392 MR. T. W. WOODHEAD ON THE 
and other cell-walls. While examining sections of the shade-form 
of Heracleum, it appeared that the curvature of the upper wall 
of the epidermal cell seemed to be directly correlated with the 
form of the palisade-cells below it. The general effect of this 
curvature is to produce a central area of increased, but not 
uniform, light intensity, which is bounded by a shadow ; and as a 
consequence of this differential lighting the modifications in the 
palisade-cells seemed attributable. In the sun-form this boundary 
Figs. 61-64. 
ài 
D D 
ur E. 
Fig. 61. T. S. * Sun"-leaf of Heracleum Sphondylium. 
62. Surface view upper epidermis of ditto. 
63. T. S. * Shade "-leaf of H. Sphondylium. 
64. Surface view upper epidermis of ditto. 
of shadow may be of service in protecting the chlorophyll 
corpuscles from the injurious effects of intense light, as the 
mechanism is such that the chlorophyll corpuscles may circulate 
in an area of relative shadow. 
One of my students (Mr. J. W. H. Johnson), while kindly 
preparing sections for me, worked out in detail several interesting 
points with reference to these modified lens mechanisms, the 
results of which will be shortly published. Haberlandt’s 
recently published monograph (41) gives an excellent exposition 
of the lens mechanisms of epidermal cells. The form in Heracleum 
is of a somewhat different type to those figured by Haberlandt, 
and more recently by Guttenberg (40) in Adoxa, &c. 
