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 contains two British genera, Lythrum and Peplis, the former of which is of peculiar interest and has been already alluded to in the opening chapter (ante p. 40).
Lythrum salicaria (Fig. 77), presents us with three distinct forms of flower, which were already recorded by Vaucher, while their functions and relations were first explained by Mr. Darwin. He distinguished them according to the length of their styles, as the Long-styled (Fig. 78), Mid-styled (Fig. 79), and Short-styled (Fig. 80). In this species it is remarkable that the seeds of the three forms differ from one another; 100 of the long-styled seeds being equal to 121 mid-styled, or 142 short-styled. The pollen grains, also, not only differ in size, the long stamens having the largest pollen grains, the middle-sized stamens middle-sized pollen grains, and the short stamens small pollen grains; but also in colour, being green in the longer stamens, and yellow in the shorter ones; while the filaments are pink in the long stamens, uncoloured in the shorter ones.
Mr. Darwin has also proved by experiment that this species does not set its seeds, if the visits of insects are prevented; in a state of nature, however, the plant is much frequented by bees, humble bees, and flies; which always alight on the upper side of the flowers on the stamens and pistil. Mr. Darwin has shown that perfect fertility can only be obtained by fertilising each form with pollen from pistils of the corresponding length.
Fig. 77. - Lythrum salacaria.
Thus the long-styled form is naturally fertilised by pollen from the long stamens of the two other forms; but it can be so, though imperfectly, by its own two sets of stamens, and by the shorter stamens of the two other forms; it can, therefore, be fertilised, to use Mr. Darwin's expression, "legitimately" in two ways, and "illegitimately" in four ways. The same is the case with the other two forms, so that eighteen modes of union are possible, of which six are natural or "legitimate," twelve are illegitimate, and more or less sterile. This case is therefore indeed most complex.
Fig. 78. - Long-styled form of Lythrum salicaria.
Fig. 79 - Mid-styled ditto.
Fig. 80. - Short-styled ditto
Mr. Darwin suggests (Jour. Linn. Soc. v. viii. 1864, p. 193) that the trimorphous condition of this plant may be advantageous, because if it were dimorphous only there would be but an equal chance in favour of any two plants being of different forms, and therefore capable of self-fertilisation; whereas, being trimorphous, the chances are two to one. In the cowslip and primrose, where large numbers of plants grow together, this, he thinks, would not be so material. However this may be, the stigma and the two groups of stamens appear to correspond with the three divisions of the body (viz. the head, thorax, and abdomen) of the bee, Cilissa melanura, by which it is almost exclusively fertilised.
The genus Lythrum is also remarkable for the great differences existing between different species. For instance, L. graefferi, like L. salicaria, is trimorphous; while L. thymifolia is dimorphous, and L. hyssophifolia is homomorphous.