.AND LIFE-HISTORY OF A TROPICAL EPIPHYLLOUS LICHEN. 107 
activity ; in some cases considerable hypertrophy occurs, as well as or preceding active 
division. 
If the above view is correct, it might also be instructive to inquire how far the colour 
of the little light which does pass through the orange-red thallus may be concerned in 
the matter, if at all. No doubt the functions of the cells are more or less powerfully 
affected, at any rate, by the shading. 
In the case of Citrus it may be that something must be placed to the absorptive power; 
how much, is very doubtful. The same hardly applies to the Alga on Michelia ; it appears 
improbable that the rhizoids, even though embedded well into the cuticle, absorb even 
water directly from the leaf-cells in any quantity. 
It seems unnecessary to discuss the possible modification of influence when the Alga 
has become invested by the lichen-forming mycelium, since a dense slough of corky 
tissues has generally become formed by that time. However, in some cases there is 
reason to suppose that, either from destructive shade or from absorption of moisture by 
the injured tissues, the affected patch gradually extends through the thickness of the 
leaf, and may even cause an irregular perforation to appear. 
In eonnexion with this part of the subject, it may be not unprofitable to examine the 
changes which occur in the cells of the Alga-thallus before and during their investment 
by the Fungus. It has been already pointed out that the thallus of both young and old 
specimens of the Alga may be green or more or less orange in colour, aecording to cir- 
cumstances, and that this results, immediately, from the presence in the cells of an 
apple-green colouring-matter diffused through the cell-protoplasm, together with isolated 
or aggregated yellow or orange-red oily drops which may become so plentiful as to mask 
the green tint. The orange-coloured drops also occur in the rhizoid-cells, but are usually 
far less abundant there. 
I have tried to ascertain the conditions whieh decide whether the oily drops become 
formed in sufficient quantity to mask the green colour, or remain in abeyance. It appears 
the rule that in the rainy season, when the sun-light is frequently obseured by clouds for 
long periods and the average temperature is on the whole lower, the green colour pre- 
dominates ; and at such times one often observes the thallus-cells to contain fewer and 
smaller drops of yellow or orange matter (Pl. XVIII. fig. 6). On the other hand, in the 
very hot, dry season, the thallus-cells frequently present scarcely any green colouring- 
matter at all, but are occupied by large orange-coloured drops, peculiarly lustrous and 
oily-looking, and no doubt consisting of fusions of the otherwise isolated drops, and which 
give a very decided deep-blue reaction with iodine. At fig. 6 6 are represented several 
cells in this condition ; but it frequently happens that the oily drops are larger even than 
in this example, and may occupy nearly the whole of the cell-cavity. An intermediate 
stage, where the aggregated oily drops form large masses in the otherwise green-coloured 
matrix, is seen at fig. Ба & 6. 
It seems impossible to avoid connecting the intensity of the sun-light, and the high 
degree of temperature, with the increase in number and size of the oily drops and the 
starchy matter mingled with them; and since both must be regarded as products of the 
