442 MESSRS. J. PARKIN AND H. H. W. PEARSON ON THE 
Thus the arched wall seems to be associated with a thinner 
outer wall and a rather deeper cell. A comparison may perbaps 
be drawn between these arched outer walls and the furrowed or 
wrinkled cuticles which are frequently found in the glabrous 
leaves of insolated plants. With regard to the latter, it has been 
suggested * that the irregularities of the surface effect the dis- 
persion of some of the light falling on the leaf, and so constitute 
a means of protection against excessive illumination. Some of 
the light-rays would undoubtedly undergo total reflection from 
the curved surfaces of these epidermal cell-walls, and it seems not 
unreasonable to ascribe to them a similar function. 
(6) Waviness of the Lateral Epidermal Walls. 
In shade-leaves the lateral walls of the lower epidermis are 
usually more wavy than those of the upper. Of the plants in 
question nineteen only have the side-walls of the upper epidermal 
cells sinuous, while in forty-four, rather more than half, this 
character is shown by the cells of the lower surface. A comparison 
has been made between wavy and straight walls with regard to the 
depth of the epidermal cell and the thickness of the outer wall, 
including both surfaces of the leaf. 
Depth of cell—average for leaves with straight lateral walls 25°7 p. 
. ne) ” wavy ” ” 23'4 p- 
Thickness of outer wall _,, ” » straight ,, » dom 
»  » » wavy ” 4 £24 
Considering that a straight lateral wall is a characteristic 
of sun-leaves, it might be expected that this would be accom- 
panied by a thicker outer wall and perhaps by a deeper 
epidermis, as this often has a water-storing function; the above 
numbers bear this out to some extent. Perhaps the thinner the 
side-wall, the greater is the inclination to be wavy. Exact 
measurements, however, have not been made to ascertain how 
this idea is supported, on the whole, by this series of plants. In 
some instances it is certainly not the vase. Anaphalis oblonga 
and Blumea flexuosa, for example, have thick, yet wavy UPP® 
epidermal lateral walls. Hairiness does not seem to influence 
directly this sinuosity. Haberlandtt considers the wavy side- 
wall to be a help in withstanding strains and bendings, a8 well 
* Kearney, Contr. U.S. Herb. v. n. 6 (1901), p. 389. 
+ Haberlandt, Phys, Pflanz.-Anat. (1896) pp. 103 & 105. 
