VARIATION, CROSS AND SELF-FERTILIZATION
One indication that a plant has been associated with people for a long time is that its genus is the name for that plant in some ancient language—so it is with pisum, the Latin word for the garden pea. Pea plants, grown for their exceptionally nutritious pods and seeds, are herbaceous annuals with compound leaves ending in tendrils that make them good climbers. This staple food source, in particular Pisum sativum, is thought to have originated in the circum-Mediterranean region and Middle East, where dried peas have been consumed for some 7000 years. Of the many varieties available in Victorian England, Darwin grew forty-one in his garden at Down—to observe and experiment with, and, of course, to eat; Emma Darwin’s recipe book includes a hearty “pea soop.”123
Her husband’s studies with peas covered many subjects, published in several books and articles over the years, beginning with his earliest investigations into the question of transmutation, or species change, in the late 1830s. With the help of his father’s gardener, John Abberley, Darwin featured Pisum in some of his very first experiments, which tested whether certain long-established plant varieties would breed true and resist crossing with related varieties. This would mean such varieties bore the hallmark of separate species, consistent with Darwin’s growing conviction that varieties were but incipient species.1124 In one test, Abberley planted rows of different pea varieties then planted out the resulting seeds of each. In another, Darwin asked him to plant individual pea plants mixed in with beans. The results in both cases supported the concept that the pea varieties bred true.2125
Pea (Pisum sativum).— … The varieties of the common garden-pea are numerous and differ considerably from one another. For comparison I planted at the same time forty-one, English and French varieties. They differed greatly in height,—namely from between 6 and 12 inches to 8 feet, —in manner of growth, and in period of maturity. Some differ in general aspect even while only two or three inches in height. The stems of the Prussian pea are much branched. The tall kinds have larger leaves than the dwarf kinds, but not in strict proportion to their height:—Hair’s Dwarf Monmouth has very large leaves, and the Pois nain hatif, and the moderately tall Blue Prussian, have leaves about two-thirds of the size of the tallest kind. In the Danecroft the leaflets are rather small and a little pointed; in the Queen of Dwarfs rather rounded; and in the Queen of England broad and large. In these three peas the slight differences in the shape of the leaves are accompanied by slight differences in colour, in the Pois géant sans parchemin, which bears purple flowers, the leaflets in the young plant are edged with red; and in all the peas with purple flowers the stipules are marked with red.
In the different varieties, one, two, or several flowers in a small cluster, are borne on the same peduncle; and this is a difference which is considered of specific value in some of the Leguminosae. In all the varieties, the flowers closely resemble each other except in colour and size. They are generally white, sometimes purple, but the colour is inconstant even in the same variety. …
The pods and seeds, which with natural species afford such constant characters, differ greatly in the cultivated varieties of the pea; and these are the valuable, and consequently the selected parts. Sugar peas, or Pois sans parchemin, are remarkable from their thin pods, which, whilst young, are cooked and eaten whole; and in this group … it is the pod which differs most; thus Lewis’s Negro-podded pea has a straight, broad, smooth, and dark-purple pod, with the husk not so thin as in the other kinds; the pod of another variety is extremely bowed; that of the Pois géant is much pointed at the extremity; and in the variety “à grands cosses” the peas are seen through the husk in so conspicuous a manner that the pod, especially when dry, can hardly at first be recognised as that of a pea.
In the ordinary varieties, the pods also differ much in size;—in colour, that of Woodford’s Green Marrow being bright-green when dry, instead of pale brown, and that of the purple-podded pea being expressed by its name;—in smoothness, that of Danecroft being remarkably glossy, whereas that of the Ne plus ultra is rugged; in being either nearly cylindrical, or broad and flat;—in being pointed at the end, as in Thurston’s Reliance, or much truncated, as in the American Dwarf. In the Auvergne pea, the whole end of the pod is bowed upwards. In the Queen of Dwarfs and in Scimitar peas, the pod is almost elliptic in shape. I here give drawings of the four most distinct pods produced by the plants cultivated by me.
