FORMS OF FLOWERS, POLLINATION
Periwinkle is a common name for plants in two related genera in this family—Vinca, a small genus of seven species of distributed from the western Mediterranean region to southwest Asia; and the sister genus Catharanthus, seven of eight species of which are Madagascar endemics.
The name “vinca,” bestowed by Linnaeus in 1767, is derived from the name given in antiquity by Pliny the Elder, vincaperivinca, still reflected in its common Italian and French names, pervinca and pervenche, respectively. Two of the European Vinca species, V. major and V. minor, have long been prized horticulturally; both are lustrous-green creeping evergreen vines sporting lavender-colored pinwheel-shaped flowers. Because the plants are low growing and spread quickly, they are often used as groundcover in garden landscapes, but have become invasive in many parts of the world.
Darwin was curious about pollination and seed production in Vinca. He noticed that Vinca major never seemed to set seed in England and figured it was for want of the right insect pollinators. He did an experiment using a slender bristle to mimic a moth pollinator, described in a letter to Daniel Oliver, Keeper of the Herbarium at the Royal Botanic Gardens, Kew: “I have passed fine bristle between anthers (not cutting or touching the flowers) in same way as proboscis of moth would pass to nectary, near the sides of the corolla; pollen sticks to bristle and a bristle thus covered from pollen of one flower is used for another flower. … I have 4 or 5 fine pods swelling.”150 He published the results in the Gardeners’ Chronicle, concluding with his usual crowd-sourcing appeal, urging readers to try the experiment and send in the results.151 At Darwin’s urging, Charles Crocker, a Kew propagator always willing to help, experimented with the related tropical periwinkles. His results, equally as positive as Darwin’s, were reported in the same journal the following month:
Following the suggestion made by Mr. Darwin at page 552, a week or two ago, I thought that I would try if the tropical kinds of Vinca could be induced to produce seed, which is never the case under cultivation if left to themselves. I impregnated eight flowers, and in the course of a few days had the satisfaction of seeing that the pistils in seven cases were swelling well. The erect double follicles are now in several instances more than an inch long; in one they are not yet ripe. The plant upon which I tried the experiment was the white-flowered variety of Vinca rosea [now Catharanthus roseus]. I used the pollen from the same plant as I wished also to see if this variety would reproduce itself by seed, or if it will revert to the normal colour of the species. I merely passed a hair down the tube of one flower after another as an insect might insert its proboscis in its search for nectar.152
Darwin’s hunch was correct—outside of their native range, these ground-hugging plants have a hard time attracting pollinators with probosces long enough to fertilize them. Lack of seed production may be why these plants are not even more invasive, spreading vegetatively for the most part, and slowly at that.
Fertilisation of Vincas.—I do not know whether any exotic Vincas seed, or whether gardeners would wish them to seed, and so raise new varieties. Having never observed the large Periwinkle or Vinca major to produce seed, and having read that this never occurs in Germany, I was led to examine the flower. The pistil, as botanists know, is a curious object, consisting of a style, thickening upwards, with a horizontal wheel on the top; and this is surmounted by a beautiful brush of white filaments. The concave tire of the wheel is the stigmatic surface, as was very evident when pollen was placed on it, by the penetration of the pollen-tubes. The pollen is soon shed out of the anthers and lies embedded in little alcoves in the white filamentous brush above the stigma. Hence it was clear that the pollen could not get on to the stigma without the aid of insects, which, as far as I have observed in England, never visit this flower. Accordingly, I took a fine bristle to represent the proboscis of a moth, and passed it down between the anthers, near the sides of the corolla; for I found that the pollen sticks to the bristle and is carried down to the viscid stigmatic surface. I took the additional precaution of passing it down first between the anthers of one flower and then of another, so as to give the flowers the advantage of a cross; and I passed it down between several of the anthers in each case. I thus acted on six flowers on two plants growing in pots; the germens of these swelled, and on four out of the six I have now got fine pods, above 1½ inch in length, with the seeds externally visible; whereas the flower stalks of the many other flowers all shanked off. I wish any one who wishes to obtain seed of any other species that does not habitually seed would try this simple little experiment and report the result. I shall sow the seeds of my Vinca for the chance of a sport: for a plant which seeds so rarely might be expected to give way to some freak on so unusual and happy an occasion.
Viola odorata. Watercolor by Elizabeth Wharton, British Flowers.