1909] CURRENT LITERATURE 473 
that the experimental work there leaves much to be desired, while the anatomical 
relations have been pretty fully cleared up. He discusses, therefore, the investi- 
gations that have attempted to eliminate the lens action of the papillose epidermis 
by making plane the surface with water, oil, gelatin, etc.; after which he addresses 
himself to the question of the residual lens action of the epidermal cells. He 
shows that in all cases with oblique illumination there remains an unequal excen- 
tric distribution of the brightness on the inner walls of the epidermal cells, though 
of less intensity than with dry epidermis. Inquiring into the sensitiveness of 
the cells in distinguishing differences of illumination, it appears that the more 
sensitive leaves have about the same capacity, 1/30, as the human eye under 
ordinary circumstances, the extremes being 1/75 and 1/12.5. But the differences 
of illumination in the case of papillose epidermis, even when wetted, is far above 
these figures, and, as HABERLANDT proceeds to show, is often greater even in 
leaves that have the epidermal cells plane outside and concave on the inner end. 
In the latter case the differences were from 1/15 to 10/1, according to the obliquity 
of the light. 
After discussing the previous more or less contradictory results of the wetting 
method, in which some of the experimental leaves were not in condition to per- 
ceive the direction of the light and to respond to it, he describes his new method. 
This consists of wetting only a part of a leaf of Tropaeolum with water made 
plane with a thin sheet of mica, and leaving the rest dry. A zone between the 
wet and dry portions is darkened with a paper screen, and the petiole is also 
properly protected. Then the two regions are illuminated obliquely from con- 
trary directions. By varying the relative areas of the wet and dry regions, and 
the intensity of the illumination, it is easy to determine the relative efficiency 
of the epidermis under the two conditions. When the areas are equal and the 
wet area receives double the intensity of light, its direction on the dry side con- 
trols the response. When the intensities are equal and the wet area is 2.2-4.8 
times the dry, the latter still dominates the reaction. 
We translate the latest statement of HABERLAND?’s theory: “The perception 
of the direction of light results, according to my theory, from the differences 
in brightness or the different distribution of intensity upon the inner walls of the 
epidermis, which may be brought about in various ways. The smaller differ- 
ences of brightnesss may be produced by the mere convexity of the inner walls 
of the epidermis. These eased iment as a = (miissen die Reizschwella 
erreichen) if the outer wall tion of the light is thus excluded; 
and they may suffice even if the normal collection of light by convex outer walls 
is made impossible by wetting. Then even the wet leaf perceives the direction 
of the light and returns, although slowly and generally incompletely, to the usual 
light position. In these cases the epidermal cells act as optical stimulators, and 
and though they may be dispensed with to a certain extent, the promptness and 
precision of the movement are enhanced by their effect in increasing the intensity 
of the stimulus. This action is especially valuable in the last phases of the adjust- 
ment, when by the diminution of the angle of incidence, the differences of bright- 
