576 
Journal of Agricultural Research 
Vol. Xt, No. ir 
In order to make a careful study of penetration, special culture methods 
were necessary. Test tubes were prepared by placing in the bottom 
rolls of filter paper (PI. 44, A) which were moistened and rendered suit¬ 
able as a medium for fungus growth by pouring a small amount of 
melted potato agar over them. These tubes were autoclaved and then 
planted in equal numbers with seeds of both resistant and susceptible 
varieties of flax. These seeds had been previously treated for five 
minutes with a 1-to-1,000 solution of mercuric chlorid. At the time of 
planting, or in some cases just after the seeds had germinated, some 
of the tubes containing each strain were innoculated with F. Uni from a 
pure culture, while others were left uninoculated to serve as controls, 
and in most cases these remained free from fungus growth (Pl. 44, A, 
b f c). Cabbage seedlings were also grown for penetration studies in 
tubes prepared as above and inoculated with F. conglutinans from a pure 
culture. The potato agar served as an excellent medium for the growth 
of the species of Fusarium, which had immediate access to the young 
seedlings growing in the tubes. As soon as any signs of wilting could be 
seen, the seedlings were carefully removed from the tubes, mounted on 
slides in a 20 per cent glycerin solution and examined carefully under the 
microscope. In this way the root hairs can be easily observed, and 
almost the entire young root system is so transparent that penetrating 
hyphae can be detected. In some cases it was necessary to separate the 
cortical layer from the inner part of the root tissue in order to be able 
to examine it closely. This was done by carefully splitting the root on 
one side with a sharp scalpel and removing the cortical layer, which was 
then spread on the slide and mounted in glycerin, as mentioned above. 
Penetration studies were also made with young flax seedlings grown in 
very loose, infected soil. It is very difficult to obtain clean root hairs 
from plants grown in soil. The best results were secured by taking 
the plant up with a large lump of soil which was then dissolved away 
without destroying all of the root hairs by placing it in a vessel of still 
water and agitating gently. These roots were then mounted and exam¬ 
ined in the same manner as those taken from the tubes. A small amount 
of eosin placed under the cover slip was often of considerable aid in 
observing root hairs and hyphse. 
A careful study of slides prepared as above described revealed root- 
hair (fig. 1, G, H, I), epidermal (fig. 2, 3), and stomatal (fig. 4), pene¬ 
tration of flax seedlings taken from tube cultures; root-hair (fig. 1) 
and epidermal (fig. 3), penetration of flax seedlings grown in infected 
soil; and root hair penetration (fig. 1, A, B) of cabbage seedlings taken 
from tube cultures. The cabbage seedlings produce a more abundant 
supply of root hairs, and are, for this reason, more desirable for the 
study of root hair penetration than are flax seedlings. By cross inocu¬ 
lation in tube culture it was found that Fusarium conglutinans could 
penetrate the root hairs of flax seedlings (fig. 1, K). Likewise, F. Uni 
