FORMS OF FLOWERS, POLLINATION
The Latin name as well as the common name of Pulmonaria refers to the lung, in a nod to herbalists of old who believed that the spotted leaves of Pulmonaria officinalis cure diseased and ulcerated lungs. This supposed relationship between plants and people is based on the medieval “Doctrine of Signatures,” which held that plants or plant parts resembling any part of human anatomy possessed curative properties for that body part.
There are around fifteen species of Pulmonaria, native to Europe and beyond, with many cultivars grown in gardens throughout the temperate world. They are perennial herbs with basal leaf rosettes and attractive flower racemes in a spiraled or “scorpioid” arrangement, often initially blue in color and transitioning to pink after pollination. Along with Primula, Linum, and other plants that Darwin studied, they present a nice example of flower dimorphism, bearing heterostylous flowers with different lengths of stamens and pistils that function to facilitate cross pollination.
Darwin’s eldest son William discovered floral dimorphism in a population of narrow-leaved lungwort (Pulmonaria angustifolia) on the Isle of Wight in spring of 1863. His father was delighted, eager to expand his studies of heterostyly and inter-fertility in primroses and flax.129 Well-stocked by William with seedlings of P. angustifolia and seeds of another species, the common blue lungwort (P. officinalis), Darwin experimented with both plants the following year. When he began his work he noted that P. angustifolia and P. officinalis are so similar that some botanists considered them as “mere varieties” of the same species. Through cross-pollination tests, however, he determined that they were truly distinct species. He expected that the flower morphs would be cross-fertile and self-sterile, consistent with other heterostylous plants he had studied. But while the long-styled form of P. angustifolia was self-sterile, he was surprised to find the short-styled form to be self-fertile to a remarkable degree.
He commented on this to Asa Gray in the United States, saying, “Did I ever tell you that a year or two ago I ascertained that Pulmonaria offers a curious case. The long-styled form being absolutely sterile with its own pollen, whilst the short-styled is almost perfectly fertile with its own pollen.” This puzzling departure from the more typical result when experimentally self-pollinating within morphs of heterostylous plants—which tend to yield sterile and often stunted offspring—prompted Darwin to declare, “This seems to me a very curious fact.”130
Pulmonaria angustifolia.—Seedlings of this plant, raised from plants growing wild in the Isle of Wight, were named for me by Dr. Hooker. It is so closely allied to the last species, differing chiefly in the shape and spotting of the leaves, that the two have been considered by several eminent botanists—for instance, Bentham—as mere varieties. But, as we shall presently see, good evidence can be assigned for ranking them as distinct. Owing to the doubts on this head, I tried whether the two would mutually fertilise one another. Twelve short-styled flowers of P. angustifolia were legitimately fertilised with pollen from long-styled plants of P. officinalis (which, as we have just seen, are moderately self-fertile), but they did not produce a single fruit. Thirty-six long-styled flowers of P. angustifolia were also illegitimately fertilised during two seasons with pollen from the long-styled P. officinalis, but all these flowers dropped off unimpregnated. Had the plants been mere varieties of the same species these illegitimate crosses would probably have yielded some seeds, judging from my success in illegitimately fertilising the long-styled flowers of P. officinalis; and the twelve legitimate crosses, instead of yielding no fruit, would almost certainly have yielded a considerable number, namely, about nine. … Therefore P. officinalis and angustifolia appear to be good and distinct species, in conformity with other important functional differences between them, immediately to be described.
The long-styled and short-styled flowers of P. angustifolia differ from one another in structure in nearly the same manner as those of P. officinalis. But in the accompanying figure a slight bulging of the corolla in the long-styled form, where the anthers are seated, has been overlooked. My son William, who examined a large number of wild plants in the Isle of Wight, observed that the corolla, though variable in size, was generally larger in the long-styled flowers than in the short-styled; and certainly the largest corollas of all were found on the long-styled plants, and the smallest on the short-styled. Exactly the reverse occurs, according to Hildebrand, with P. officinalis. Both the pistils and stamens of P. angustifolia vary much in length … so that the stigma in the one form does not stand on a level with the anthers in the other. The long-styled pistil is sometimes thrice as long as that of the short-styled; but from an average of ten measurements of both, its length to that of the short-styled was as 100 to 56. The stigma varies in being more or less, though slightly, lobed.
My son collected in the Isle of Wight on two occasions 202 plants, of which 125 were long-styled and 77 short-styled; so that the former were the more numerous. On the other hand, out of 18 plants raised by me from seed, only 4 were long-styled and 14 short-styled. The short-styled plants seemed to my son to produce a greater number of flowers than the long-styled; and he came to this conclusion before a similar statement had been published by Hildebrand with respect to P. officinalis. My son gathered ten branches from ten different plants of both forms and found the number of flowers of the two forms to be as 100 to 89, 190 being short-styled and 169 long-styled. With P. officinalis the difference, according to Hildebrand, is even greater, namely, as 100 flowers for the short-styled to 77 for the long-styled plants. …
Pulmonaria angustifolia. Long-styled, left. Short-styled, right.
The fertility of the two legitimate unions to that of the two illegitimate together is as 100 to 35, judged by the proportion of flowers which produced fruit; and as 100 to 32, judged by the average number of seeds per fruit. But the small number of fruit yielded by the 18 long-styled flowers in the first line was probably accidental, and if so, the difference in the proportion of legitimately and illegitimately fertilised flowers which yield fruit is really greater than that represented by the ratio of 100 to 35. The 18 long-styled flowers illegitimately fertilised yielded no seeds,—not even a vestige of one. Two long-styled plants which were placed under a net produced 138 flowers, besides those which were artificially fertilised, and none of these set any fruit; nor did some plants of the same form which were protected during the next summer. Two other long-styled plants were left uncovered (all the short-styled plants having been previously covered up), and humble-bees, which had their foreheads white with pollen, incessantly visited the flowers, so that their stigmas must have received an abundance of pollen, yet these flowers did not produce a single fruit. We may therefore conclude that the long-styled plants are absolutely barren with their own-form pollen, though brought from a distinct plant. In this respect they differ greatly from the long-styled English plants of P. officinalis which were found by me to be moderately self-fertile; but they agree in their behavior with the German plants of P. officinalis experimented on by Hildebrand. …
The great difference in the fertility of the long and short-styled flowers, when both are illegitimately fertilised, is a unique case, as far as I have observed with heterostyled plants. The long-styled flowers when thus fertilised are utterly barren, whilst about half of the short-styled ones produce capsules, and these include a little above two-thirds of the number of seeds yielded by them when legitimately fertilised. The sterility of the illegitimately fertilised long-styled flowers is probably increased by the deteriorated condition of their pollen; nevertheless, this pollen was highly efficient when applied to the stigmas of the short-styled flowers.
Salvia coccinea. Hand-colored engraving by William Jackson Hooker, from The Botanical Magazine 1828 55: 2864.