214 THE PRESIDENTIAL ADDRESS. (Sess. uxxr. 
has grown plants at a window knows that the leaf-surfaces 
always turn to the light. How does the plant perceive the 
differences in illumination to which it reacts? Recent work 
seems to bring us nearer an explanation... The epidermis or 
skin of the leaf consists of translucent cells without chloro- 
phyll; they are fuil of sap of varying refractive index, and 
the walls are flat or curved in different degree, or thickened 
in places. These cells by their construction act as lenses of 
kinds, and are the light-sense-organs of the plant. When 
light strikes an epidermal cell of a normally expanded leaf 
vertically, a brightly illuminated field is formed in the centre 
of the protoplasmic lining of the inner wall of the epidermal 
cell, its intensity and sharpness varying with the optical 
character of the cells as lenses: around this central lighit- 
spot isa darker zone. To this the leafis attuned. When 
the direction of incident light is changed, the position of the 
bright field on the inner w all is displaced, and the difference 
is perceived by the plant—the change acts as the stimulus 
which starts the movement of the leaf to bring its upper 
surface again at right angles to the meident light. The 
degree in which the epidermal cells are constructed as lenses 
varies; sometimes single cells only so act. This view of 
light-perception, only lately put forward, appears to be well- 
founded, and it opens up a wide field for further investigation 
and explains many well-known features of leaves. As an 
example: the velvety surface of the leaves of many wood- 
plants is caused by numberless papille, and capping each of 
these there are now found light-sense-organs which are thus 
raised to escape submergence in water lying on the leat- 
surface by which the working of their optical apparatus 
would be interfered with. Similarly, the waxy coating of 
leaves, which prevents their being wetted, is apparently an 
arrangement to prevent their being blinded by rain, and the 
blinding of leaves by dust is probably one of the causes of 
failure of tree-growth in towns. That plants possess such 
light-sense-organs may surprise us, but in them we again 
find a parallel development with the ccelli of some of the 
lower members of the animal kingdom. 
The later part of the address was devoted to the subject 
of convergence of form (homoplasy) and divergence of form 
of plants. A series of examples were shown of similar 
