668 MR. L. A. BOODLE ON THE STRUCTURE OF LEAVES 
upright, distinctly xerophytic leaves. It should be compared 
with fig. 3. The leaves produced later in the season may be 
called half-xerophytic, and, though small, they agree in habit 
with the leaves of the original neighbours of this plant. Hypo- 
derm is present, but often does not extend for more than about 
half the length of the transverse section of a pinnule. These 
leaves, when fertile, mostly produced the long sori described as 
characteristic of the species; while in the greenhouse the sori 
were quite small and very few, and showed distinct reduction of 
the marginal indusium, as seen in fig. 5, where the left-hand 
diagram is from a greenhouse-leaf, that on the right from an 
outdoor-leaf. Both introrse and extrorse indusia are shaded. In 
the greenhouse-leaves, in parts of certain sori both indusia may 
Fig. 5. 
Margin of pinnule, showing indusia in greenhouse-leaf and 
outdoor-leaf respectively. 
be practically absent. This reduction of the indusia is of some 
importance, as the characters of the indusia are used for classifi- 
catory purposes. 
The production of thicker leaves, which are richer in palisade- 
ussue, on transferring the bracken-plant from the greenhouse to 
the garden, may be compared with Bonnier’s experiments on the 
transplantation of different plants from a lowland to an alpine 
situation *. 
The facts brought forward in the present paper establish a 
high degree of plasticity for Pleris aquilina. Though this applies 
to the particular plants dealt with, it cannot be definitely asserted 
that much less plastic forms, or even fixed forms (as regards 
structure), may not exist within the species, e.g. among the 
* Bonnier, Comptes Rendus, cx. p. 363. 
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