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-Iike 

 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. 



Later 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 



