CLIMBING PLANTS, CROSS AND SELF-FERTILIZATION
Lathyrus is found mainly in temperate regions of the world, cultivated widely for ornament, cover crops, fodder, and food. The best recognized members of the genus are sweet peas and everlasting peas, grown since the seventeenth century for their elegant, fragrant flowers in varied colors. Leaf tendrils are prominent in most species in the genus, but one, yellow pea (Lathyrus aphaca), has stipules that function as leaves while the leaves have developed into tendrils, and another, grass vetchling (L. nissolia), has grass-like leaves and no tendrils, prompting Darwin to suggest it may represent reversion to a primordial ancestor. Darwin grew both native and cultivated species, and his varied Lathyrus studies—rotating movement of seedlings, cleistogamous flowers, evolution and reversion of tendrils, cross- and self-pollination, and more—are described in no fewer than five of his books.
Nearly all the species of Lathyrus possesses tendrils; but L. nissolia is destitute of them. This plant has leaves, which must have struck every one with surprise who has noticed them, for they are quite unlike those of all common papilionaceous plants, and resemble those of a grass. In another species, L. aphaca, the tendril, which is not highly developed (for it is unbranched, and has no spontaneous revolving-power), replaces the leaves, the latter being replaced in function by large stipules. Now if we suppose the tendrils of L. aphaca to become flattened and foliaceous, like the little rudimentary tendrils of the bean, and the large stipules to become at the same time reduced in size, from not being any longer wanted, we should have the exact counterpart of L. nissolia, and its curious leaves are at once rendered intelligible to us.
It may be added, as serving to sum up the foregoing views on the origin of tendril-bearing plants, that L. nissolia is probably descended from a plant which was primordially a twiner; this then became a leaf-climber, the leaves being afterwards converted by degrees into tendrils, with the stipules greatly increased in size through the law of compensation. After a time, the tendrils lost their branches and became simple; they then lost their revolving-power (in which state they would have resembled the tendrils of the existing L. nissolia), and afterwards losing their prehensile power and becoming foliaceous would no longer be thus designated. In this last stage (that of the existing L. nissolia) the former tendrils would reassume their original function of leaves, and the stipules which were recently much developed being no longer wanted, would decrease in size. If species become modified in the course of ages, as almost all naturalists now admit, we may conclude that L. nissolia has passed through a series of changes, in some degree like those here indicated.
Darwin’s most sustained interest in the genus Lathyrus was the puzzling prevalence of self-fertilization in cultivated varieties, challenging his belief that species must at least occasionally be cross-fertilized (see Ipomoea). Insect visits to their attractive “papilionaceous” flowers, with their colorful paired wing-petal landing pads, rarely led to cross-pollination in Darwin’s environs. Darwin’s son Francis and his wife Amy reported nectar robbery by bees in L. japonicus in Wales.82 (As it happens, Darwin had collected this very species at Cape Tres Montes in Patagonia83—a cross-hemispheric distribution that demonstrates remarkable dispersal abilities.) But nectar robbery could hardly be responsible for the general failure of Lathyrus species to cross-pollinate, though when it occurred crossing would be reduced all the more.
In Darwin’s experimental plantings, he found that cultivated varieties of the sweet pea remained true even when grown side by side with other varieties. He raised several generations, producing both cross- and self-fertilized plants by hand-pollinating, and confirmed that products of crossing improved in height and vigor—proof that the usual benefits were there when they did cross. When Darwin noticed a leafcutter bee entering a flower in a different way from the bumblebees, triggering the mechanism that deposited pollen on the bee, he suspected that lack of crossing came down to absence of natural pollinators. Writing to Italian botanist Federico Delpino, a frequent correspondent, he wondered if different colored varieties of L. odoratus in Italy showed signs of hybridizing if left unprotected from visiting insects. Delpino confirmed that there in its native range, local growers had to plant L. odoratus varieties in isolation to prevent intercrossing.84 Its local pollinator, the leafcutter bee Megachile ericetorum, does not occur in Britain.
