CROSS AND SELF-FERTILIZATION, PLANT MOVEMENT
Cyclamen numbers about twenty-three species ranging from Europe to the Middle East and north Africa. The name is derived from the Greek kyklos, circular, referring to the disk-like shape of the tuber. The plants are prized for their boldly patterned leaves and dramatic flowers, nodding with backswept petals. After pollination, as the fruit develops, nearly all Cyclamen species exhibit a curious change: the upright flower stem (peduncle) lengthens as it goes into a slow-motion, spiraling nose-dive, carrying the seed capsule down under the leaf litter and, in some cases, right into the ground. This might seem like a kind of “self-sowing,” but it is thought to function more as a way to provide ants ready access to the ripening capsule—Cyclamen seeds are myrmecochorous, meaning they are dispersed by ground-dwelling ants, which are rewarded for their efforts with a nutritious morsel called an elaiosome that’s attached to each seed. Darwin was aware of the curious descent of Cyclamen seed capsules—his grandfather Erasmus Darwin commented upon it in his epic poem The Loves of the Plants.1
Charles mainly studied Persian cyclamen, Cyclamen persicum, the one species that does not coil its peduncles, though they do curve as the seed capsules are lowered to the ground. He grew this species in his greenhouse where he was able to observe and experiment on several levels. He crossed and self-pollinated flowers and then raised offspring from the resulting seeds over several years, noting that cross fertilization yielded robust plants, whereas self-fertilized plants produced “miserable specimens.”
Ten flowers crossed with pollen from plants known to be distinct seedlings, yielded nine capsules, containing on an average 34.2 seeds, with a maximum of seventy-seven in one. Ten flowers self-fertilised yielded eight capsules, containing on an average only 13.1 seeds, with a maximum of twenty-five in one. This gives a ratio of 100 to 38 for the average number of seeds per capsule for the crossed and self-fertilised flowers. The flowers hang downwards, and as the stigmas stand close beneath the anthers, it might have been expected that pollen would have fallen on them and that they would have been spontaneously self-fertilised; but these covered-up plants did not produce a single capsule. On some other occasions, uncovered plants in the same greenhouse produced plenty of capsules, and I suppose that the flowers had been visited by bees, which could hardly fail to carry pollen from plant to plant.
The seeds obtained in the manner just described were placed on sand, and after germinating were planted in pairs—three crossed and three self-fertilised plants on the opposite sides of four pots. When the leaves were 2 or 3 inches in length, including the foot-stalks, the seedlings on both sides were equal. In the course of a month or two, the crossed plants began to show a slight superiority over the self-fertilised, which steadily increased; and the crossed flowered in all four pots some weeks before, and much more profusely than the self-fertilised. The two tallest flower-stems on the crossed plants in each pot were now measured, and the average height of the eight stems was 9.49 inches. After a considerable interval of time, the self-fertilised plants flowered, and several of their flower-stems were roughly measured, and their average height was a little under 7.5 inches; so that the flower-stems on the crossed plants to those on the self-fertilised were at least as 100 to 79. …
These plants were left uncovered in the greenhouse; and the twelve crossed plants produced forty capsules, whilst the twelve self-fertilised plants produced only five; or as 100 to 12. But this difference does not give a just idea of the relative fertility of the two lots. I counted the seeds in one of the finest capsules on the crossed plants, and it contained seventy-three; whilst the finest of the five capsules produced by the self-fertilised plants contained only thirty-five good seeds. In the other four capsules most of the seeds were barely half as large as those in the crossed capsules. In the following year, the crossed plants again bore many flowers before the self-fertilised bore a single one. The self-fertilised plants were miserable specimens, whilst the crossed ones looked very vigorous.
Darwin’s observations on this cyclamen’s leaf movement revealed a zigzag pattern, rising in the evening and falling in the morning. He watched, fascinated, as the peduncles of the fertilized flowers slowly lengthened and bowed their developing seed capsule to the soil in a graceful curve. His experiments with potted plants, brought in and out of a dark cupboard, confirmed that it was not gravity that pulled the capsules down, but a distinct form of plant movement, a modified form of the common circular motion (circumnutation) he dubbed “apheliotropism”—movement away from the sun.
Whilst this plant is in flower, the peduncles stand upright, but their uppermost part is hooked so that the flower itself hangs downwards. As soon as the pods begin to swell, the peduncles increase much in length and slowly curve downwards, but the short, upper, hooked part straightens itself. Ultimately the pods reach the ground, and if this is covered with moss or dead leaves, they bury themselves. We have often seen saucer-like depressions formed by the pods in damp sand or sawdust; and one pod (.3 of inch in diameter) buried itself in sawdust for three-quarters of its length. We shall have occasion hereafter to consider the object gained by this burying process. The peduncles can change the direction of their curvature, for if a pot, with plants having their peduncles already bowed downwards, be placed horizontally, they slowly bend at right angles to their former direction towards the centre of the earth. We therefore at first attributed the movement to geotropism; but a pot which had lain horizontally with the pods all pointing to the ground, was reversed, being still kept horizontal, so that the pods now pointed directly upwards; it was then placed in a dark cupboard, but the pods still pointed upwards after four days and nights. The pot, in the same position, was next brought back into the light, and after two days there was some bending downwards of the peduncles, and on the fourth day two of them pointed to the centre of the earth, as did the others after an additional day or two. Another plant, in a pot which had always stood upright, was left in the dark cupboard for six days; it bore 3 peduncles, and only one became within this time at all bowed downwards, and that doubtfully. The weight, therefore, of the pods is not the cause of the bending down. This pot was then brought back into the light, and after three days, the peduncles were considerably bowed downwards. We are thus led to infer that the downward curvature is due to apheliotropism; though more trials ought to have been made.
In order to observe the nature of this movement, a peduncle bearing a large pod which had reached and rested on the ground, was lifted a little up and secured to a stick. A filament was fixed across the pod with a mark beneath, and its movement, greatly magnified, was traced on a horizontal glass during 67 h. The plant was illuminated during the day from above. A copy of the tracing is given [here]; and there can be no doubt that the descending movement is one of modified circumnutation, but on an extremely small scale. … Considering the great length and thinness of the peduncles and the lightness of the pods, we may conclude that they would not be able to excavate saucer-like depressions in sand or sawdust, or bury themselves in moss, etc., unless they were aided by their continued rocking or circumnutating movement.
Cyclamen persicum: downward apheliotropic movement of a flower-peduncle, traced on a horizontal glass from 1 P.M. Feb. 18th to 8 A.M. 21st.
Cypripedium calceolus. Watercolor by Elizabeth Wharton, British Flowers.