662 MR. L. A. BOODLE ON THE STRUCTURE OF LEAVES 
Stahl * mentions that in Ilex Aquifolium the sun-leaves have a 
continuous hypoderm, while, in the shade-leaves, hypoderm is 
differentiated only in the neighbourhood of the midrib, the stronger 
veins and the margin of the leaf. The differences relating to the 
thickness of the leaf, the differentiation of the palisade, &c. in 
the bracken are less remarkable. Similar cases are described by 
Lamarli¢re and Stahl. Lamarlié¢re ¢ found that in the oak and the 
beech the shade-leaves bad one layer of palisade and the sun-leaves 
two (both leaves being taken from the same tree), and that in Salix 
rosmarinifolia the palisade has a greater thickness in insolated 
leaves than in shaded leaves in the ratio of 80 to 12. Again, 
the thickness of the leaf was found by Stahl tf, in the case of 
several flowering-plants, of which he gives measurements, to be 
greater in a specimen growing in a sunny locality than in one in 
the shade, the size (superficial area) of the leaf and the thickuess 
of the leaf being to a certain extent inversely proportional to one 
another. He also adds that Pteris aquilina shows similar varia- 
tions in the thickness of the leaf. 
Before considering the question of the plasticity of the bracken 
we must take some further observations into account. In habitats 
having an intermediate character, the structure of the leaves 
is, speaking generally, of an intermediate type likewise. The 
outer wall of the upper epidermis is of medium thickness, hypo- 
derm may be present over about half the total area of the leaf, 
or may be continuous but not very well differentiated (e.g., most 
of its cells containing a very small amount of chlorophyll), and, 
with respect to the differentiation of palisade and the total thick- 
ness of the mesophyll, the leaf takes a position between the 
extreme types of structure. 
In most intermediate localities, however (e. 7., between scattered 
trees or close to awood or a hedge-bank, which form a wind-screen), 
it is impossible to gauge the real nature of the habitat, so there 
is no need to discuss these in detail. But in other cases, where 
there was evidence of special conditions affecting certain plants 
or part of a plant, one obtains useful data. I will therefore quote 
a few such cases. 
* Stahl, “ Einfluss d. Standortes auf d. Ausbildung d. Laubblitter,” Jenaische 
Zeitschy. xvi. 1883, p. 176. 
t Lamarliére, “ Recherches physiologiques sur les feuilles ete.,” Revue 
Générale de Bot. iv. 1892, p. 484. 
} Stahl, Zoe. cit. p. 182. 
