140 
W. J. Dowson, 
host cells could then be stained with orange G if desired. This was 
seldom done, however, as an excellent differentiation was given by the 
deep blue of the mycelial hyphae. and the pale blue of the host cells. 
With gentian violet and orange G, the hyphae retained the violet while 
the host cells took up the orange stain. With fuchsin and „lichtgrün“ 
the hyphae were stained red, and the cells of the leaf green. In all 
cases the nuclei of the host cells retained the first stain of the com¬ 
bination; but it was föund impossible to stain the nuclei of the hyphae 
owing apparently to the fact that firstly the membrane of the hyphae also 
retained the stain to the same degree as the nuclei, and that secondly 
the stain washed out from the membrane apparently at the same rate as 
from the nuclei. In a transverse section through a spot, the hyphae at 
the edge which were yet young and densely filled with cytoplasm stained 
deeply, while those in the centre appeared fainter in colour, owing to the 
small amount of cytoplasm present in the older cells of the hyphae. The 
microtome sections were cut from 5—10 thick and stained in Dela- 
fields haematoxylin, Heidenhain, and the combinations, gentian-violet 
and orange G, and Fuchsin and „lichtgrün“. The iron-containing stains 
proved unsatisfactory in most cases, a great deal of black precipitate 
being thrown down in the hyphae. In no cases were haustoria made 
out to be penetrating the host cells; but very often at the edge fo 
a disease area the hyphae were seen to branch in a curious manner 
round about the host cells (fig. 50 a, b, c ). Many parallel hyphae with 
curious thickenings and swellings were found at these points arising 
from one main hypha. These parallel-running branches seemed to 
wrap round the cells and reminded one somewhat of the internal 
anatomy of a lichen thallus. The resemblance between a lichen and 
the young edge of the disease spots, and the young infection areas 
was to be further observed in the relation between the hyphal cells 
and host cells. In good preparations the host cells in and about an 
infection area i. e. an area with young infection hyphae, or the edge of 
a disease spot of any age, were not shrunk or abnormal in any way by 
the presence of these hyphae, and a symbiotic relationship seemed to ex¬ 
ist between host and fungus at these points (fig. 51 a and b). It was 
only in the centre of fairly old disease areas that the host cells were 
destroyed. The destruction of the host tissues at these points was nearly 
complete, the host“ cells being replaced by a much branched mycelium. 
The harder parts of the vascular bundles of the host and the epidermis 
were the only tissues to be recognised in these areas. In places beneath 
the epidermis of both sides the parasitic hyphae had formed clumps of a 
pseudoparenchymatous nature; it was from these that the numerous conidio- 
phores, and aerial hyphae were given off through the widened and split 
stomata to the exterior. The mycelium was throughout intercellular, and 
multiseptate. The nuclei were made out in one case which was a han¬ 
ging drop-culture killed in chrom-acetic acid, and stained in Heiden¬ 
hain (fig. 52) the membranes of some of the young hyphae not being 
stained so that under the oil immersion the nuclei of the individual cells 
could be made out. The cells of the hyphae appeared to be uninuclear. 
In the above quoted paper by Reed and Cooley on Heterosfiorium 
variabile the parasitic mycelium is given as intracellular although their 
microphotograph is by no means convincing. 
