44 
L. A. Boodle. 
growing on walls. The leaves vary in appearance, possibly in 
relation to the average dampness of the wall among other factors. 
They are usually more upright than in plants grown in soil, often 
small and distinctly succulent {e.g. as much as three times as thick 
as a normal leaf and slightly brittle), and in that case show great 
elongation of the palisade-cells, as in plants treated with salt. 
They usually show less reduction of intercellular spaces than in the 
latter, but greater thickening of the outer wall of the upper epi¬ 
dermis (103//, was the greatest thickness noted). 
From the foregoing it is seen that under xerophytic conditions 
and under halophytic conditions (natural or artificial) the Wall¬ 
flower tends to become succulent, the structure being, on the whole, 
similar in the two cases. 1 
The xerophytic character of halophytes has been pointed out 
by Schimper and others, and also the fact that several examples 
are known of one and the same species occurring as a halophyte and as 
a xerophyte (by the sea and in Alpine habitats, but not in intermediate 
situations), as is the case with Plantago marit'una? The structure 
of a halophyte or a xerophyte, as the case may be, is often suitable 
for existence in the alternative situation. Warming 3 points out that 
many plants usually classed as halophytes should be regarded as 
being simultaneously halophytes and xerophytes, e.g. those growing 
on sand-dunes close to the sea. 
In view of this fact, Warming, while admitting the resemblance 
between the two classes of plants, has attempted to find whether 
there are any points of structural difference between true halophytes 
and true xerophytes. He finds, in the halophytes he dealt with, 
that on an average the outer wall of the epidermis is not specially 
thick, 4 while it is eminently so in xerophytes ; he also thinks that a 
point of difference is to be found in the nature of the many-layered 
palisade-tissue. Where the cells are much elongated, it is the outer 
• 
1 The palisade on the upper side of the leaf usually consists of 
more numerous layers in plants from a wall and from near 
the sea than in an artificially salted plant. 
3 Schimper, loc. cit., p. 28. 
3 Warming, Halofyt-Studier, D. Kgl. Dauske Vidensk. Selsk. 
Skr., 6 Roekke, naturvidensk. og mathem.. Afd. VIII., 4. 
pp. 235. 239, 243. I am indebted to Mr. W. C. Worsdell, 
F.L.S. for kindly lending me the manuscript of a trans¬ 
lation of this work, which he has made. 
4 Several plants, however, from the Mangrove-formation figured 
by Schimper (loc. cit., Taf. IV.) have a very thick outer wall. 
