Journal of Agricultural Research 
Vol. XI, No. it 
578 
was evidently able to penetrate cabbage seedlings as they were killed 
by it in tube cultures. However, no penetration was observed in the 
latter as examination was limited. When flax was planted on “cabbage- 
sick soil” and the reverse, no wilting or yellowing occurred. The fungus, 
very likely, enters the root hairs to some extent, but probably is unable 
to invade the tissues of the plant for the same reason that F. Uni is unable 
to invade the tissues of the resistant flax plant. It was shown by the 
Fig. 2.— Fusarium Uni penetrating epidermis of young flax root grown in loose, infected soil. 
tube-culture method (PI. 44, A, a, d) that F. Uni can penetrate the 
young seedlings of the resistant strain of flax as readily as it can pene¬ 
trate the seedlings of the susceptible strain under those conditions. 
By what exact means the fungus is able to penetrate the cell walls 
of the host is not known. Perhaps the most feasible explanation is that 
given by Ward ( 26) that the fungus protoplasm overcomes the resistance 
of the cells of the host 
by means of enzyms 
or toxins. This might 
be interpreted as 
meaning that the fun¬ 
gus secretes an enzym 
which has a solvent 
action on the cell wall, 
or that it may secrete 
a toxin which prevents 
Fig. 3.— Fusarium Uni penetrating epidermis of young flax seedling g,n,y reaction on the part 
in test-tube culture. 
plasm by killing or weakening it, thus making possible the invasion of the 
cell. Perhaps both of these phenomena occur simultaneously. In the case 
of root-hair penetration there is a slight depression at the point of entrance 
of the fungus, and the diameter of the opening made by a hypha is some¬ 
what less than the regular diameter of the hypha in question. Stomatal 
penetration was found on the stem of a young flax seedling near the 
point where the root began branching. Seedlings taken from the soil 
