"Power of
Movement" that injuring the root-tip on one side, by cutting or burning
it, induced a similar curvature. On the other hand it was shown that
curvature could be produced in roots by cementing cards, not to the
naked surface of the root-tip, but to pieces of gold-beaters skin
applied to the root; gold-beaters skin being by itself almost without
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 from 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 experiments 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 ("Bot. Zeitung", 1897, page
81.) 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 Darwi