Oct. ii, 1915 
Phytophthora infestans in Irish Potato 
97 
latent in apparently healthy potato plants. It is, of course, obvious 
that Massee makes two radical departures from well-established prin¬ 
ciples: First, that the rapid dissemination of spores is not sufficient to 
cause an epidemic; and, second, that mycelium remains latent in the 
potato tissues. 
The development of an epidemic by means of conidia under field condi¬ 
tions has been carefully followed and described in an earlier part of this 
paper, and the results fully confirm Ward (36) and others. That conidia 
or asexual spores are able to cause epidemics in the case of a great number 
of parasitic fungi is well known and needs no further argument. Had 
Massee demonstrated histologically the presence of latent mycelium in the 
apparently healthy potato plant as a whole, the latent-mycelium theory 
would have been worthy of more careful consideration. 
WILSON'S SCLEROTIA-LIKE BODIES OF THE POTATO FUNGUS 
Another singular theory to account for the perpetuation of P. infestans 
is that proposed by Wilson (37). He believed he had found sclerotia-like 
bodies on the potato tuber and plant as a whole which were the resting 
organs of the potato fungus. This theory was later indorsed, strangely 
enough, by Plowright (28) and W. G. Smith (31). The latter stated that 
it was his conviction that the bodies Wilson found were of fungous origin, 
and possibly those figured by Martius (19). These sclerotial bodies were 
later proved by Murray and Flight (22) to be calcium-oxalate crystals. 
Eater Wilson (38) reported a more fictitious discovery, that of a muco- 
plasm existing in the potato plant, which was able to give origin to late- 
blight. 
CONIDIA BORNE IN THE SOIL RENEWING INFECTION 
De Bary early suggested that the fungus might perpetuate itself by 
means of the conidia, although he considered it very improbable that 
primary infection often, if ever, takes place in this way. Jensen (14) 
claims to have found a case where the shoots were killed before they 
reached the surface of the soil, and the spores on these shoots infected 
the stem of a healthy plant growing in close proximity. Clinton (8) 
also cites a case where conidia borne under wet cotton possibly functioned 
in causing infection in one of his pot cultures. In this paper are recorded 
further experiments showing that the fungus fruits with great ease on 
the cut surfaces of the seed tuber and on infected sprouts in the soil, 
although so far no case has been found where such spores functioned in 
producing infection above the surface of the soil. It is not impossible, 
however, that it might happen, and Hecke (12) records such a case. 
As stated above, it is not improbable that spores produced on the cut 
surface of diseased tubers or sprouts may cause infection in some cases, 
yet the author can not hold with Hecke (12) and Clinton (8) that primary 
infection due to conidia occurs uniformly throughout a field. In an 
