VARIATION, CLIMBING PLANTS
The genus Solanum has a worldwide distribution, with the greatest species diversity found in tropical South America. Linnaeus named the type species S. nigrum in 1753, and today more than 1400 species are recognized, some growing as trees and shrubs while others are herbs or vines. Solanum and other members of the family are of great economic importance, a group that includes such crops as potatoes, tomatoes, tobacco, and eggplants, as well as many cultivated ornamentals and medicinal plants.
In his early evolutionary speculations, domestication gave Darwin insight into the formation of varieties and species, and Solanum was a prime example. In January 1835, traveling aboard the HMS Beagle in the Chonos Archipelago off the Chilean coast, he came upon an unpromising species of wild potato growing abundantly on Guayteca Island. “The tallest plant was four feet in height,” he later wrote in his Journal of Researches. “The tubers were generally small, but I found one of an oval shape, two inches in diameter: they resembled in every respect, and had the same smell as English potatoes; but when boiled they shrunk much and were watery and insipid.”133 Darwin’s potato is now known as Solanum ochoanum, a name honoring the distinguished Peruvian potato expert Carlos Ochoa, who rediscovered the plant years later, growing right where Darwin found it.134 Darwin recalled this wild species when describing the remarkable diversity of domestic potato varieties in Variation.135
He went on to grow many varieties in his garden—a double benefit as subjects for scientific study and food on the table. (Potato Rissoles were a favorite.136) Comparing the varieties, Darwin noted the traits that varied and those that did not. The stems, leaves, and for the most part the flowers were all far more similar among varieties than were the tubers—unsurprisingly, Darwin argued, since it’s the tubers that are under selection by people, not the leaves or flowers.
Potato (Solanum tuberosum).—There is little doubt about the parentage of this plant; for the cultivated varieties differ extremely little in general appearance from the wild species, which can be recognised in its native land at the first glance. The varieties cultivated in Britain are numerous; thus Lawson gives a description of 175 kinds. I planted eighteen kinds in adjoining rows; their stems and leaves differed but little, and in several cases there was as great a difference between the individuals of the same variety as between the different varieties. The flower varied in size, and in colour between white and purple, but in no other respect, except that in one kind the sepals were somewhat elongated. One strange variety has been described which always produces two sorts of flowers, the first double and sterile, the second single and fertile. The fruit or berries also differ, but only in a slight degree. The varieties are liable in very different degree to the attack of the Colorado potato-beetle.
The tubers, on the other hand, present a wonderful amount of diversity. This fact accords with the principle that the valuable and selected parts of all cultivated productions present the greatest amount of modification. They differ much in size and shape, being globular, oval, flattened, kidney-like, or cylindrical. One variety from Peru is described as being quite straight, and at least six inches in length, though no thicker than a man’s finger. The eyes or buds differ in form, position, and colour. The manner in which the tubers are arranged on the so-called roots or rhizomes is different; thus, in the gurken-kartoffeln they form a pyramid with the apex downwards, and in another variety they bury themselves deep in the ground. The roots themselves run either near the surface or deep in the ground. The tubers also differ in smoothness and colour, being externally white, red, purple, or almost black, and internally white, yellow, or almost black. They differ in flavour and quality, being either waxy or mealy; in their period of maturity, and in their capacity for long preservation.
As with many other plants which have been long propagated by bulbs, tubers, cuttings, &c., by which means the same individual is exposed during a length of time to diversified conditions, seedling potatoes generally display innumerable slight differences. Several varieties, even when propagated by tubers, are far from constant … Dr. Anderson procured seed from an Irish purple potato, which grew far from any other kind, so that it could not, at least in this generation, have been crossed, yet the many seedlings varied in almost every possible respect, so that “scarcely two plants were exactly alike.” Some of the plants which closely resembled each other above ground produced extremely dissimilar tubers; and some tubers which externally could hardly be distinguished, differed widely in quality when cooked.
Darwin ran several other garden and greenhouse studies of temperate and tropical Solanum species, ranging from pollination and grafting to cotyledon movement and climbing habits. In the latter case, he was especially struck by Solanum jasminoides (now S. laxum), a cultivated species from South America with a strong leaf-climbing tendency. The petioles of this scrambling climber act a bit like tendrils, wrapping themselves about supports, becoming thicker and more robust in the process. Darwin studied cross-sections of these hefty petioles, marveling that they can become thicker than even the main stem itself—a plant “singular” in morphology and physiology.
Solanum jasminoides.— Some of the species in this large genus are twiners; but the present species is a true leaf-climber. A long, nearly upright shoot made four revolutions, moving against the sun, very regularly at an average rate of 3 hrs. 26 m. The shoots, however, sometimes stood still. It is considered a greenhouse plant; but when kept there, the petioles took several days to clasp a stick: in the hothouse, a stick was clasped in 7 hrs. In the greenhouse, a petiole was not affected by a loop of string, suspended during several days and weighing 2½ grains; but in the hothouse, one was made to curve by a loop weighing 1.64 gr.; and, on the removal of the string, it became straight again.
The flexible petiole of a half or a quarter grown leaf which has clasped an object for three or four days increases much in thickness, and after several weeks becomes so wonderfully hard and rigid that it can hardly be removed from its support. On comparing a thin transverse slice of such a petiole with one from an older leaf growing close beneath, which had not clasped anything, its diameter was found to be fully doubled, and its structure greatly changed. In two other petioles similarly compared, and here represented, the increase in diameter was not quite so great. In the section of the petiole in its ordinary state (A), we see a semilunar band of cellular tissue (not well shown in the woodcut) differing slightly in appearance from that outside it and including three closely approximate groups of dark vessels. Near the upper surface of the petiole, beneath two exterior ridges, there are two other small circular groups of vessels. In the section of the petiole (B) which had clasped during several weeks a stick, the two exterior ridges have become much less prominent, and the two groups of woody vessels beneath them much increased in diameter. The semilunar band has been converted into a complete ring of very hard, white, woody tissue, with lines radiating from the centre. … This petiole after clasping the stick had actually become thicker than the stem from which it arose; and this was chiefly due to the increased thickness of the ring of wood. … It is a singular morphological fact that the petiole should thus acquire a structure almost identically the same with that of the axis; and it is a still more singular physiological fact that so great a change should have been induced by the mere act of clasping a support.
Solanum jasminoides, with one of its petioles clasping a stick.
Solanum jasminoides. A. Section of a petiole in its ordinary state. B. Section of a petiole some weeks after it had clasped a stick.
Spiranthes cernua. Hand-colored engraving drawn by Walter Hood Fitch, from The Botanical Magazine 87: 5277.