Sleejnng Plants 397 



applied to the root ; gold-beaters skin being by itself almost with- 

 out eiFect. But it must be allowed that, as regards touch, it is not 

 clear how the addition of shellac and card can increase the degree of 

 contact. Tliere is however some evidence that very close contact 

 with a solid body, such as a curved fi-agnient of glass, produces 

 curvature : and this may conceivably be the explanation of the effect 

 of gold-beaters skin covered with shellac. But on the whole it is 

 perhaps safer to classify the shellac experiments with the results of 

 undoubted injury rather than with those of contact. 



Another subject on which a good deal of labour was expended 

 is the sleep of leaves, or as Darwin called it their nyctitropic 

 movement. He showed for the first time how widely spread this 

 phenomenon is, and attempted to give an explanation of the use to 

 the plant of the power of sleeping. His theory was that by becoming- 

 more or less vertical at night the leaves escape the chilling effect of 

 radiation. Our method of testing this view was to fix some of the 

 leaves of a sleeping plant so that they remained horizontal at night 

 and therefore fully exposed to radiation, while their fellows Avere 

 partly protected by assuming the nocturnal position. The experi- 

 ments showed clearly that the horizontal leaves were more injured 

 than the sleeping, i.e. more or less vertical, ones. It may be objected 

 that the danger from cold is very slight in warm countries where 

 sleeping plants abound. But it is quite possible that a lowering of 

 the temperature which produces no visible injury may nevertheless 

 be hurtful by checking the nutritive processes (e.g. translocation of 

 carbohydrates), which go on at night. Stahl^ however has ingeniously 

 suggested that the exposure of the leaves to radiation is not directly 

 hurtful because it lowers the temperature of the leaf, but indirectly 

 because it leads to the deposition of dew on the leaf-surface. He 

 gives reasons for believing that dew-covered leaves are unable to 

 transpire efficiently, and that the absorption of mineral food-material 

 is correspondingly checked. Stahl's theory is in no way destructive 

 of Darwin's, and it is possible that nyctitropic leaves are adapted 

 to avoid the indirect as well as the direct results of cooling by radia- 

 tion. 



In what has been said I have attempted to give an idea of some 

 of the discoveries brouglit before the world in the Power of Move- 

 nieut^ and of the subsequent history of the problems. We must now 

 pass on to a consideration of the central thesis of the book, — the 

 relation of circumnutation to the adaptive curvatures of plants. 



1 Bot. Zeitung, 18'J7, p. 81. 



' In 1881 Professor Wiesner publiahed his Das Bewegungsverm'dgen dcr PJlaiixen, a 

 book devoted to the criticism of The Power of Movement in Plants. A letter to Wiesner, 

 published in Life and Letters, in. p. 336, shows Darwin's warm appreciation of his critic's 

 work, and of the spirit in which it is written. 



