Chap. VI. USE OF SLEEP MOVEMENTS. 287 



thin pins, so as to stand from J to f inch above the 

 cork. This difference in the result, which is in itself 

 curious as showing what a very slight difference in 

 the conditions iniluences the amount of injury in- 

 flicted, may be attributed, as we believe, to the sur- 

 rounding warmer air not circulating freely beneath the 

 closely pinned leaves and thus slightly warming them. 

 This conclusion is supported by some analogous facts 

 hereafter to be given. 



We will now describe in detail the experiments 

 which were tried. These were troublesome from our 

 not being able to predict how much cold the leaves of 

 the several species could endure. Many plants had 

 every leaf killed, both those which were secured in 

 a horizontal position and those which were allowed to 

 sleep — that is, to rise up or sink down vertically. 

 Others again had not a 'single leaf in the least in- 

 jured, and these had to be re-exposed either for a 

 longer time or to a lower temperature. 



Oxalis acetosella. — A very large pot, thickly covered with 

 between 800 and 400 leaves, had been kept all winter in the 

 greenhouse. Seven leaves were pinned horizontally open, 

 and were exposed on March 16th for 2 h. to a clear sky, the 

 temperature on the surrounding grass being - 4° C. (24° to 

 25° F.). Next morning all seven leaves were found quite 

 killed, so were many of the free ones which had previously 

 gone to sleep, and about 100 of them, either dead or browned 

 and injured, were picked off. Some leaves showed that they 

 had been slightly injured by not expanding during the whole 

 of the next day, though they afterwards recovered. As all the 

 leaves which were pinned open were killed, and only about a 

 third or fourth of the others were either killed or injured, we 

 had some little evidence that those which were prevented from 

 assuming their vertically dependent position suffered most. 



The following night (17th) was clear and almost equally cold 

 (— 3° to — 4° 0. on the grass), and the pot was again exposed 

 but this time for only 30 m. Eight leaves had been pinned out, 



