FORMS OF FLOWERS, POLLINATION
Pansy, Johnny-jump-up and heartsease are common names for Viola varieties widely grown in gardens. In Variation, Darwin noted the history of Viola cultivation dating as far back as 1687, with many new varieties “energetically commenced” by nurserymen beginning in 1811. Viola tricolor, heartsease, was a Victorian-era staple of the flower garden, and the species from which most garden pansies were developed. It is therefore unsurprising, as a plant well familiar to his audience, that Darwin cited heartsease as an example of the tell-tale signs of artificial selection in Origin. The myriad cultivated heartsease varieties differ most in their flowers, the feature valued by people, he pointed out, but not much in their leaves:
There is another means of observing the accumulated effects of selection—namely, by comparing the diversity of flowers in the different varieties of the same species in the flower-garden; the diversity of leaves, pods, or tubers, or whatever part is valued, in the kitchen-garden … See how different the leaves of the cabbage are, and how extremely alike the flowers; how unlike the flowers of the heartsease are, and how alike the leaves.153
He delved in more deeply in Variation, tracing the steps by which garden-variety heartsease were developed: “The first great change was the conversion of the dark lines in the centre of the flower into a dark eye or centre, which at that period had never been seen, but is now considered one of the chief requisites of a first-rate flower.”154 Darwin went on to acknowledge the confusion of determining the parentage of cultivated varieties, with several descended from wild species, and more or less intercrossed to boot.
Darwin experimented with cross- and self-pollination in violets for several years beginning in the early 1860s, and he appreciated that the small, self-fertile, “cleistogamic” flowers found in many violet species assured abundant production of seeds. Viola, it happens, has perhaps the largest number of cleistogamous species of any flowering plant genus. Darwin dissected several of them, writing to Joseph Hooker in 1862, “I have been amusing myself by looking at the small flowers of Viola … What queer little flowers they are.”155
It was known even before the time of Linnaeus that certain plants produced two kinds of flowers, ordinary open, and minute closed ones; and this fact formerly gave rise to warm controversies about the sexuality of plants. These closed flowers have been appropriately named cleistogamic by Dr. Kuhn. They are remarkable from their small size and from never opening, so that they resemble buds; their petals are rudimentary or quite aborted; their stamens are often reduced in number, with the anthers of very small size, containing few pollen-grains, which have remarkably thin transparent coats, and generally emit their tubes whilst still enclosed within the anther cells; and, lastly, the pistil is much reduced in size, with the stigma in some cases hardly at all developed. These flowers do not secrete nectar or emit any odour; from their small size, as well as from the corolla being rudimentary, they are singularly inconspicuous. Consequently, insects do not visit them; nor if they did, could they find an entrance. Such flowers are therefore invariably self-fertilised; yet they produce an abundance of seed. In several cases, the young capsules bury themselves beneath the ground, and the seeds are there matured. These flowers are developed before, or after, or simultaneously with the perfect ones. Their development seems to be largely governed by the conditions to which the plants are exposed, for during certain seasons or in certain localities only cleistogamic or only perfect flowers are produced. …
Viola canina.—The calyx of the cleistogamic flowers differs in no respect from that of the perfect ones. The petals are reduced to five minute scales; the lower one, which represents the lower lip, is considerably larger than the others, but with no trace of the spur-like nectary; its margins are smooth, whilst those of the other four scale-like petals are papillose. … The stamens are very small, and only the two lower ones are provided with anthers, which do not cohere together as in the perfect flowers. The anthers are minute, with the two cells or loculi remarkably distinct; they contain very little pollen in comparison with those of the perfect flowers. The connective expands into a membranous hood-like shield which projects above the anther-cells. These two lower stamens have no vestige of the curious appendages which secrete nectar in the perfect flowers. The three other stamens are destitute of anthers and have broader filaments, with their terminal membranous expansions flatter or not so hood-like as those of the two antheriferous stamens. … In the cleistogamic flowers, the pollen-grains, as far as I could see, never naturally fall out of the anther-cells, but emit their tubes through a pore at the upper end. I was able to trace the tubes from the grains some way down the stigma. The pistil is very short, with the style hooked, so that its extremity, which is a little enlarged or funnel-shaped and represents the stigma, is directed downwards, being covered by the two membranous expansions of the antheriferous stamens. It is remarkable that there is an open passage from the enlarged funnel-shaped extremity to within the ovarium; this was evident, as slight pressure caused a bubble of air, which had been drawn in by some accident, to travel freely from one end to the other: a similar passage was observed by Michalet in V. alba. The pistil therefore differs considerably from that of the perfect flower; for in the latter it is much longer, and straight with the exception of the rectangularly bent stigma; nor is it perforated by an open passage. …
The seeds produced by the cleistogamic and perfect flowers do not differ in appearance or number. On two occasions, I fertilised several perfect flowers with pollen from other individuals and afterwards marked some cleistogamic flowers on the same plants; and the result was that 14 capsules produced by the perfect flowers contained on an average 9.85 seeds; and 17 capsules from the cleistogamic ones contained 9.64 seeds,—an amount of difference of no significance. It is remarkable how much more quickly the capsules from the cleistogamic flowers are developed than those from the perfect ones.
Vitis vinifera. Watercolor painting by Baldassare Cattrani, in Exoticarum atque indigenarum plantarum.