OF THE BRACKEN IN RELATION TO ENVIRONMENT. 661 
referring to figs. 1 and 2 (p. 663), though these diagrams repre- 
sent a somewhat different case to be described below, and are 
not quite extremes as regards dimensions. 
A few measurements may be given here. The thickness of 
the outer wall of the upper epidermis (taking the average for a 
few cases) is 5°7 w for leaves from very exposed situations *, and 
29 » for leaves from a very sheltered and shaded locality. The 
entire thickness of the leaf, again taking two similarly opposite 
types of habitat, is 318 » for the exposed one and 163» for the 
sheltered one. Thus the difference in both cases is roughly in 
the ratio of 2 t0 1. The exposed leaf may be called markedly 
xerophytic, chiefly on account of its hypoderm, while the other 
type of leaf is a distinct shade-leaf. 
In the exposed leaf the veins are accompanied by colourless 
cells, which have slightly or considerably thickened walls, and are 
in contact with the epidermis on the lower side and with the 
hypoderm on the upper side?. Owing to (1) the increased 
mechanical support of this arrangement, (2) the greater thick- 
ness of the leaf and of the walls of the upper epidermis, and 
(3) perhaps the somewhat denser mesophyll, the exposed leaf is 
very firm to the touch, while the sheltered leaf is quite soft. A 
considerable number of cases were tested with regard to this, and 
it was found possible to estimate the internal structure fairly well 
from the degree of hardness of the leaf. 
In various localities rough observations were made by walking 
through patches of brackenand feeling the leaves, with the purpose 
of discovering a plant whose leaves were manifestly much softer 
or much harder than was typical for the habitat; but, in localities 
belonging to the two extreme types referred to above, no such 
case was found. ; 
That the leaves of the same species should have a distinct 
hypoderm in some cases and practically none in others, Is an 
example of a rather wide range of structure, and, as will be 
described more fully below, the same range may be attained by 
different leaves of the same plant, or even by different pinne of 
the same leaf. Instances of a similar kind are on record; thus 
* This scarcely differs from the average thickness (5°5 ») found by Parkin 
and Pearson (“The Botany of the Ceylon Patanas, II,” Journ. Linn. Soc., 
Bot. vol. xxxv. (1903) p. 440) in the case of the flora of the Ceylon patents a 
t The midrib has a large amount of thick-walled tissue, which, in the shelter 
leaf, is less in amount and less thickened. 
