Sleeping Plants 397 
applied to the root; gold-beaters skin being by itself almost with- 
out effect. 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. There is however some evidence that very close contact 
with a solid body, such as a curved fragment 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 were 
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 brought before the world in the Power of Move- 
ment? 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, 1897, p. 81. 
? In 1881 Professor Wiesner published his Das Bewegungsvermégen der Pflanzen, a 
book devoted to the criticism of The Power of Movement in Plants. A letter to Wiesner, 
published in Life and Letters, m1, p. 336, shows Darwin’s warm appreciation of his critic’s 
work, and of the spirit in which it is written. 
