The Movements of Leaves

Fritz Mueller has sent me some additional observations on the movements of leaves, when exposed to a bright light.² Such movements seem to be as well developed and as diversified under the bright sun of Brazil, as are the well-known sleep or nyctitropic movements of plants in all parts of the world. This result has interested me much, as I long doubted whether paraheliotropic movements were common enough to deserve to be separately designated. It is a remarkable fact that in certain species these movements closely resemble the sleep movements of allied forms. Thus the leaflets of one of the Brazilian Cassiæ assume when exposed to sunshine nearly the same position as those of the not distantly allied Hæmatoxylon when asleep, as shown in Fig. 153 of “The Movements of Plants.”³ Whereas the leaflets of this Cassia sleep by moving down and rotating on their axes, in the same peculiar manner as in so many other species of the genus. Again, with an unnamed species of Phyllanthus, the leaves move forwards at night, so that their midribs then stand nearly parallel to the horizontal branches from which they spring; but when they are exposed to bright sunshine they rise up vertically, and their upper surfaces come into contact, as they are opposite. Now this is the position which the leaves of another species, namely Phyllanthus compressus, assume when they go to sleep at night.⁴ Fritz Müller states that the paraheliotropic movements of the leaves of a Mucuna,⁵ a large twining Papilionaceous plant, are strange and inexplicable; the leaflets sleep by hanging vertically down, but under bright sunshine the petiole rises vertically up, and the terminal leaflet rotates by means of its pulvinus through an angle of 180^o, and thus its upper surface stands on the same side with the lower surfaces of the lateral leaflets. Fritz Müller adds, “I do not understand the meaning of this rotation of the terminal leaflet, as even without such a movement it would be apparently equally well protected against the rays of the sun. The leaflets, also, on many of the leaves on the same plant assume various other strange positions.” With one species of Desmodium, presently to be mentioned as sleeping in a remarkable manner, the leaflets rise up vertically when exposed to bright sunshine, and the upper surfaces of the lateral leaflets are thus brought into contact. The leaves of Bauhinia grandiflora go to sleep at an unusually early hour in the evening, and in the manner described at p. 373 of “The Movements of Plants,” namely, by the two halves of the same leaf rising up and coming into close contact: now the leaves of Bauhinia Brasiliensis do not sleep, as far as Fritz Müller has seen, but they are very sensitive to a bright light, and when thus exposed the two halves rise up and stand at 45^o or upwards above the horizon.⁶

Fritz Müller has sent me some cases, in addition to those given in my former letter of March 3, of the leaves of closely-allied plants which assume a vertical position at night by widely different movements; and these cases are of interest as indicating that sleep-movements have been acquired for a special purpose. We have just seen that of two species of Bauhinia the leaves of one sleep conspicuously, while those of a second species apparently do not sleep at all. The leaves of Euphorbia jacquiniæflora depend vertically at night, whereas those of a dwarfish Brazilian species rise vertically up at night.⁷ The leaves of this Euphorbia stand opposite one another—a position which is rather rare in the genus; and the rising movement may be of service to the plant, as the upper surfaces of the opposite leaves mutually protect one another by coming into contact. In the genus Sida the leaves of two species rise, while those of a third Brazilian species sink vertically down at night.⁸ Two species of Desmodium are common plants near Fritz Müller’s house: in one the leaflets move simply downwards at night; but in the other not only do the three leaflets move vertically down, while the main petiole rises vertically up, as is likewise the case with D. gyrans, but in addition the lateral leaflets rotate so as to stand parallel with the terminal leaflet, behind which they are more or less completely hidden.⁹ This, as far as I have seen, is a new kind of nyctitropic movement; but it leads to a result common to several species, namely, that of packing the three leaflets closely together and placing them in a vertical position.

Charles Darwin

Down, Beckenham, Kent, April 14