_d._ PARASYMBIOSIS OF FUNGI. There occur on lichens, certain parasites classed as fungi which at an early stage are more or less parasymbionts of the host; as growth advances they may become parasitic and cause serious damage, killing the tissues on which they have settled. Zopf[950] found several instances of such parasymbiosis in his study of fungal parasites, such as _Rhymbocarpus punctiformis_, a minute Discomycete which inhabits the thallus of _Rhizocarpon geographicum_. By means of staining reagents he was able to trace the course of the parasitic hyphae, and found that they travelled towards the gonidia and clasped them lichen-wise without damaging them, since these remained green and capable of division. At no stage was any harm caused to the host by the alien organism. Another instance he observed was that of _Conida rubescens_ on the thallus of _Rhizocarpon epipolium_. By means of fine sections through the apothecia of _Conida_ and the thallus of the host, he proved the presence of numerous gonidia in the subhymenial tissue, these being closely surrounded by the hyphae of the parasite, and entirely undamaged: they retained their green colour, and in size and form were unchanged. Zopf[951] at first described these parasites as fungi though later[951] he allows that they may represent lower forms of lichens. Tobler[952] has added two more of these parasymbiotic species on the border line between lichens and fungi, similar to those described by Zopf. One of these, _Phacopsis vulpina_, belonging to the fungus family Celidiaceae, is parasitic on _Letharia vulpina_. The fronds of the host plant are considerably altered in form by its presence, being more branched and curly. Where the parasite settles a swelling arises filled with its hyphae, and the host gonidia almost disappear from the immediate neighbourhood, only a few “nests” being found and these very mucilaginous. These nests as well as single gonidia are surrounded by _Phacopsis_ hyphae which have gradually displaced those of the _Letharia_ thallus. The gonidia are excited to division and increase in number on contact with either lichen or fungus hyphae, but in the latter case the increase is more abundant owing doubtless to a more powerful chemical irritant in the fungus. As development advances, the _Phacopsis_ hyphae multiply to the exclusion of both lichen hyphae and gonidia from the area of invasion. Finally the host cortex is split, the fungus bursts through, and the tissue beneath the parasite becomes brown and dead. _Phacopsis_ begins as a “parasymbiont,” then becomes parasitic, and is at last saprophytic on the dead cells. The hyphae travel down into the medulla of the host and also into the soredial outgrowths, and are dispersed along with the host. The effect of _Verrucula_ on the host thallus may also be cited[953].