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Show 284 MODIFIED CIRCUMNUTATION. CHAP. VI. 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 pulvin~s _is present;_* b~t this statement applies only to penodw and nyctitr?p1c movements, as may be inferred from other cases giVen by Frank.t . 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 o~e point in common, that ~heu upper_ ~urfaces avmd t'acing the zenith, often with th~ additw~al fact that they come into close contact. with opposite leaves or leaflets, clearly indicates, as 1~ seems to us, that the object gained is the prote~twn of t~e _uppe~ sur· faces from being chilled at night ·by radmtwn. rh.ere is nothing improbable in the upper surface needmg 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.:j: Seedlings are often protected from radiation by a very thin covering of straw; and fruit-trees on walls by a few fir-branches, o~· oven ~y a fishing-net, suspended over them. Th_ere I.s a van~ty of the gooseberry,§ the flowers of whwh from bemg produced before the leaves, are not protecte~ by them from radiation, and consequently often fail to yield fruit. .An excellent observer II has remarked * Pfeffer, 'Die Period. Beweg. der Blattorgane.' 1875, p. 159. t 'Die Nat. Wauercchte Rich· tung von Pflanzentheilen,' 1870, p. 52. t Martins in 'Bull. Soc. Bot. de France,' tom. xix. 1872. Wells, in his famous ' Essay on Dew ' remarks that nn exposed tber~omoter rises as sooa us even a fleecy cloud, hig_h in the sky, pas::,es over th0 zemth. , § , Loudon's Gardener's :Mag., vol. iv. 1828_, p. 11~. , G dener's 11 Mr. H1vers m ar Chron.,' 1866, p. 73~. CHAP. VI. USE OF SLEEP MOVEMENTS. 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 Linnrous, had the principle of radia~ tion 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, Desmodiurn 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 tho surrounding air, that it could hardly make any sensible difference ~o them, whether they stood horizontally and radiated ~nto the open sky, or vertically and radiated chiefly In a lateral direction towards neighbouring plants and ot~er objects. We endeavoured, therefore, to a.scertam something on this head by preventing the leaves ol•• :::J I II • •• t •, ::; I :l· tl .•." .I ,", 1,11 |