Notes . 
321 
sown on the injured place. It was found that the cells immediately surrounding the 
place of injury were rendered susceptible to the attacks of a * biologic form 1 which is 
unable to attack uninjured leaves of the plant in question. 
In the third part of the paper, dealing with general considerations, the following 
hypothesis is advanced as to the actual manner in which the injury to a leaf causes 
it to become susceptible to a ‘ biologic form ’ otherwise unable to infect it. It is 
supposed that the leaf-cells of each species of host-plant contain a substance or sub- 
stances — possibly an enzyme — peculiar to each species which, when the leaf is 
uninjured and the cells are vigorous, are able to prevent the successful attack of any 
mildew except the one ‘ biologic form 5 which has become specialized to overcome the 
resistance. When the vitality of the leaf, however, becomes affected by injury, this 
substance is destroyed, or becomes weakened, in the leaf-cells in the neighbourhood 
of the injury, so that the conidia of other 1 biologic forms ’ are now able to infect them. 
The author suggests that injuries to leaves, caused in nature by hail, storms of 
wind, attacks of animals, &c., may produce the same effect as the artificial injuries 
described above in rendering the injured leaf susceptible to a Fungus otherwise unable 
to infect it. Conidia produced on these injured places would be able to infect 
uninjured leaves, and would spread indefinitely. Such may be the explanation of 
a common phenomenon — the sudden appearance of disease caused by parasitic Fungi 
on plants hitherto immune. 
A case is described which, it is believed, gives evidence that the injuries produced 
by Aphides caused leaves previously ‘ immune ’ to become susceptible. 
In the concluding remarks, reference is made to the antagonistic forces concerned 
in the evolution of a ‘ biologic form/ viz. ‘ specializing factors ’ and ‘ generalizing 
factors.’ 
Attention is also drawn to the close parallel between (1) the behaviour of the 
Fungus in the experiments in which the conidia were sown on the tissues of the leaf 
exposed by the cut ; and (2) the biological facts obtaining in the class of parasitic 
Fungi known as ‘wound parasites’ (. Nectria , Peziza willkommii , &c.), which are able 
to infect their hosts only through a wound. 
ERNEST S. SALMON, Cambridge. 
ON THE STRUCTURE OF THE PALAEOZOIC SEED LAGENO STOMA 
LOMAXI, WITH A STATEMENT OF THE EVIDENCE UPON WHICH IT IS 
REFERRED TO LYGINODENDRON. — The present communication deals with the 
structure of Lagenosioma Lomaxi> a fossil seed from the lower coal-measures, and with 
the evidence upon which the authors refer it to the well-known carboniferous plant, 
Lyginoden dr on . 
It is found that this species of Lagenosioma , especially in its young form, was 
enclosed in a husk or cupule, borne on a short pedicel. 
The seed, which is of Cycadean character, is fully described, and its relation to 
other fossil and recent seeds discussed. 
1 Abstract, reprinted from the Proceedings of the Royal Society. 
[Annals of Botany, Vol. XVIII. No. LXX April, 1304.] 
