INSECTIVOROUS PLANTS
Pinguicula grows in low-nutrient moist environments, similar to other carnivorous plants. The rosette-forming light-green leaves glisten with luscious glands, inspiring the Latin name, which refers to fat (pinguis), and the common name butterwort for the buttery-looking surface that attracts insects like fly-paper. With approximately ninety species in the genus, several are native to Europe. Darwin was able to observe and work his experiments on three of them, primarily the common circumboreal species P. vulgaris.
Butterworts were a late addition to Darwin’s insectivorous plant project—he only became aware of their fly-paper qualities in spring of 1874, from family friend William Marshall. He swung into action but knew it would delay publication of his book. “I am now hard at work getting my book on Drosera &c. ready for Printers,” he wrote to Asa Gray, at Harvard, “but it will take some time for I am always finding out new points to observe … Day before Yesterday I found out that Pinguicula digests and then absorbs animal matter; I know that this holds good for albumen, gelatin and insects, but I am now in the midst of my observations.”120
Movement of the leaves of Pinguicula had never been suspected or described before Darwin’s research, and it enhanced his fascination with the animal-like movement of plants and their digestive abilities. Marshall and his sister Theodora, along with other contacts, friends, and family (including Darwin’s future daughter-in-law, Amy Ruck), gladly sent him leaves with insects and seeds trapped upon them, leading Darwin to declare that Pinguicula is “not only insectivorous but graminvorous and granivorous!”121
Glands on the leaves secrete mucilage with digestive enzymes, and the more the glands are excited, the more mucilage is produced. With live plants under his care, Darwin observed the inward curling of the leaf margins, an action that moves insects onto more glands, triggering more secretion. The curling also pools the digestive enzymes in the concavity, preventing them from leaking off the edges or getting washed away in the rain. Darwin marveled at how he could entice the leaves to respond, timing them as they slowly curled their edges over fragments of proffered flies, roast meat, assorted seeds, pollen, albumen, and other treats. He also measured the roots, their scant size reinforcing his idea that they provide little nutrition for the plant: “We may, therefore, conclude that Pinguicula vulgaris, with its small roots, is not only supported to a large extent by the extraordinary number of insects which it habitually captures, but likewise draws some nourishment from the pollen, leaves, and seeds of other plants which often adhere to its leaves. It is therefore partly a vegetable as well as an animal feeder.”122
Pinguicula vulgaris—This plant grows in moist places, generally on mountains. It bears on an average eight, rather thick, oblong, light green leaves, having scarcely any footstalk. A full-sized leaf is about 1½ inch in length and ¾ inch in breadth. The young central leaves are deeply concave and project upwards; the older ones towards the outside are flat or convex and lie close to the ground, forming a rosette from 3 to 4 inches in diameter. The margins of the leaves are incurved. Their upper surfaces are thickly covered with two sets of glandular hairs, differing in the size of the glands and in the length of their pedicels. The larger glands have a circular outline as seen from above and are of moderate thickness; they are divided by radiating partitions into sixteen cells, containing light-green, homogeneous fluid. They are supported on elongated, unicellular pedicels (containing a nucleus with a nucleolus) which rest on slight prominences. The small glands differ only in being formed of about half the number of cells, containing much paler fluid, and supported on much shorter pedicels. Near the midrib, towards the base of the leaf, the pedicels are multicellular, are longer than elsewhere, and bear smaller glands. All the glands secrete a colourless fluid, which is so viscid that I have seen a fine thread drawn out to a length of 18 inches; but the fluid in this case was secreted by a gland which had been excited. The edge of the leaf is translucent and does not bear any glands; and here the spiral vessels, proceeding from the midrib, terminate in cells marked by a spiral line, somewhat like those within the glands of Drosera. …
A friend sent me on June 23 thirty-nine leaves from North Wales, which were selected owing to objects of some kind adhering to them. Of these leaves, thirty-two had caught 142 insects, or on an average 4.4 per leaf, minute fragments of insects not being included. Besides the insects, small leaves belonging to four different kinds of plants, those of Erica tetralix being much the commonest, and three minute seedling plants, blown by the wind, adhered to nineteen of the leaves. One had caught as many as ten leaves of the Erica. Seeds or fruits, commonly of Carex and one of Juncus, besides bits of moss and other rubbish, likewise adhered to six of the thirty-nine leaves. The same friend, on June 27, collected nine plants bearing seventy-four leaves, and all of these, with the exception of three young leaves, had caught insects; thirty insects were counted on one leaf, eighteen on a second, and sixteen on a third. …
We thus see that numerous insects and other objects are caught by the viscid leaves; but we have no right to infer from this fact that the habit is beneficial to the plant, any more than in the before given case of the Mirabilis, or of the horse-chestnut. But it will presently be seen that dead insects and other nitrogenous bodies excite the glands to increased secretion; and that the secretion then becomes acid and has the power of digesting animal substances, such as albumen, fibrin, &c. Moreover, the dissolved nitrogenous matter is absorbed by the glands, as shown by their limpid contents being aggregated into slowly moving granular masses of protoplasm. The same results follow when insects are naturally captured, and as the plant lives in poor soil and has small roots, there can be no doubt that it profits by its power of digesting and absorbing matter from the prey which it habitually captures in such large numbers. It will, however, be convenient first to describe the movements of the leaves.
That such thick, large leaves as those of Pinguicula vulgaris should have the power of curving inwards when excited has never even been suspected. It is necessary to select for experiment leaves with their glands secreting freely, and which have been prevented from capturing many insects; as old leaves, at least those growing in a state of nature, have their margins already curled so much inwards that they exhibit little power of movement, or move very slowly. …
We learn from [experiment] that the margins of the leaves curl inwards when excited by the mere pressure of objects not yielding any soluble matter, by objects yielding such matter, and by some fluids-namely an infusion of raw meat and a week solution of carbonate of ammonia. A stronger solution of two grains of this salt to an ounce of water, though exciting copious secretion, paralyses the leaf. Drops of water and of a solution of sugar or gum did not cause any movement. Scratching the surface of the leaf for some minutes produced no effect. Therefore, as far as we at present know, only two causes—namely slight continued pressure and the absorption of nitrogenous matter—excite movement. It is only the margins of the leaf which bend, for the apex never curves towards the base. The pedicels of the glandular hairs have no power of movement. …
… We have seen that when large bits of meat, or of sponge soaked in the juice of meat, were placed on a leaf, the margin was not able to embrace them, but, as it became incurved, pushed them very slowly towards the middle of the leaf, to a distance from the outside of fully .1 of an inch (2.54 mm.), that is, across between one-third and one-fourth of the space between the edge and midrib. Any object, such as a moderately sized insect, would thus be brought slowly into contact with a far larger number of glands, inducing much more secretion and absorption, than would otherwise have been the case. That this would be highly serviceable to the plant, we may infer from the fact that Drosera has acquired highly developed powers of movement, merely for the sake of bringing all its glands into contact with captured insects. So again, after a leaf of Dionaea has caught an insect, the slow pressing together of the two lobes serves merely to bring the glands on both sides into contact with it, causing also the secretion charged with animal matter to spread by capillary attraction over the whole surface. In the case of Pinguicula, as soon as an insect has been pushed for some little distance towards the midrib, immediate re-expansion would be beneficial, as the margins could not capture fresh prey until they were unfolded.
Leaf margin inflected over a row of small flies, left, and against two bits of meat, right
Pisum sativum. Watercolor by Elizabeth Blackwell, A Curious Herbal.