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Show 286 :MODIFIED CIRCUMNUTA'l'ION. CIIAl'. VI. of several plants from going to sloop, and by exposing to a clear sky when the temperature was Leneath the freezing-point, thes , as well as tho oth 'r leaves on the same plants which had already assumed their nocturnal vertical positio11. Our xperimcnts show that leaves thus compelle<l to remain horizontal at night, suffered much more injury from frost than those which were allowed to assume their normal vertical position. It may, however, be said thnt conclusions drawn from such observations are not applicable to sleeping plants, the inbtLitants of countries where frosts do not occur. But in every country, and at all seasons, leaves must Le exposed to nocturnal chills through ra<liation, which might bo in some degree injurious to them, and which they would escape by assuming a vertical position. In our experiments, leaves were prevented from assuming theil' nyctitropic position, gcncmll y by being fastened with the £nest entomological pins (which did not sensibly injure them) to thin sheets of cork snppOI'ted on sticks. But in some instances they wore fastened clown by narrow strips of card, and in others by their petioles being vasscd through slits in the cork. 'l'he loaves were at first f,Lstcnecl close to the cork, for as this is a bad conductor, and as the leaves were not exposed for long periods, we thought that the cork, which had be n k pt in the bouse, would very slightly warm them; so that if they were inju.rcd by the frost in a greater degree than the free vertwal leaves, the evidence would be so much tho stronger that the horizontal position was injurious. B~1t we found that when there was any "light difl'oronco m the result, which could be detecto<l only occn,sionally, the leaves which had been fastened closely down suffered rather more than those fu.stened with very long and CRAP. VI. USE OF SLEEP MOVEl\1ENTS. 287 thin pins, so as to stand from ! to 'i inch above the cork. This difference in the result, which is in itself curious as showing what a very slight difference in t~e conditions infh~ences the amount of injury inflicted, may be attnbutcd, as we bclicv , to the surrounding warmer air not circulating fr ely beneath the closely pinned leaves and thus slightly warming them. This conclusion is supported by some analogous facts hereafter to be given. ~e will no~v d scribe in detail the experiments whiCh were tncd. These were troublesome from our not being able to pr diet how much cold the leaves of the several speci s could endure. 1.\-Iany plants had ~very leaf killed., both those which w re s cured in ~horizontal position and those which w re allowed. to sleep-that. is, to rise up or sink down vertically. ?thers agam had. not a single leaf in the least inJUred, a~d. these had to be rc-cxposcd. either for a longer time or to a lower temperature. Oa;alis, acetosella.-A very large pot, thickly covered with between oOO and 400 loaves, had been kept all winter in tho greenhouse. Seven leaves were pinned horizontally open and were exposed on March 16th for 2 h. to a clear sky th~ ~~p;rature on tho . s~urounding grass being - 4° c. (24o to kill .). Ne~t mornm~ all seven leaves were found quito ed, so were many of the free ones which had previously :~e .t~ sleep, and about 100 of them, either dead or browned had :ured~ were .P~cked off. Some leaves showed that they of th en shghtly lllJured by not expanding durino- tho whole leave: ne~t day, thou.gh they afterwards recovered. 0 As all tho thi d W~lCh were pmned open were killed, and only about a h dr or 10urth of th e o the rs wer.e e1· t 11 or ki.l led or l.l lJ.U red w<e : so.me litt~e evidence that those which wore prevented f~·om sTumh Ing then vertically dependent !)Osition suflcred most (- aoe foil · · ' · t _ow;ng mght (17th) was clear and almost equally cold but thi 0 t' 4 0. on the grass), and tho pot was again exposed s Ime for only 30m. Eight leaves had been pinned out: I '' :_., .. ,,: I ,,, ,... ': il |