AND LIFE-HISTORY OF A TROPICAL EPIPHYLLOUS LICHEN. 105 
cuticle, and this applies henceforth to the whole structure. In vertical sections through 
leaf and thallus (Pl. XVIII. fig. 8) this is plainly seen; the cuticle of the leaf is appa- 
rently continuous with the cuticle formed by the thickened outer walls of the thallus. 
In Wichelia I have never observed this process to extend further. 
In Citrus, however, an apparent extension of the above process takes place; and I have 
introduced a drawing to show this at fig.18. The Algal thallus—quite typical in general 
characters, so far as I have observed—when considerably developed, is invariably found 
under the proper cuticle of the upper surface of the leaf. I have carefully attended to - 
this point, and I am convinced of the generality of it; unfortunately no proof of how it 
occurs in the first instance has been obtained. 
From an early stage the thallus appears as a boss of closely packed cells (chiefly ramifi- 
cations Фос. of the rhizoids) in several irregular layers, actively vegetating between the 
outer cuticularized walls and the inner ordinary walls of the epidermis-cells. The trichomes, 
again, pierce the cuticle immediately above their points of origin: this is true both for 
the fertile and barren hairs. In no instance have I observed the lower cells to penetrate 
into the leaf, though (as shown in the figure) the lower walls of the laterally ruptured 
epidermis-cells become brown and tend to disorganization. 
In this case one may suppose that the zoospore, on coming to rest prior to germination, 
actually dissolves away the cuticle and outer layer of cell-walls, and that the luxuriant 
thallus results from the continued growth in the well-sheltered space beneath ; but it seems 
impossible to avoid the further inference, that an actual destruction of the lateral wall of 
the epidermis-cells occurs here by parasitic influence on the part of the Alga. In some 
measure supporting this view is the fact that these subcuticular growths have less 
colouring-matter than the epiphyllous disks on Michelia. But another explanation of 
the process seems possible, and even probable. 
'The:glands of Citrus are apt to burst and rupture the upper epidermis of the leaf. А 
zoospore might find in the damp, shady hollow thus produced no unsuitable spot for its 
germination and growth; and the lateral rupture of epidermal cells may be due to ten- 
sion produced by the pressure of the growing Algal mass. It is not unworthy of remark 
that the points of exit of the two “ hairs " (in fig. 13) correspond more or less with the 
shrivelled gland ; and the same has been observed in other cases. 
Another fact comes out clearly from such a section as is represented in fig. 18. Not 
only the cuticle extending over the thallus-mass, but all the leaf-cells immediately beneath 
are affected more or less injuriously, as shown by the sienna-brown colour with which 
they are tinted. This sienna coloration extends to the cell-walls and contents of several 
series of cells immediately beneath the body of the thallus, but is quite absent from those 
cells which are not covered by the Cryptogam. For the sake of simplicity only, I have not 
drawn the chlorophyll granules and other cell-contents ; but it will be understood that 
the normal, healthy and chlorophyll-containing cells have been left colourless, whereas 
all that lenticular tract of cells coloured brown in fig. 13 is clearly and seriously altered. 
The cell-walls no longer give the reaction of cellulose, but resist the action of sulphurie 
acid, dyes, &c.; while the cell-contents are completely broken down, in a few cases to 
mere masses of resinous material Nor is this all; the brown-coloured cell-walls are 
s 2 
