FORMS OF FLOWERS, POLLINATION, CROSS AND SELF-FERTILIZATION
With more than 700 species and innumerable cultivars, salvias are prized for their bold colorful flowers and fragrant foliage. Herbalists of old found them attractive for another reason—the Latin name is derived from salvare, “to heal,” and salvus, “uninjured,” in reference to the medicinal uses of the foliage in compresses for wounds and other treatments dating back to antiquity. Darwin, however, was interested in the plant’s wounds—he noted that bumblebees often avoid entering the mouth of Salvia flowers, preferring to bore into the flower base close to the spot where the nectar lies hidden within the corolla—nectar robbery, not very helpful for pollination.
Perhaps some of your readers may like to hear a few more particulars about the humble-bees which bore holes in flowers, and thus extract the nectar. This operation has been performed on a large scale in the Zoological Gardens:—Near the refection-house there is a fine bed of Stachys coccinea, every flower in which has one, and sometimes two, small irregular slits, or orifices, on the upper side of the corolla near its base. I observed some plants of Marvel of Peru, and of Salvia coccinea, with holes in similar positions; but in Salvia Grahami they were without exception cut through the calyx.
Those bees that do enter the flower in the way the plants preferred them to, so to speak, encounter a wonderful pollination adaptation: two stamens fused such that one protrudes from the other like a handle, acting as a lever when the insect pushes past it to get to the nectary, tipping the attached pollen-laden stamen down onto its back where pollen is smeared. It was just the sort of adaptation that thrilled Darwin, who declared the fused stamens “as perfect a structure as can be found in any orchid.”131
This mechanism was first described by German botanist Friedrich Hildebrand, who wrote Darwin in 1864 that his research into the pollination mechanisms of Salvia, Pulmonaria, Linum, and other plants was inspired by Darwin’s orchid book.132 (Hildebrand, whom Darwin cites extensively in his 1876 book Cross and Self Fertilisation, coined the term “heterostyly,” which Darwin thought was an improvement over his own terms “di- and trimorphic” to describe these flowers.) Bees in a hurry didn’t seem to appreciate the biomechanical beauty of Salvia, however perfect Darwin found it. He observed that flowers of the vivid scarlet sage (Salvia coccinea) were often perforated with one or two slits on the upper side of the corolla near the base, and both the calyx and corolla of blackcurrant or Graham’s sage, now Salvia microphylla, were invariably perforated by what he labeled “pick-pocket bees.”
Wondering to what extent seeds were produced when insects failed to pollinate the flowers, he worked under a net, artificially self-pollinating some and leaving others alone. It turns out that Salvia coccinea does not need the bees to set seed, though they do better when pollinated as described in Darwin’s study. How did this compare with seed-set in outcrossed flowers? To Darwin’s surprise, plenty of seeds were produced regardless of how they were pollinated, but then, after growing them out, he found that the cross-fertilized plants grew significantly larger and produced nearly twice as many flowers as the self-pollinated ones—yet another demonstration of the benefits of outcrossing.
Salvia coccinea.—This species, unlike most of the others in the same genus, yields a good many seeds when insects are excluded. I gathered ninety-eight capsules produced by flowers spontaneously self-fertilised under a net, and they contained on an average 1.45 seeds, whilst flowers artificially fertilised with their own pollen, in which case the stigma will have received plenty of pollen, yielded on an average 3.3 seeds, or more than twice as many. Twenty flowers were crossed with pollen from a distinct plant, and twenty-six were self-fertilised. There was no great difference in the proportional number of flowers which produced capsules by these two processes, or in the number of the contained seeds or in the weight of an equal number of seeds.
Seeds of both kinds were sown rather thickly on opposite sides of three pots. When the seedlings were about 3 inches in height, the crossed showed a slight advantage over the self-fertilised. When two-thirds grown, the two tallest plants on each side of each pot were measured; the crossed averaged 16.37 inches, and the self-fertilised 11.75 in height; or as 100 to 71. When the plants were fully grown and had done flowering, the two tallest plants on each side were again measured …
… Each of the six tallest crossed plants exceeds in height its self-fertilised opponent; the former averaged 27.85 inches, whilst the six tallest self-fertilised plants averaged 21.16 inches; or as 100 to 76. In all three pots the first plant which flowered was a crossed one. All the crossed plants together produced 409 flowers, whilst all the self-fertilised together produced only 232 flowers; or as 100 to 57. So that the crossed plants in this respect were far more productive than the self-fertilised.
Solanum tuberosum. Water and bodycolor on vellum by English School artist, Album of Garden Flowers.