440 MESSRS. J. PARKIN AND H. H. W. PEARSON ON THE 
other extreme are Careya arborea, Embelia viridiflora, and 
Osyris arborea; in these cases the mesophyll is over twenty 
times as thick as the upper epidermis. 
(2) Thickness of the Outer Wall of the Upper Epidermis. 
The average is 5'5 p. Only thirteen plants have a decidedly 
thin outer wall—below 3°5 »—and this is connected with a hairy 
upper surface in all but three instances, viz., Cassia mimusoides, 
Hypericum mysorense, and Owalis corniculata, which have 
glabrous leaves. Twenty-one plants have the outer wall between 
7 and 10°5 p, and in six others it is more than 10°5 p. About 
the thickest outer wall—14°3 p—is exhibited by Oldenlandia 
Heynet (Pl. 11. fig. 4). 
A glabrous upper surface is the rule. About a quarter of these 
plants have a hairy one, and of these only two are anomalous in 
having this associated with a thick outer wall. These are 
Knoxia platycarpa var. hirsuta and Woodfordia floribunda, 
neither of which, however, is very hairy on the upper surface. 
The average thickness of the outer wall in leaves with glabrous 
upper epidermis is 6°3 », and with hairy upper epidermis 3°6 p. 
As a rule the cuticle is quite distinct. Often half, or even 
more, of the whole thickness of the wall is cuticularized (Pl. 12. 
figs. 1 & 2). Exact measurements of the two constituents of 
the wall have, however, not been made. In two or three cases 
the cuticularized portion penetrates the side-walls, e. g. Gaultheria 
fragrantissima and Rhodomyrtus tomentosa, In the first instance 
(Pl. 12. fig. 5) the pegs of cuticularized cellulose, as shown in 
section, reach to nearly the base of the side-walls ; in the second 
(Pl. 12. fig. 4) rather more than halfway. Some thick outer 
walls have, however, only a relatively narrow cuticularized part, 
e. g., Bupleurum mucronatum (P). 11. fig. 1), Oldenlandia 
Heynei (Pl. 11. fig. 4), and Hedyotis Lawsonie (Pl. 11. fig. 2). 
(3) Depth of the Lower Epidermis (exclusive of the thickness of 
its outer wall). 
The lower epidermis shows as a rule, as one might expects 
shallower cells with thinner outer walls. Sometimes, however 
it is nearly or quite similar to the upper epidermis, and this 
likeness, as the table shows, is usually associated with an erect 
or semi-erect attitude of the leaf. 
The average depth of the lower epidermis is 18°5 p, not much 
more than half that of the upper. In no case does it exceed at 
