284 MODIFIED CIECUMNUTATION. 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 pulvinus is present ; * but 

 this statement applies only to periodic and nyctitropic 

 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 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 sur- 

 faces 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.J 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 radiatipn, and consequently often fail to 

 yield fruit. An excellent observer || has remarked 



* Pfeffer, 'Die Period. Beweg. Dew,' remarks that an exposed 



dor Blattorgane.' 1875, p. 159. thermometer rises as soon as even 



t ' Die Nat. Wagereehte Eich- a fleecy cloud, higli in the sky, 



tung von Pflanzentheiltn,' 1870, passes over the zenith. 



p. 52. § 'Loudon's Gardener's Mag.,' 



i Martins in 'Bull. Soc. Bot. vol. iv. 1828, p. 112. 



de France,' torn. xix. 1872. || Mr. Rivers in 'Gardener's 



Wells, in his famous 'Essay on Ghron.,' 1866, p. 732. 



