There is, however, one important distinction between them, namely, that movements effected by growth on the alternate sides, are confined to young growing leaves, whilst those effected by means of a pulvinus last for a long time. We have already seen well-marked instances of this latter fact with cotyledons, and so it is with leaves, as has been observed by Pfeffer and by ourselves. The long endurance of the nyctitropic movements when effected by the aid of pulvini indicates, in addition to the evidence already advanced, the functional import-
* This distinction was first pointed out (according to Pfeffer, 'Die Periodischen Bewegungen der Blattorgane,' 1875, p. 161) by Dassen in 1837.
** 'Flora,' 1873, p. 433.
*** 'Bot. Zeitung,' 1879, Dec. 19th, p. 830.
[page 284] ance of such movements to the plant. There is another difference between the two sets of cases, namely, that there is never, or very rarely, any torsion of the leaves, excepting when a pulvinus is present;* but this statement applies only to periodic and nyctitropic movements as may be inferred from other cases given by Frank.** The fact that the leaves of many plants place themselves at night in widely different positions from what they hold during the day, but with the one point in common, that their upper surfaces avoid facing the zenith, often with the additional fact that they come into close contact with opposite leaves or leaflets, clearly indicates, as it seems to us, that the object gained is the protection of the upper surfaces from being chilled at night by radiation. There is nothing improbable in the upper surface needing protection more than the lower, as the two differ in function and structure. All gardeners know that plants suffer from radiation. It is this and not cold winds which the peasants of Southern Europe fear for their olives.*** Seedlings are often protected from radiation by a very thin covering of straw; and fruit-trees on walls by a few fir-branches, or even by a fishing-net, suspended over them. There is a variety of the gooseberry,**** the flowers of which from being produced before the leaves, are not protected by them from radiation, and consequently often fail to yield fruit. An excellent observer***** has remarked
* Pfeffer, 'Die Period. Beweg. der Blattorgane.' 1875, p. 159.
** 'Die Nat. Wagerechte Richtung von Pflanzentheilen,' 1870, p. 52
*** Martins in 'Bull. Soc. Bot. de France,' tom. xix. 1872. Wells, in his famous 'Essay on Dew,' remarks that an exposed thermometer rises as soon as even a fleecy cloud, high in the sky, passes over the zenith.
**** 'Loudon's Gardener's Mag.,' vol. iv. 1828, p. 112.
***** Mr. Rivers in 'Gardener's Chron.,' 1866, p. 732. [page 285]
that one variety of the cherry has the petals of its flowers much curled backwards, and after a severe frost all the stigmas were killed; whilst at the same time, in another variety with incurved petals, the stigmas were not in the least injured.
This view that the sleep of leaves saves them from being chilled at night by radiation, would no doubt have occurred to Linnaeus, had the principle of radiation been then discovered; for he suggests in many parts of his 'Somnus Plantarum' that the position of the leaves at night protects the young stems and buds, and often the young inflorescence, against cold winds. We are far from doubting that an additional advantage may be thus gained; and we have observed with several plants, for instance, Desmodium gyrans, that whilst the blade of the leaf sinks vertically down at night, the petiole rises, so that the blade has to move through a greater angle in order to assume its vertical position than would otherwise have been necessary; but with the result that all the leaves on the same plant are crowded together as if for mutual protection.
We doubted at first whether radiation would affect in any important manner objects so thin as are many cotyledons and leaves, and more especially affect differently their upper and lower surfaces; for although the temperature of their upper surfaces would undoubtedly fall when freely exposed to a clear sky, yet we thought that they would so quickly acquire by conduction the temperature of the surrounding air, that it could hardly make any sensible difference to them, whether they stood horizontally and radiated into the open sky, or vertically and radiated chiefly in a lateral direction towards neighbouring plants and other objects.