Pole Evans , — The Cereal Rusts . 
443 
The Fixing Solutions. 
1. Chrom-acetic solutions. On the whole the best results were 
obtained with this solution, in strengths varying from i per cent, to one- 
tenth per cent, solutions. Whenever possible the fixing was done under 
the air-pump, and the fixed portions were allowed to stay in the solution 
from twelve to forty-eight hours. 
2 . Flemming’s Chrom-osmium acetic solution. The weaker formula 
was most frequently used, and sometimes it was diluted down to one-half 
its volume with water. The chief objections to these solutions are: — 
(a) The blackening due to the osmium. 
(b) Their slow penetrating power. 
(c) They cause the protoplasm of the hyphae to become vacuolated. 
3. Picro-formol solution : — 
Commercial formol . . -30 c.c. 
Water ...... 20 c.c. 
Acetic acid ..... 5 c.c. 
saturated with picric acid. 
The advantages of this solution over the two- previous ones are : — 
(a) Its quick penetrating power. 
(b) Its not causing the material to become brittle on cutting. 
After washing and hardening imthe usual way, the material was taken 
into paraffin by means of Xylol or cedar-wood oil. The sections were cut 
from 5 to 10 ju thick, and all the staining done on the slide. 
The chief stains used were : — 
1. Diamant IAichsin and Lichtgriin. 
2. Safranin, gentian-violet and orange G. 
3. Iron-alum-haematoxylin with safranin, eosin or orange G. 
General. 
The Uredineae, like most other true parasites, possess two methods of 
penetrating their host. 
In the one case the germ-tube emitted from the spore penetrates the 
firm membrane or cuticle. This usually takes place with the sporidia 
developed from the teleutospores. 
In the other case, the germ-tubes penetrate their host by means of 
a natural opening or orifice, the stoma. This is always the method adopted 
in the case of uredospores and aecidiospores. 
The method of infection by means of uredospores was known to 
de Bary, who has given a general description of the sequence of events 
involved. Several other investigators have from time to time given various 
descriptions of infection phenomena. But no one seems to have realized 
K k 2 
