714 TRANSACTIONS OF SECTION K. 
hyphez, which may be followed out some distance back into the older parts of 
the thallus, swell up and stain more deeply than the others. They are at first 
uninuclear, but soon become multinuclear as they increase in size. Fusions 
with neighbouring cells are common, but no transference of nuclei has been 
observed. Soon we get a mass of closely interwoven cells full of cytoplasm and 
containing numerous nuclei. No coiled carpogonia can be made out, but taken 
as a whole these darkly stained cells can be seen to form part of a connected 
system of branched hyphze coming from the medulla further back, and passing 
into the cortical margin. The multinuclear condition seems to be due to simul- 
taneous nuclear divisions in the cells, and not to any passage of nuclei from 
cell to cell. When nuclear division is still active, long unbranched but multi- 
cellular hyphe can be seen to grow out towards the cortex, forcing their way 
through the latter generally at an oblique angle. These are from structure and 
appearance functionless trichogynes. They gradually disappear. Certain of 
the large cells—the ‘ascogonia ’—now grow out, and the nuclei formed by simul- 
taneous division—that is to say, female nuclei—pass into the ascogenous hyphe, 
thus formed, in pairs. From these the asci appear to derive their first nucleus 
in the usual way. 
2. The Structure and Life-history of Verrucaria margaracea, an 
Aquatic Lichen. By Miss EB. M. Poutton, M.Sc. 
The thallus is a small, more or less circular disc, attached to pebbles at the 
bottom of streams. It is very thin, and its morphological characters change 
with advancing age. It was found that the ascospores were unseptate, unisep- 
tate, biseptate, or triseptate, depending upon their maturity. They could, 
however, germinate in any of these states. They invariably germinated en masse 
while within the perithecium, the tuft of entangled hyphe thus produced being 
expelled through the ostiole and forming an admirable device for catching the 
Algal cells floating in the water. 
3. The Biology of the Apple Canker Fungus, Nectria ditissima, Tul. 
By 8. P. Witrsnire. ‘ 
Nectria ditissima is a genuine wound parasite. The normal reaction of the 
cortex to injury is the formation of a phellogen layer over the exposed area, and 
so inoculations can only be successful when the injury is deep enough for the 
fungus to reach the wood; otherwise the diseased portion is surrounded by the 
phellogen, the tree thus healing itself of the disease. 
The fungus travels across the cortex through the intercellular spaces, across 
the phloem and cambium by mechanically breaking through the cell walls (dis- 
integration of the tissue soon following), and then traverses the woody elements 
and pith through the pits in the walls. The medullary rays do not form a special 
means of entrance to the wood. 
The reactions of the host against the disease are the formation of 
(1) Phellogen, at the limits of the infected region in the cortex; 
(2) Abnormal wood of cells very similar to those of the medullary rays, and 
containing ‘gum sacs’ ; 
(3) Wound gum in the wood vessels—which substance, however, ca 
penetrated by the hyphe. 
The mycelium may possibly be able to travel about the stem and break tout 
to form a fresh canker, but such is probably abnormal, the extent of the hyphe 
usually remaining quite local. No trace of an invisible hibernating mycelium 
has been found in apparently healthy shoots. 
The chief means of inoculation in nature are injuries made by (1) frost; 
(2) Schizoneura lanigera—the woolly aphis. In both cases the bark is burst by 
the swelling of various tissues. 
The relatively immune varieties of apple readily infect if inoculated through 
suitable injuries. Investigations into the thickness of the bark, the rate of 
reaction to injury, and the growing power of the fungus in the sap have given 
negative results, suggesting that the determining factor is physiological. 
be 
