116 MR. W. G. P. ELLIS ON A TRICHODERMA 
below, and had produced on this leaf an appearance that 
exactly resembled the ravages of a Peronospora, but was really 
due to the parasitic Botrytis, which I allowed to form its conidia 
so as to remove any possibility of doubt. 
At or near the growing-points of Pellia, and less often on the 
margin of the browned, apparently dead thallus, there appeared 
small green outgrowths (PI. 6. figs. 3, 4, and 5). These grew and 
produced elongated slender branches of the thallus, forming in 
fact the rejuvenating shoots that supply such favourable material 
for sections to demonstrate the direct infection of the host. 
Anybody who has passed their hand over a close-growing pateh 
of Pellia has probably seen thrown off from the thallus small 
isolated pieces each capable of independent existence and growth, 
and resembling in fact the buds on Lycopodium Selago stem, and 
the bulbs and bulbils of higher plants. These rejuvenating 
shoots are probably related to, if not morphologically identical 
with, these propagative shoots, and were readily produced by 
keeping a piece of thallus moist in a covered glass dish. They 
seem to arise from the apical embryonic tissue, but whether such 
tissue is capable of withstanding the general disease of the thallus 
I cannot say. May it not indeed involve the whole question of 
predisposition? for here is an instance of the immunity of 
vigorous living cells towards a fungus, to whose attacks older, 
less vital cells have become an easy prey. 
I am very glad of this opportunity of acknowledging the very 
great assistance so kindly given me. To Professor Marshall 
Ward I am indebted for much information, for suggestions, and 
help as difficulties arose. I owe thanks also to Mr. R. I. Lynch, 
F.L.S., Curator of the Botanic Garden, Cambridge, and to his 
foreman, Mr. G. Lamb, for their kind assistance in so many 
ways and for providing me with that supply of material in good 
condition, so essential in any investigations of this kind. 
Summary. 
The thallus of Pellia epiphylla was found to suffer from 3 
disease evidently epidemie in nature, caused by a fungus whose 
septate mycelium was found in the tissues of the host. The 
fungus was isolated and found to be the conidial form of an 
Ascomycete, and from its structure and conidia, and life-history 
as far as it could be made out, it appears to be similar to, if not 
identical with, the Trichoderma-phase of Hypocrea, but T? 
resting-stage has yet been found. It was cultivated on nutrient 
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