AND LIFE-HISTORY OF A TROPICAL EPIPHYLLOUS LICHEN. 101 
also, the cell-walls become cuticularized, and a tissue is produced in many respects resem- 
bling true cork. In other cases, or at other points of the invested thallus, when the 
Fungus is very luxuriant, the cell-contents diminish in quantity and become broken down 
into irregular masses of orange-red oily matter; or they disappear altogether and the cell- 
cavity becomes filled with air, or air-containing masses of very fine filaments. These 
changes appear to result from the breaking-up of the larger filaments, which traverse the 
junctions of the cell (figs. 57 & 60) and make their way into the mass of rhizoids below 
(cf. also figs 65 & 66). 
This more extensive invasion of the thallus (the protrusion of hyphæ between the cells 
and into the looser spaces among the rhizoids) is also brought about by branches of the 
mycelium which spread from the edges of the investing mass. Іп figs. 4 & 59 are shown 
well-marked and typical cases of this. The now richly developed superficial mycelium, 
which has spread over the whole area, sending branches down between the cells and 
forming the earlier pycnidia, on reaching the edges of the thallus, forms tufts of vigorous 
hyphze, some of which radiate off on the surface of the leaf, and continue the superficial 
mycelium, while others make their way under the thallus, and proceed to ramify between 
the cells. 
It thus happens that vertical sections of the thallus in this grey condition present the 
appearance depicted in figs. 64 & 65, where are seen the remains of the invested Alga, 
forming a sort of dome to a loose mass of air-containing Fungus-hyphe, resting on the 
surface of the leaf, and having formed (in fig. 65) a pyenidium (which is partly cut away); 
in this example the Algal cells, forming a sort of © gonidial layer " on the upper surface, 
have orange-red contents; but cases occur where these are green, as also those of the 
rhizoid cells which are to be found among the hyphs. Such would be the case, for 
instance, if the thallus of fig. 62 had been cut across the structure exactly as in fig. 65, 
but the red colour replaced by green. 
At a stage later than this, when the Lichen may be considered fully formed, the mass 
of air-containing tissue * has much increased, and the contents of the Algal cells often 
become nearly all destroyed; true perithecia are then formed, in which asci &c. appear 
in due course. If the above description has been followed, it now becomes easy to under- 
stand how the advent of the investing mycelium may affect the colour, consistency, and 
form of the Algal organism chiefly concerned, and types of which are figured and 
described at the commencement of this paper. 
It is conceivable that the parasitic mycelium (as it must be considered) may attack 
the host Alga at any stage, and that the lease of life enjoyed by the compound body, or 
“lichen,” may depend on this, among other circumstances, e. g. the relative vigour and 
amount of the host, or parasite, or both. As a matter of observation, the Fungal hyphe 
may commence to invest the Alga at any period, in some cases while still but a zoospore 
come to rest (fig. 54), or when only a very few cells represent the thallus (fig. 55); and 
in these cases there can be no doubt of its injurious influence. The weak, struggling 
cells, possibly stimulated at first, soon succumb, after making a few feeble and, it may 
be, irregular divisions (fig. 55). A very common case is that shown in fig. 56; and 
* This also often effervesces in acetic acid, and contains CaCO, in minute granules. 