Pods and Peas. I. Queen of Dwarfs. II. American Dwarf. III. Thurston’s Reliance. IV. Pois Géant sans parchemin. a. Dan O’Rourke Pea. b. Queen of Dwarfs Pea. c. Knight’s Tall White Marrow. d. Lewis’s Negro Pea.
With respect to the varieties not naturally intercrossing, I have ascertained that the pea, which in this respect differs from some other Leguminosae, is perfectly fertile without the aid of insects. Yet I have seen humble-bees whilst sucking the nectar depress the keel-petals, and become so thickly dusted with pollen, that it could hardly fail to be left on the stigma of the next flower which was visited. Nevertheless, distinct varieties growing closely together rarely cross; and I have reason to believe that this is due to their stigmas being prematurely fertilised in this country by pollen from the same flower. The horticulturists who raise seed-peas are thus enabled to plant distinct varieties close together without any bad consequences; and it is certain, as I have myself found, that true seed may be saved during at least several generations under these circumstances.
The common pea is perfectly fertile when its flowers are protected from the visits of insects; I ascertained this with two or three different varieties, as did Dr. Ogle with another. But the flowers are likewise adapted for cross-fertilisation; Mr. Farrer specifies the following points, namely: “The open blossom displaying itself in the most attractive and convenient position for insects; the conspicuous vexillum; the wings forming an alighting place; the attachment of the wings to the keel, by which any body pressing on the former must press down the latter; the staminal tube enclosing nectar, and affording by means of its partially free stamen with apertures on each side of its base an open passage to an insect seeking the nectar; the moist and sticky pollen placed just where it will be swept out of the apex of the keel against the entering insect; the stiff elastic style so placed that on a pressure being applied to the keel it will be pushed upwards out of the keel; the hairs on the style placed on that side of the style only on which there is space for the pollen, and in such a direction as to sweep it out; and the stigma so placed as to meet an entering insect,—all these become correlated parts of one elaborate mechanism, if we suppose that the fertilisation of these flowers is effected by the carriage of pollen from one to the other.” Notwithstanding these manifest provisions for cross-fertilisation, varieties which have been cultivated for very many successive generations in close proximity, although flowering at the same time, remain pure. …
It is remarkable, considering that the flowers secrete much nectar and afford much pollen, how seldom they are visited by insects either in England or, as H. Müller remarks, in North Germany. I have observed the flowers for the last thirty years, and in all this time have only thrice seen bees of the proper kind at work (one of them being Bombus muscorum), such as were sufficiently powerful to depress the keel, so as to get the undersides of their bodies dusted with pollen. These bees visited several flowers and could hardly have failed to cross-fertilise them. Hive-bees and other small kinds sometimes collect pollen from old and already fertilised flowers, but this is of no account. The rarity of the visits of efficient bees to this exotic plant is, I believe, the chief cause of the varieties so seldom intercrossing. That a cross does occasionally take place, as might be expected from what has just been stated, is certain, from the recorded cases of the direct action of the pollen of one variety on the seed-coats of another. …
We understand today that Abberley’s results stem from the fact that pea flowers typically self-pollinate. Varieties will readily cross when hand-pollinated, as Austrian monk Gregor Mendel showed. Like Darwin, Mendel chose peas for his experiments because he could grow them easily and readily observe traits of interest. He cross-bred tall and dwarf pea varieties, green and yellow peas, purple and white flowers, wrinkled and smooth peas, and observed the resulting offspring—experiments that led to the foundation of modern genetics. Contrary to popular belief, Darwin was unaware of the monk’s pea-crossing papers, and even if he had read them he probably would not have appreciated their significance—that insight came forty years later, when biologists could understand Mendel in light of late nineteenth-century discoveries of chromosomes and meiosis.
Primula veris. Water and bodycolor on vellum by Dame Ann Hamilton, Drawings of Plants.