562 
Journal of Agricultural Research 
Vol. XIV, No. la 
Before leaving this subject some mention should be made of the root 
hair infections observed by Woronin ( 16 ) and others. The writer has 
also observed and studied these infections. They are often very abundant 
and conspicuous. It is not surprising that they should have impressed 
students of dubroot. The writer has never been able to bring proof, 
however, that a single one of these infections is caused by P. brassicae . 
In every case where it has been possible to follow the further develop¬ 
ment of the amebse and plasmodia observed in the root hairs they have 
been shown to belong to another organism. Two species of Olpidium 
are known to infect the root hairs of cabbage plants in Europe. One of 
these, Olpidium brassicae , was observed and described by Woronin ( 16 ). 
The other, O . borzii , has been studied by Nemec ( 14 ). The writer 
has found both of these species on cabbage roots in this country. It is, 
in fact, difficult to find a cabbage plant entirdy free from 0. brassicae . 
0. borzii is less common, but is by no means rare. Both species have 
been observed to produce long tubes out of which the zoospores pass. 
The writer has grown many cabbage plants in soil free of P. brassicae, and 
has observed in these plants all the different stages of root-hair infection 
that one finds on the roots of plants grown in infected soil. The living 
plasmodia in the root hairs dosely resemble those of P. brassicae . But 
when killed with Flemming's weaker solution and stained with Flem¬ 
ming's triple stain, they show certain characteristic differences that dis¬ 
tinguish them from P. brassicae . The distribution of the nuclei in these 
plasmodia is more regular than in P. brassicae , and the structure of the 
cytoplasm is also different. Plate 76, B, shows a cross section of a young 
root infected with Olpidium brassicae . The dark round bodies are the 
plasmodia. This parasite gets into the root hairs and into other cells in 
all parts of the primary cortex. It sometimes occurs sparingly also in 
the outer cells of the secondary cortex. P. brassicae is seldom found in 
abundance in the cells of the primary cortex or in the outer cells of the 
secondary cortex. It parasitizes the central cylinder and the inner 
portions of the secondary cortex. O. brassicae is never found in these 
tissues, so far as the writer has observed. The fields occupied by the 
two parasites overlap somewhat, but for the most part they are distinct. 
There seems to be no good reason why P. brassicae should not get into 
root hairs and other epidermal cells of the primary cortex. No doubt it 
does pass through these cells on its way to the central cylinder. The 
writer is forced to conclude, however, that the root hairs are of no im¬ 
portance to P. brassicae and that Woronin was in error in believing that 
the organism he observed in root hairs belonged to this parasite. The 
same conclusion has already been reached by Favorski (4), who made a 
careful study of the root-hair infections and had at his disposal the slides 
made by Woronin. 
