, so that it must be first touched by any insect alighting on the flower. Gradually, however, the corolla elongates, carrying up the stamens with it, until at length they come opposite the stigma, as shown in Fig. 35. Thus, if the flower has not already been fertilised by insects, it is almost sure to fertilise itself.
1 In the Thymes there are likewise some small flowers which contain no stamens.
Fig. 32. - Thymus serpyllum, in the first condition, with ripe stamens.
Fig. 33. - Thymus serpyllutn, in the second condition, with mature stigma,
Fig. 34. - Myosotis versicolor (young flower).
Fig. 35. - Myosotis versicolor (older flower).
I now pass to the third of the principal modes by which self-fertilisation is prevented. In the flowers hitherto described, while the several species offer the most diverse arrangements, we have met with no differences within the limits of the same species, excepting those dependent upon sex. But there are other species which possess flowers of two or more kinds, sometimes, as in the violet, adapted to different conditions, but more frequently so constituted as to ensure cross-fertilisation. In some of the violets ( V. odorata, canina, etc.), besides the blue flowers with which we are all so familiar, there are other, autumnal, flowers almost without petals and stamens; which indeed have scarcely the appearance of true flowers, but in which numerous seeds are produced. "Ckistogamous" flowers, as these have been called, occur also in Lamium amplexicaule (Figs. 36 and 37), Oxalis acetosella, Trifolium subterraneum, and other plants belonging to very different groups. They were, I believe, first observed by Dillenius in Ruellia ("Hortus Elthamensis," vol. ii. p. 239). As, however, they have no relation to our present subject, I shall not now dwell upon them.
Fig. 36. - Cleistogamous flower of Lamium amplexicaule.
Fig. 37. - Section of ditto.
I pass on, therefore, to the genus Primula. If a number of specimens of primroses or of cowslips be examined, we shall find that about half of them have the pistil at the top of the tube, and the stamens half-way down (as is shown in Fig. 38), while the other half have, on the contrary, the stamens at the top of the tube, and the pistil halfway down (as shown in Fig. 39). Corresponding differences occur in Polyanthus and Auricula, and have long been known to gardeners, and even to schoolchildren, by whom the two forms are distinguished as "thrum-eyed" and "pin-eyed." As already mentioned, plants which present these differences are known as Heteromorphous (in opposition to those which are Homomorphous, or have only one kind of flower), while those with two forms are called Dimorphous, those with three, Trimorphous.
Fig. 38. - Primula (long-styled form).
Fig. 39. - Primula (short-styled form).
Sprengel himself had noticed a case of Dimorphism in Hottonia, and shrewdly observed that there was probably some reason for it, but was unable to suggest any explanation.
In Lythrum the existence of different forms had been observed by Vaucher in 1841, and in the genus Oxalis by Jacquin, who regarded them as indicative of different species; but it was reserved for the genius and perseverance of Mr. Darwin to explain ("Jour. Linn. Soc." 1862, p. 77) the significance of this curious phenomenon, and the important part it plays in the economy of the flower. Now that Mr. Darwin has pointed this out, it is sufficiently obvious: An insect thrusting its proboscis down a primrose of the long-styled form (Fig. 38) would dust its proboscis at a part which, when it visited a short-styled flower (Fig. 39), would come just opposite the head of the pistil, and could not fail to deposit some of the pollen on the stigma. Conversely, an insect visiting a short-styled plant, would dust its proboscis at a part further from the tip; which, when the insect subsequently visited a long-styled flower, would again come just opposite to the head of the pistil. Hence we see that by this beautiful arrangement, insects must carry the pollen of the long-styled form to the short-styled, and vice versa.