473 
Wager. — The Perception of Light in Plants. 
epidermal cells and the lower show a convergence, due partly to the slightly 
arched outer and inner walls, partly to an irregular sculpturing in the middle 
region of the outer wall. This produces a number of little spots and lines 
of light which unite together to form a more or less homogeneous band of 
light which falls on the lower wall. The position which the leaves take up 
is, however, as C. and F. Darwin have shown (loc. cit., p. 269), probably 
mainly due to epinastic growth, and seems ‘ to be very little affected by 
geotropism or heliotropism \ 
Experimental Observations. 
The experimental observations upon which Haberlandt’s hypothesis are 
based consist in the elimination of the lens function either by submerging 
the leaf in water, or by covering the surface of the leaf with a layer of water 
under a thin strip of mica. In all cases where the leaf was properly wetted 
and the lens function eliminated, Haberlandt found that the heliotropic 
movements were inhibited (loc. cit., 1905). In Tropaeolum majus the leaves 
have a waxy covering which prevents wetting, and thus allows the lens 
function to be maintained even when submerged ; the leaves are, under these 
conditions, still able to respond to the light stimulus. If the surface of the 
leaf is washed in dilute alcohol, however, it is easily wetted ; its lens function 
is eliminated, and it no longer responds to the light. These experiments 
appear to be conclusive, but more recent observations show that leaves do 
not always behave, in this way, and that some other factors than the elimina- 
tion of the lens function must be taken into account in explaining the loss 
of heliotropic response. 
Fitting 1 suggested that possibly some deep-seated disturbance in the 
life of the plant had been brought about by the water. Haberlandt, 2 how- 
ever, showed that a leaf wetted on the under side exhibited no loss of 
power to reach the fixed light position. This does not prove that some 
modification may not be brought about by the contact of water with the 
upper surface. That it is extremely important the leaf should not remain 
wet for any length of time is seen in the very perfect arrangements, such as 
waxy bloom, smooth epidermis, &c., for preventing it. Haberlandt 3 himself 
shows that when a leaf has been covered with water for some time, it is 
either not able to gain its power of response to light, or only imperfectly, 
although the lens function remains as good as ever. 
In a later experiment with leaves of Tropaeolum majus. Haberlandt 4 
found, however, that when the upper surface only of the leaf was wetted he 
1 Bot. Zeit, 1906. 
2 Die Bedeutung der papillosen Laubblattepidermis fiir die Lichtperzeption. Biol. Centralbl., 
xxvii, 297, 1907. 
3 Ein experiment taler Beweis fiir die Bedeutung der papillosen Laubblattepidermis als Licht- 
sinnesorgan. Ber. d. d. bot. Gesell., xxiv, 361-6, 1906 ; and Bedeutung, &c., p. 297. 
4 Bedeutung, &c., p. 300. 
K k 2 
