PLANT MOVEMENT
Lupines are one of a veritable pack of plants whose names make reference to wolves—from the Latin word for wolf, lupus. In this case, the name may reference the palmately divided leaves, evoking a wolf’s paw, or the ancient (and misguided) belief that these plants are ravenous as wolves in depleting soils of nutrients. Many are found in disturbed habitat with infertile soils, but their effect is quite the opposite of depletion—these nitrogen-fixing legumes help restore barren soils.
The genus is expansive—more than 200 species are found from Europe to north Africa and Eurasia and throughout the Americas (the center of lupine diversity). While there are a few woody lupines, most are herbaceous annuals and perennials of open fields and meadows. Many species are toxic, but others have been used in many ways by people; some have been grown since antiquity for their edible seeds, others are planted as livestock fodder or cover crops. A multitude of ornamental varieties have been produced, prized for their large spikes of brightly colored flowers. In Darwin’s time, lupines were a mainstay of the English garden, and it was in the gardens of his childhood home in Shrewsbury and nearby Maer, home of the Wedgwoods, his wife Emma’s family, that he began observing pollination of lupines and other flowers in the summers of 1840 and 1841, flush with the excitement of his recent conversion to the heretical idea of species change (known as transmutation in his day and evolution today).90
Darwin came to learn that lupine stamens are distinctive, with five large colorful sagittate anthers and five smaller and differently colored ones, a curious dimorphism that quickly caught his imagination. He had first read about this in an 1841 treatise by Swiss pastor and botanist Jean Pierre Étienne Vaucher and wondered if it was another adaptation to promote outcrossing, like heterostyly. How widespread was this phenomenon? “Can you think of plants which have differently coloured anthers or pollen in same flowers,” he asked Joseph Hooker in August 1862. “It would be a safe guide to dimorphism.—Do just think of this.”91 Hooker was able to provide him with a list of cases. Darwin threw himself into crossing experiments, including among his many experimental plants two lupine species: common yellow and blue lupines, Lupinus luteus and L. pilosus. As described in Cross and Self Fertilisation, his lupines developed fruits and seeds freely whether crossed or self-pollinated, but cross-pollination trials led to a much more vigorous second generation.
Darwin returned to lupines in his later studies of movement in plants. He found that the leaves of some species trace extraordinary ellipses (circumnutation) in the course of the day but do not “sleep” (nyctitropism) at night. Others do sleep in various ways, moving up or down, or rotating from a horizontal to a vertical attitude, their many-pointed leaves aptly likened by him to so many vertically hung “stars” at night. Ever attuned to evolutionary transitions, Darwin recognized that yellow lupine was a one-plant showcase for diversified nyctitropic behavior. He wrote, “Four leaves on the same plant, which had their leaflets horizontal at noon, formed vertical stars at night; and three other leaves equally horizontal at noon, had all their leaflets sloping downwards at night. So that the leaves on this one plant assumed at night three different positions. Though we cannot account for this fact, we can see that such a stock might readily give birth to species having widely different nyctitropic habits.”92
Lupinus —The palmate or digitate leaves of the species in this large genus sleep in three different manners. One of the simplest is that all the leaflets become steeply inclined downwards at night, having been during the day extended horizontally. This is shown in the accompanying figures, of a leaf of L. pilosus, as seen during the day from vertically above, and of another leaf asleep with the leaflets inclined downwards. As in this position they are crowded together, and as they do not become folded like those in the genus Oxalis, they cannot occupy a vertically dependent position; but they are often inclined at an angle of 50° beneath the horizon. In this species, whilst the leaflets are sinking, the petioles rise up, in two instances when the angles were measured to the extent of 23°. The leaflets of L. sub-carnosus and arboreus, which were horizontal during the day, sank down at night in nearly the same manner; the former to an angle of 38° and the latter of 36° beneath the horizon; but their petioles did not move in any plainly perceptible degree. It is, however, quite possible, as we shall presently see, that if a large number of plants of the three foregoing and of the following species were to be observed at all seasons, some of the leaves would be found to sleep in a different manner.
Lupinus pilosus: A, leaf seen from vertically above in daytime; B, leaf asleep, seen laterally at night.
In the two following species, the leaflets, instead of moving downwards, rise at night. With L. hartwegii, some stood at noon at a mean angle of 36° above the horizon, and at night at 51°, thus forming together a hollow cone with moderately steep sides. The petiole of one leaf rose 14° and of a second 11° at night. With L. luteus, a leaflet rose from 47° at noon to 65° above the horizon at night, and another on a distinct leaf rose from 45° to 69°. The petioles, however, sink at night to a small extent, viz., in three instances by 2°, 6°, and 9° 30 seconds. Owing to this movement of the petioles, the outer and longer leaflets have to bend up a little more than the shorter and inner ones, in order that all should stand symmetrically at night. …
Lupinus pubescens: A, leaf viewed laterally during the day; B, same leaf at night; C, another leaf with the leaflet forming a vertical star at night.
We now come to a remarkable position of the leaves when asleep, which is common to several species of Lupines. On the same leaf, the shorter leaflets, which generally face the centre of the plant, sink at night, whilst the longer ones on the opposite side rise; the intermediate and lateral ones merely twisting on their own axes. But there is some variability with respect to which leaflets rise or fall. As might have been expected from such diverse and complicated movements, the base of each leaflet is developed (at least in the case of L. luteus) into a pulvinus. The result is that all the leaflets on the same leaf stand at night more or less highly inclined, or even quite vertically, forming in this latter case a vertical star. This occurs with the leaves of a species purchased under the name of L. pubescens; and in the accompanying figures we see at A the leaves in their diurnal position; and at B the same plant at night with the two upper leaves having their leaflets almost vertical. At C another leaf, viewed laterally, is shown with the leaflets quite vertical. It is chiefly or exclusively the youngest leaves which form at night vertical stars. But there is much variability in the position of the leaves at night on the same plant; some remaining with their leaflets almost horizontal, others forming more or less highly inclined or vertical stars, and some with all their leaflets sloping downwards, as in our first class of cases. It is also a remarkable fact, that although all the plants produced from the same lot of seeds were identical in appearance, yet some individuals at night had the leaflets of all their leaves arranged so as to form more or less highly inclined stars; others had them all sloping downwards and never forming a star; and others, again, retained them either in a horizontal position or raised them a little.
Maurandya scandens. Hand-colored engraving, drawn by Sydenham Edwards, from The Botanical Magazine, 13: 460.