476 Wager . — The Perception of Light in Plants. 
merits were made with Tropaeolum majiis. They show clearly enough that 
in the case of the leaves of this plant the dry leaf is more sensitive to 
heliotropic stimulation than the wetted leaf ; in other words, the leaf in 
which the lens function is not interfered with is more sensitive than the one 
in which it is eliminated. This, however, does not prove that the lens 
function stands in direct causal relation to the heliotropic response. The 
power to respond when covered with water seems to vary with different 
leaves, and it is quite probable that the leaves of Tropaeolum majus are 
sensitive to water in some way which interferes with the heliotropic response 
other than elimination of the lens function. 
Nordhausen 1 brought about the elimination of the lens action by means 
of a thin layer of 5 to 10 per cent, of gelatine spread over the surface of the 
leaf. As the refractive index (1*341 in 5 per cent., 1-347 in 10 per cent, 
solution) is very near that of the cell contents, there can be no question of 
reversed illumination. Experimenting on similar plants to those used by 
Haberlandt, Nordhausen shows that the response to light is not inhibited, 
and therefore he concludes that the lens function is not directly connected 
with the perception of light by the leaf-blade. 
Haberlandt (loc. cit., 1909 ) criticizes Nordhausen’s experiments and 
shows that, as regards the lens function, leaves covered with a 5 to 10 per 
cent, solution of gelatine behave very much like those covered with water, 
and therefore in certain cases a light response would be obtained. 
From a few experiments which I have made to test the heliotropic 
response when leaves are submerged in water, results were obtained which 
seemed to show, when taken in conjunction with those already described by 
Haberlandt and others, that the evidence on the experimental side is at 
present so contradictory and unsatisfactory that a very much more complete 
investigation of the various functions involved will be necessary before any 
satisfactory conclusions can be arrived at. 
Ranunculus Ficaria. A complete plant was submerged in water and 
exposed to one-sided illumination. The leaf-stalks curved very markedly 
towards the light, and the leaf-blade was placed more or less at right angles 
to the falling light rays. The response was just as active and as rapid as in 
plants not submerged. Leaves removed from the plant did not respond so 
quickly, but in all young leaves there was a definite response in a few hours. 
The cells both of the upper and lower epidermis of the leaves are 
irregular in outline, with both inner and outer walls arched. With a small 
stop there is a slight differential illumination of the basal walls, but not with 
an open stop. Below the basal wall is an irregular light area surrounded 
by a dark one (Fig. 17), with brighter spots here and there where the cell- 
wall was slightly more curved. When the outer wall was covered with water 
1 Ueber die Bedeutung der papillosen Epidermis als Organ fur die Lichtperzeption der Laub- 
blatter. Ber. d. d. bot. Gesell., xxv, 398, 1907. 
