CLIMBING PLANTS, MOVEMENT
The Nasturtium family has but a single genus, Tropaeolum, comprising some eighty species widespread in Central and South America. The popular garden nasturtium (T. majus), canary bird vine (T. peregrinum), and flame nasturtium (T. speciosum) are prized by gardeners worldwide for both their culinary use—peppery in flavor, like watercress in the Mustard family (genus Nasturtium, not coincidentally)—and vividly colored flowers. Those striking colors inspired a sanguinary reference by Linnaeus in bestowing the name Tropaeolum, which is derived from the Greek tropaion, “trophy.” This references the ancient Roman custom of the trophy-pole, on which vanquished enemies’ armor and weapons were hung. The round leaves of the type species T. majus were imagined to resemble shields and its flowers blood-stained helmets.
Had Linnaeus waited another decade before naming Tropaeolum, an observation made by his nineteen-year-old daughter Elisabet Christina might have inspired a more interesting moniker. In the fading light of dusk one evening, she noticed that the flowers suddenly gleamed, almost seeming to emit flashes or sparks of light. Her observations were soon published,144 touching off intense scientific speculation and poetic inspiration over what became known as the “Elizabeth Linnaeus Phenomenon.” Erasmus Darwin wrote in The Loves of the Plants how in the twilight, “A faint glory trembles” around Tropaeolum, over whose “fair form the electric lustre plays,” and in the notes cited Elisabet Christina’s discovery of what many took to be an electrical phenomenon.145 The Romantic poets were inspired in turn. Coleridge, for example, drew on The Loves of the Plants in penning the lines: “‘Tis said, in Summer’s evening hour / Flashes the golden-colour’d flower / A fair electric flame.”146 More recent studies confirm an explanation first suggested by the German polymath Johann Wolfgang von Goethe: the scintillating gleam is more optical illusion than botanical electricity, created when the brightly colored flowers are viewed obliquely against a leafy green background in low light.147
Charles Darwin embraced a different kind of botanical “electricity,” pertaining more to impulses, analogous to the animal nervous system, thought to be behind the touch-sensitivity and rapid movement of some plants. If he knew of Elisabet Christina Linnaea and the case of the curiously scintillating Tropaeolum, it is not clear that he ever commented on it, his interest in nasturtiums falling squarely within the realms of pollination, inheritance, and movement. In Cross and Self Fertilisation, he noted that nasturtiums are “manifestly adapted for cross-fertilisation by insects” due to their protandrous flowers, with stamens developing before the pistils so that insects bring pollen from a younger (male) flower to an older (female) one. His experiments resulted in a batch of crossed plants producing 243 seeds, while the same number of self-fertilized plants yielded only 155 seeds—consistent with his overarching hypothesis of the benefits of outcrossing. Further research on several species and varieties involved movement, documenting how young revolving internodes and petioles enable these leaf-climbers to ascend supporting objects (as described in Climbing Plants), and the heliotropic (turning toward sunlight) and nyctitropic (nocturnal “sleep”) movement of leaves (as described in Movement).
Tropaeolum tricolorum, var. grandiflorum.—The flexible shoots, which first rise from the tubers, are as thin as fine twine. One such shoot revolved in a course opposed to the sun, at an average rate, judging from three revolutions, of 1 hr. 23 m.; but no doubt the direction of the revolving movement is variable. When the plants have grown tall and are branched, all the many lateral shoots revolve. The stem, whilst young, twines regularly round a thin vertical stick, and in one case I counted eight spiral turns in the same direction; but when grown older, the stem often runs straight up for a space, and, being arrested by the clasping petioles, makes one or two spires in a reversed direction. Until the plant grows to a height of two or three feet, requiring about a month from the time when the first shoot appears above ground, no true leaves are produced, but, in their place, filaments coloured like the stem. The extremities of these filaments are pointed, a little flattened, and furrowed on the upper surface. They never become developed into leaves. As the plant grows in height new filaments are produced with slightly enlarged tips; then others, bearing on each side of the enlarged medial tip a rudimentary segment of a leaf; soon other segments appear, and at last a perfect leaf is formed, with seven deep segments. So that on the same plant we may see every step, from tendril-like clasping filaments to perfect leaves with clasping petioles. After the plant has grown to a considerable height and is secured to its support by the petioles of the true leaves, the clasping filaments on the lower part of the stem wither and drop off; so that they perform only a temporary service.
