1921] McWHORTER—MOSSES AND LICHENS 323 
in which the moss appears plastered over by the lichens. The apical 
development of the moss has been stopped. The lichen hyphae 
could be traced through the old moss tissue where they forced their 
way intercellularly. 
A very constant feature of the lichen growth on mosses is the 
clinging of the lichen hyphae to the thickened walls of the moss. 
This seems to be of great significance in the eventual destruction 
of the moss colony. It is not the meristematic tissue that seems 
particularly desirable to the lichen fungus, but the thickenings of the 
moss walls. The case seems homologous with the destruction of 
wood by a polyporous fungus, where the lignified part of the wood is 
especially attacked, and the cellulose walls are left almost un- 
touched. In the mosses the young walls are pure cellulose; the 
thickenings are of pectin. Thin sections of all the moss-lichen 
colonies studied showed the hyphae imbedded in the pectin. 
There is no evidence that the hyphae have been covered over by 
the forming pectin layers, but it seems obvious that the hyphae 
have taken their position by dissolving out the pectin. The 
figures of Amphiloma on Grimmia show a case of this, but 
Amphiloma is more destructive than most lichens, and in places 
has completely destroyed the moss. When sharply stained in 
safranin and analine blue, the pectinized part of the walls stains a 
strong red, so that penetration of the bluish stained hyphae may 
be plainly followed. In some colonies, even when the lichens 
appear to be literally plastered over the mosses, the lichen hyphae 
were found to be confined to the pectinized regions, and the cellulose 
walls to be intact; then the lichens are exerting a smothering 
effect carried on through a saprophytic rather than a parasitic 
action. 
Lichen fungi sometimes become truly parasitic on their moss 
hosts. This is especially true of Amphiloma, which is shown in 
fig. 6. Here the lichen is an intracellular parasite. Amphiloma 
haustoria soon break down the plastids, even in old moss tissues. 
Amphiloma seldom attacks the meristematic tissues. Under 
some conditions Physcia obscura may send hyphae of non-rhizoidal 
nature into the meristematic moss tissues. Physcia also may so 
incorporate moss into its thallus, that the epidermis of the lichen 
