102 MR. H. M. WARD ON THE STRUCTURE, DEVELOPMENT, 
here, too, the Algal disk, though bright green for a time (as if the presence of the Fungus 
implied the absorption of fatty matters &c., and impelled the green chlorophyll to 
increased labour), soon becomes a mere mass of empty cells. In such cases the Fungus 
appears unable to develope pycnidia or perithecia*, though it commonly produces dense 
mases of conidia, each of which may extend it; indeed the production of such dense 
clusters seems to ensure a vigorous and rapid spread of radiating mycelium, extending 
on all sides and soon reaching other disks. 
Where the Alga has arrived at a more advanced stage, however, it lives with the Fungus 
investing its cells, as we have seen, for some time; the amount of work it can do for 
itself during the presence of the mycelium (judging, of course, by the condition and 
growth of the cells) appears to depend simply on the degree of subjection into which the 
latter has forced it. Ifthe very young Alga is at once and completely invested by the 
mycelium in abundance, no fertile hairs or zoospores of any sort appear to be formed, 
and very few rhizoids or barren hairs; if, however, a large thallus is invaded from one 
edge, the slowly advancing Fungus does little more than follow the thallus for some time, 
and it is certain that the Alga may retain vigour in its cells up to the formation of the 
complete Lichen. But it will be easy to understand that in such cases as the last, growth 
being modified at the points of severe investment, the circular or regular outline of the 
thallus may become affected. It is chiefly by these means, in fact, that the stellate and 
irregularly branched forms are produced, the portions not yet encroached upon growing 
out beyond the points of attack. 
In other cases the Alga goes through its whole period of development without becoming 
captive to the mycelium, or the latter only invests it to such a small extent that the 
extension of vegetative and reproductive normal growths is not perceptibly interfered 
with. It may be objected that the above cases simply prove the presence of an Alga and 
a destructive parasite, and do not support the doctrine of “ commensalism”’ applied to the 
nature of Lichens. It must be borne in mind, however, that although the completely 
invested Alga at length ceases to exercise its functions as an independent organism, there 
can be no doubt that many of its cells are living up to the latest period in the life of the 
Fungus ($. e. when the perithecia appear) ; and it does not seem logical to draw any sharp 
line at this period. 
THE “ LICHEN.” 
I have purposely reserved the details of structure of the completely invested thallus, 
and its two formst of fruit-body, for description under this head, although it is clear that 
no line can be drawn showing exactly where this stage differs from the later ones already 
referred to. 
If thin vertical sections through the complete thallus (fig. 3) and black dots of the 
Lichen be carefully examined, one obtains preparations such as are represented in figs. 64, 
65, and 66, the two former passing through the smaller black dots or pycnidia, the latter 
through the large, true “ fruit-body," or perithecium. As regards the smaller body, the 
* Perithecia were never found until a vigorous mycelium had been formed on and in the thallus, 
t In one doubtful example only have I seen a possible formation of spermatia in addition. 
