236 REV. G. HENSLOW ON THE ORIGIN OF PLANT-STRUCTURES 
M. Dufour* found experimentally that the thickness of the 
external and lateral walls of epidermal cells is greater under sun- 
light than is that of the same species when grown in the shade; 
and the cuticle is also much more developed under sunlight. 
It need hardly be pointed out that in the desert the sunlight 
and glare reflected from the sand are very powerful, and therefore, 
ceteris paribus, the intensification of those elements mentioned 
is just what would be, on @ priori grounds, expected from 
M. Dufour’s experiments. But the thickness of the cuticle 
tends powerfully to prevent the loss of water, which is the end 
and aim of all desert plants in their adaptations to the climate. 
Dr. Volkens + observed that a considerable number of xero- 
phile plants are protected against a too energetic transpiration 
by the existence on the surface, outside the cuticle, of a thick 
layer of a sort of varnish, very brilliant in appearance. It is in 
great part soluble in alcohol, and without doubt of a resinous 
nature. An interesting fact, from the point of view of geogra- 
phical botany, is that such plants are peculiar to the southern 
hemisphere. 
The presence of wax on the surface of leaves was long ago 
observed by Mulder to be closely correlated with chlorophyll ; 
so that when we find the palisadic layers much increased, as 
they are in desert plants, we should, @ priori, expect a specially 
increased Jayer of wax. The production of this substance is 
dependent ultimately upon the increased amount of light [and 
heat ?|, which appears to deoxidize chlorophyll, and to leave wax 
as one of the products ; hence the epidermis becomes colourless 
and the cuticle coated with wax, as the wax increases with the 
loss of water. Upon this Mr. Herbert Spencer observes :—“ The 
deposit of waxy substances next to the outer surface of the 
cuticular layer in leaves is probably initiated by the evaporation 
[transpiration ?] which it eventually checks ” +. 
When the external walls of epidermal cells are flat, the cuticle 
is generally smooth; when, on the contrary, each cell forms a 
convexity towards the exterior, it is nearly always ornamented, 
either with parallel and straight or undulating strie, or they 
may be more or less reticulated, &. The above features 
* Ann. des Sci. Nat., Bot. sér. 7, tom. v. p. 311, t. viii. 
t “Ueber Pflanzen mit lackirten Blattern,” Berichte der deutschen botan. 
Gesellschaft, Bd. viii. Heft 4, p- 120 (1890). 
} ‘ Principles of Biology,’ ii. p. 245. 
