Now I want to tell you, for my own pleasure, about the movements of Desmodium.
1. When the plant goes to sleep, the terminal leaflets hang vertically down, but the petioles move up towards the axis, so that the dependent leaves are all crowded round it. The little leaflets never go to sleep, and this seems to me very odd; they are at their games of play as late as 11 o'clock at night and probably later. (718/1. Stahl ("Botanische Zeitung," 1897, page 97) has suggested that the movements of the dwarf leaflets in Desmodium serve to shake the large terminal leaflets, and thus increase transpiration. According to Stahl's view their movement would be more useful at night than by day, because stagnation of the transpiration-current is more likely to occur at night.)
2. If the plant is shaken or syringed with tepid water, the terminal leaflets move down through about an angle of 45 deg, and the petioles likewise move about 11 deg downwards; so that they move in an opposite direction to what they do when they go to sleep. Cold water or air produces the same effect as does shaking. The little leaflets are not in the least affected by the plant being shaken or syringed. I have no doubt, from various facts, that the downward movement of the terminal leaflets and petioles from shaking and syringing is to save them from injury from warm rain.
3. The axis, the main petiole, and the terminal leaflets are all, when the temperature is high, in constant movement, just like that of climbing plants. This movement seems to be of no service, any more than the incessant movement of amoeboid bodies. The movement of the terminal leaflets, though insensible to the eye, is exactly the same as that of the little lateral leaflets--viz. from side to side, up and down, and half round their own axes. The only difference is that the little leaflets move to a much greater extent, and perhaps more rapidly; and they are excited into movement by warm water, which is not the case with the terminal leaflet. Why the little leaflets, which are rudimentary in size and have lost their sleep-movements and their movements from being shaken, should not only have retained, but have their spontaneous movements exaggerated, I cannot conceive. It is hardly credible that it is a case of compensation. All this makes me very anxious to examine some plant (if possible one of the Leguminosae) with either the terminal or lateral leaflets greatly reduced in size, in comparison with the other leaflets on the same leaf. Can you or any of your colleagues think of any such plant? It is indirectly on this account that I so much want the seeds of Lathyrus nissolia.
I hear from Frank that you think that the absence of both lateral leaflets, or of one alone, is due to their having dropped off; I thought so at first, and examined extremely young leaves from the tips of the shoots, and some of them presented the same characters. Some appearances make me think that they abort by becoming confluent with the main petiole.
I hear also that you doubt about the little leaflets ever standing not opposite to each other: pray look at the enclosed old leaf which has been for a time in spirits, and can you call the little leaflets opposite? I have seen many such cases on both my plants, though few so well marked.
LETTER 719. TO J.D. HOOKER. Down, October 23rd [1873].
How good you have been about the plants; but indeed I did not intend you to write about Drosophyllum, though I shall be very glad to have a specimen. Experiments on other plants lead to fresh experiments. Neptunia is evidently a hopeless case. I shall be very glad of the other plants whenever they are ready. I constantly fear that I shall become to you a giant of bores.
I am delighted to hear that you are at work on Nepenthes, and I hope that you will have good luck. It is good news that the fluid is acid; you ought to collect a good lot and have the acid analysed. I hope that the work will give you as much pleasure as analogous work has me. (719/1.