286 MODIFIED ClftCUMNUTATION. CHAP. VI 



of several plants from going to sleep, and by exposing 

 to a clear sky when the temperature was beneath 

 the freezing-point, these, as well as the other leaves 

 on the same plants which had already assumed their 

 nocturnal vertical position. Our experiments show 

 that leaves thus compelled 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 that 

 conclusions drawn from such observations are not 

 applicable to sleeping plants, the inhabitants of 

 countries where frosts do not occur. But in every 

 country, and at all seasons, leaves must be exposed to 

 nocturnal chills through radiation, which might be in 

 some degree injurious to them, and which they would 

 escape by assuming a vertical position. 



In our experiments, leaves were prevented from 

 assuming their nyctitropic position, generally by 

 being fastened with the finest entomological pins 

 (which did not sensibly injure them) to thin sheets 

 of cork supported on sticks. But in some instances 

 they were fastened down by narrow strips of card, 

 and in others by their petioles being passed through 

 slits in the cork. The leaves were at first fastened 

 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 been kept in the house, would 

 very slightly warm them ; so that if they were injured 

 by the frost in a greater degree than the free vertical 

 leaves, the evidence would be so much the stronger 

 that the horizontal position was injurious. But we 

 found that when there was any slight difference in the 

 result, which could be detected only occasionally, the 

 leaves which had been fastened closely down suffered 

 rather more than those fastened with very long and 