Lathyrus odoratus—Almost everyone who has studied the structure of papilionaceous flowers has been convinced that they are specially adapted for cross-fertilisation, although many of the species are likewise capable of self-fertilisation. The case therefore of Lathyrus odoratus or the sweet-pea is curious, for in this country it seems invariably to fertilise itself. I conclude that this is so, as five varieties, differing greatly in the colour of their flowers but in no other respect, are commonly sold and come true; yet on inquiry from two great raisers of seed for sale I find that they take no precautions to insure purity—the five varieties being habitually grown close together. I have myself purposely made similar trials with the same result. Although the varieties always come true, yet, as we shall presently see, one of the five well-known varieties occasionally gives birth to another which exhibits all its usual characters. …
In order to ascertain what would be the effect of crossing two varieties, some flowers on the Purple sweet-pea, which has a dark reddish-purple standard-petal with violet-coloured wing petals and keel, were castrated whilst very young and were fertilised with pollen of the Painted Lady. This latter variety has a pale cherry-coloured standard, with almost white wings and keel. On two occasions, I raised from a flower thus crossed plants perfectly resembling both parent-forms; but the greater number resembled the paternal variety. So perfect was the resemblance that I should have suspected some mistake in the label, had not the plants, which were at first identical in appearance with the father or Painted Lady, later in the season produced flowers blotched and streaked with dark purple. This is an interesting example of partial reversion in the same individual plant as it grows older. The purple-flowered plants were thrown away, as they might possibly have been the product of the accidental self-fertilisation of the mother-plant, owing to the castration not having been effectual. But the plants which resembled in the colour of their flowers the paternal variety or Painted Lady were preserved, and their seeds saved. Next summer many plants were raised from these seeds, and they generally resembled their grandfather the Painted Lady, but most of them had their wing-petals streaked and stained with dark pink; and a few had pale purple wings with the standard of a darker crimson than is natural to the Painted Lady, so that they formed a new sub-variety. Amongst these plants, a single one appeared having purple flowers like those of the grandmother, but with the petals slightly streaked with a paler tint: this was thrown away. Seeds were again saved from the foregoing plants and the seedlings thus raised still resembled the Painted Lady, or great-grandfather; but they now varied much, the standard petal varying from pale to dark red, in a few instances with blotches of white; and the wing-petals varied from nearly white to purple, the keel being in all nearly white.
As no variability of this kind can be detected in plants raised from seeds, the parents of which have grown during many successive generations in close proximity, we may infer that they cannot have intercrossed. What does occasionally occur is that in a row of plants raised from seeds of one variety, another variety true of its kind appears; for instance, in a long row of scarlets (the seeds of which had been carefully gathered from Scarlets for the sake of this experiment) two Purples and one Painted Lady appeared. Seeds from these three aberrant plants were saved and sown in separate beds. The seedlings from both the Purples were chiefly Purples, but with some Painted Ladies and some Scarlets. The seedlings from the aberrant Painted Lady were chiefly Painted Ladies with some Scarlets. Each variety, whatever its percentage may have been, retained all its characters perfect, and there was no streaking or blotching of the colours, as in the foregoing plants of crossed origin. …
From the evidence now given, we may conclude that the varieties of the sweet-pea rarely or never intercross in this country; and this is a highly remarkable fact, considering, firstly, the general structure of the flowers; secondly, the large quantity of pollen produced, far more than is requisite for self-fertilisation; and thirdly, the occasional visits of insects. That insects should sometimes fail to cross-fertilise the flowers is intelligible, for I have thrice seen humble-bees of two kinds, as well as hive-bees, sucking the nectar, and they did not depress the keel-petals so as to expose the anthers and stigma; they were therefore quite inefficient for fertilising the flowers. One of these bees, namely, Bombus lapidarius, stood on one side at the base of the standard and inserted its proboscis beneath the single separate stamen, as I afterwards ascertained by opening the flower and finding this stamen prised up. Bees are forced to act in this manner from the slit in the staminal tube being closely covered by the broad membranous margin of the single stamen, and from the tube not being perforated by nectar-passages. On the other hand, in the three British species of Lathyrus which I have examined, and in the allied genus Vicia, two nectar-passages are present. Therefore, British bees might well be puzzled how to act in the case of the sweet-pea. … One of my sons caught an elephant sphinx-moth whilst visiting the flowers of the sweet-pea, but this insect would not depress the wing-petals and keel. On the other hand, I have seen on one occasion hive-bees, and two or three occasions the Megachile willughbiella in the act of depressing the keel; and these bees had the under sides of their bodies thickly covered with pollen and could not thus fail to carry pollen from one flower to the stigma of another. Why then do not the varieties occasionally intercross, though this would not often happen, as insects so rarely act in an efficient manner? … Whatever the cause may be, we may conclude, that in England the varieties never or very rarely intercross. But it does not follow from this that they would not be crossed by the aid of other and larger insects in their native country, which in botanical works is said to be the south of Europe and the East Indies. Accordingly, I wrote to Professor Delpino, in Florence, and he informs me “that it is the fixed opinion of gardeners there that the varieties do intercross, and that they cannot be preserved pure unless they are sown separately.”
Linaria vulgaris. Hand-colored engraved plate from Flora Londinensis, published by William Curtis.