VARIATION, FORMS OF FLOWERS, POLLINATION, PLANT MOVEMENT
The kitchen garden at Down House, Darwin’s family home in Kent, England, always presented low-hanging fruit, so to speak, for Darwin’s botanical investigations, and several species were studied from different biological angles. Strawberries, Fragaria, are a good example—hybridization, flower structure, and movement of the runners (stolons) during growth provided research topics at different times, all exemplifying for him distinct lines of evidence for evolution by natural selection.
Darwin argued that domestication, through artificial selection, was a compelling analog to the way natural selection operates—one reason that On the Origin of Species opens with a chapter on “Variation Under Domestication.” The key word there is “variation,” as heritable variation is raw material for selection, artificial or natural. Strawberries are not discussed in Origin, but in his 1868 book The Variation of Animals and Plants Under Domestication, Darwin invited readers to compare the large and plump garden strawberries then available with their diminutive wild relatives. What accounts for the difference? Improvement through crossing (which mixes things up and introduces more variation) combines with selection to yield ever-larger fruits over time.
Investigating strawberry species and varieties, and their propensity to hybridize, Darwin employed his favored crowd-sourcing method—publishing a letter asking for help. In the Journal of Horticulture, Cottage Gardener, and Country Gentleman, he wrote:
Will any of your correspondents who have attended to the history of the Strawberry kindly inform me whether any of the kinds now, or formerly, cultivated have been raised from a cross between any of the Woods or Alpines with the Scarlets, Pines, and Chilis? Also, whether any one has succeeded in getting any good from a cross between the Hautbois and any other kind? … I should feel greatly indebted to any one who would take the trouble to inform me on this head.72
A related interest was the curious fact that strawberries vary in their flower structure, first noticed by French botanist Antoine Nicolas Duchesne, who cultivated a diversity of strawberry species and varieties in the Royal Botanical Gardens at Versailles. Some individuals are unisexual, bearing male-only or female-only flowers (dioecious, in modern terms), while others are hermaphroditic, with flowers bearing both stamens and pistils (monecious). In this, Darwin saw evolution in action, noting in Forms of Flowers (1877) of strawberries’ “tendency to the separation of the sexes.”
Strawberries (Fragaria).— This fruit is remarkable on account of the number of species which have been cultivated, and from their rapid improvement within the last fifty or sixty years. Let any one compare the fruit of one of the largest varieties exhibited at our Shows with that of the wild wood strawberry, or, which will be a fairer comparison, with the somewhat larger fruit of the wild American Virginian Strawberry, and he will see what prodigies horticulture has effected. The number of varieties has likewise increased in a surprisingly rapid manner. Only three kinds were known in France, in 1746, where this fruit was early cultivated. In 1766, five species had been introduced, the same which are now cultivated, but only five varieties of Fragaria vesca, with some sub-varieties, had been produced. At the present day, the varieties of the several species are almost innumerable.
Much has been written on the sexes of strawberries; the true Hautbois properly bears the male and female organs on separate plants and was consequently named by Duchesne dioica; but it frequently produces hermaphrodites; and Lindley, by propagating such plants by runners, at the same time destroying the males, soon raised a self-prolific stock. The other species often show a tendency towards an imperfect separation of the sexes, as I have noticed with plants forced in a hot-house. Several English varieties, which in this country are free from any such tendency, when cultivated in rich soils under the climate of North America, commonly produce plants with separate sexes. Thus a whole acre of Keen’s Seedlings in the United States has been observed to be almost sterile from the absence of male flowers; but the more general rule is that the male plants overrun the females. Some members of the Cincinnati Horticultural Society, especially appointed to investigate this subject, report that “few varieties have the flowers perfect in both sexual organs,” &c. The most successful cultivators in Ohio plant for every seven rows of “pistillata,” or female plants, one row of hermaphrodites, which afford pollen for both kinds; but the hermaphrodites, owing to their expenditure in the production of pollen, bear less fruit than the female plants.
Darwin later turned his attention to the growth of strawberry stolons or runners, horizontal stems that reach out to root beyond the main stems, enabling the plant to spread vegetatively. He had been studying the circumnutation of aerial stems and wondered if stolons also rotate as they grow. It turned out that they do. In Movement of Plants (1880), Darwin described how strawberry stolons twist and rotate as they maneuver around objects in their path. He documented this movement by mounting a sheet of glass above or beside a plant and applying black sealing wax to a fine glass filament attached to the part of the plant he wanted to watch. He would then mark a black dot on a white card fixed in position on a stick just beneath the needle. Noting the location of the dot on the glass plane, he would mark each movement over a period of time, usually over one or two days. He would then connect the dots to make a record of the movement.73
Stolons consist of much elongated, flexible branches, which run along the surface of the ground and form roots at a distance from the parent-plant. They are therefore of the same homological nature as stems. …
Fragaria (cultivated garden var.): A plant growing in a pot had emitted a long stolon; this was supported by a stick, so that it projected for the length of several inches horizontally. A glass filament bearing two minute triangles of paper was affixed to the terminal bud, which was a little upturned; and its movements were traced during 21 h., as shown in [the left-hand figure]. In the course of the first 12 h., it moved twice up and twice down in somewhat zigzag lines, and no doubt travelled in the same manner during the night. On the following morning after an interval of 20 h., the apex stood a little higher than it did at first, and this shows that the stolon had not been acted on within this time by geotropism; nor had its own weight caused it to bend downwards.
On the following morning (19th) the glass filament was detached and refixed close behind the bud, as it appeared possible that the circumnutation of the terminal bud and of the adjoining part of the stolon might be different. The movement was now traced during two consecutive days. … During the first day, the filament travelled in the course of 14 h. 30 m. five times up and four times down, besides some lateral movement. On the 20th, the course was even more complicated, and can hardly be followed in the figure; but the filament moved in 16 h. at least five times up and five times down, with very little lateral deflection. The first and last dots made on this second day, viz., at 7 A.M. and 11 P.M., were close together, showing that the stolon had not fallen or risen. Nevertheless, by comparing its position on the morning of the 19th and 21st, it is obvious that the stolon had sunk; and this may be attributed to slow bending down either from its own weight or from geotropism.
Fragaria: circumnutation of stolon, kept in darkness, traced on vertical glass, from 10.45 A.M. May 18th to 7.45 A.M. on 19th.
Fragaria: circumnutation of the same stolon as in the last figure, observed in the same manner, and traced from 8 A.M. May 19th to 8 A.M. 21st.
Gloriosa superba. Watercolor on vellum by Pancrace Bessa for Herbier General de l’Amateur.