Succulent Leaves in the Wall-Flower. 
45 
ones that are most affected in xerophytes, the inner in halophytes. 1 
Now in the Wall-flower it is a common occurrence for the inner 
palisade-cells to be the longest, both in the leaf made succulent by 
salt and in the leaf of a xerophytic plant from a wall. 
Thus this plant when growing as axerophyte forms an exception 
to Warming’s generalisation. Assuming that this rule is of fairly 
wide application, a possible explanation of the exceptional structure 
of this plant may be suggested. The wild form of the Wall-flower 
may have originally been a halophyte, and the garden-form when 
grown under xerophytic conditions 3 may revert to halophytic 
structure, as being the special type of xerophytic structure that 
these conditions might be expected to induce in such a plant. 3 
I have not so far been able to examine the structure of the leaves 
of the wild form. In natural habitats it grows on rocks (sometimes 
near the sea), so the halophytic origin of the species is not excluded. 
In external appearance the leaves from natural habitats are some¬ 
times similar to those produced by the cultivated form when 
growing on walls. 
That all halophytes, even such as grow in the wet soil of the 
Mangrove-formation, show some kind of xerophytic character, c.g. 
succulence, reduction of transpiring surface, dense hairy covering 
or a coating of wax, has been explained by Schimper 4 as a pro¬ 
tective arrangement against too great transpiration, which would 
1 Warming (loc. cit., p. 243) makes the following suggestion : 
“ This structure seems to indicate that turgescence is 
greatest in the inner part of the leaf, perhaps because the 
substance causing the turgescence (the salt, presumably) is 
chiefly carried to or comes to act upon the innermost 
cells.” Possibly increased turgescence may be the direct 
cause of artificially produced succulence, as indicated by 
Brick (loc. cit.) and others. Succulence can be induced by 
other salts besides sodium chloride, e.g. potassium nitrate, 
as shown by Schimper (loc. cit., p. 21). In the succulent 
leaves of the Wall-flower the expansion of the mesophyll 
appears to have stretched the epidermis slightly, and the 
“ hyponastic ” curvature of the leaf may be due to the lower 
epidermis being capable of greater extension than the 
upper, as it has thinner walls. 
2 There is a possibility that some substance easily extracted 
by the plant from the mortar may produce pseudo-maritime 
conditions. Potassium-contents appeared to be somewhat 
more marked than in normal plants. It may be noted, 
though only having an indirect bearing 011 the question, 
that leaves of Linaria Cymbalaria from the same wall had 
a palisade, usually two cells thick the outer cells being the 
longest, where there was a difference. 
3 An interesting problem is here suggested, viz. whether the 
plasticity of a species has any relation to its past history 
as regards changes of habitat. 
4 Schimper, loc. cit., pp. 12, 26. etc. 