These filaments or rudimentary leaves, as well as the petioles of the perfect leaves, whilst young, are highly sensitive on all sides to a touch. The slightest rub caused them to curve towards the rubbed side in about three minutes, and one bent itself into a ring in six minutes; they subsequently became straight. When, however, they have once completely clasped a stick, if this is removed, they do not straighten themselves. The most remarkable fact, and one which I have observed in no other species of the genus, is that the filaments and the petioles of the young leaves, if they catch no object, after standing for some days in their original position, spontaneously and slowly oscillate a little from side to side, and then move towards the stem and clasp it. They likewise often become, after a time, in some degree spirally contracted. They therefore fully deserve to be called tendrils, as they are used for climbing, are sensitive to a touch, move spontaneously, and ultimately contract into a spire, though an imperfect one. The present species would have been classed amongst the tendril-bearers, had not these characters been confined to early youth. During maturity it is a true leaf-climber. …
Tropaeolum majus (cultivated var.) —Several plants in pots stood in the greenhouse, and the blades of the leaves which faced the front-lights were during the day highly inclined and at night vertical; whilst the leaves on the back of the pots, though of course illuminated through the roof, did not become vertical at night. We thought, at first, that this difference in their positions was in some manner due to heliotropism, for the leaves are highly heliotropic. The true explanation, however, is that unless they are well illuminated during at least a part of the day they do not sleep at night; and a little difference in the degree of illumination determines whether or not they shall become vertical at night.
A large pot with several plants was brought on the morning of Sept. 3rd out of the greenhouse and placed before a north-east window, in the same position as before with respect to the light, as far as that was possible. On the front of the plants, 24 leaves were marked with thread, some of which had their blades horizontal, but the greater number were inclined at about 45°, beneath the horizon; at night all these, without exception, became vertical. Early on the following morning (4th) they reassumed their former positions, and at night again became vertical. On the 5th the shutters were opened at 6.15 A.M., and by 8.18 A.M., after the leaves had been illuminated for 2 h. 3 m. and had acquired their diurnal position, they were placed in a dark cupboard. They were looked at twice during the day and thrice in the evening, the last time at 10.30 P.M., and not one had become vertical. At 8 A.M. on the following morning (6th) they still retained the same diurnal position and were now replaced before the north-east window. At night all the leaves which had faced the light had their petioles curved and their blades vertical; whereas none of the leaves on the back of the plants, although they had been moderately illuminated by the diffused light of the room, were vertical. They were now at night placed in the same dark cupboard; at 9 A.M. on the next morning (7th) all those which had been asleep had reassumed their diurnal position. The pot was then placed for 3 h. in the sunshine, so as to stimulate the plants; at noon they were placed before the same north-east window, and at night the leaves slept in the usual manner and awoke on the following morning. At noon on this day (8th) the plants, after having been left before the north-east window for 5 h. 45 m. and thus illuminated (though not brightly, as the sky was cloudy during the whole time), were replaced in the dark cupboard, and at 3 P.M. the position of the leaves was very little, if at all, altered, so that they are not quickly affected by darkness; but by 10.15 P.M. all the leaves which had faced the north-east sky during the 5 h. 45 m. of illumination stood vertical, whereas those on the back of the plant retained their diurnal position. On the following morning (9th) the leaves awoke as on the two former occasions in the dark, and they were kept in the dark during the whole day; at night a very few of them became vertical, and this was the one instance in which we observed any inherited tendency or habit in this plant to sleep at the proper time. That it was real sleep was shown by these same leaves reassuming their diurnal position on the following morning (10th) whilst still kept in the dark.
Vicia faba. Watercolor on paper by Charles Germain de Saint-Aubin, Receuil De Plantes Copiées D ‘Aprés Nature Par De Saint Aubin Dessinateur Du Roy Louis XV.