20 COMPARATIVE MORPHOLOGY OF FUNGI 



phanerogams and causing the wilt of flax seedlings in Flanders. Its 

 zygotes are also binucleate in the young stages (Nemec, 1922). 



Reesia amoeboides on Lemna sp. in Germany has the same life cycle 

 as Olpidium. It is the first form in which copulation of planogametes 

 was observed by Fisch (1884), but until the work of Kusano, his work 

 was considered erroneous. Monochytrium Stevensianum (Griggs, 1910) 

 on Ambrosia artemisifolia in the United States appears to be similar, 

 but its life cycle is incompletely known. Also noteworthy is the parti- 

 ally known Pleolpidium which parasitizes fungal hyphae, such as P. 

 irregulare on Pythium. In some respects it reminds one of the Woroni- 

 naceae but differs from them in the uniflagellate zoospores. Dangeard 

 (1889, 1895a) made similar observations for Sphaerita. 



Synchytriaceae. — In contrast to the Olpidiaceae, the thallus of this 

 family does not change to a sporangium but to a sporangial sorus par- 

 tially protruding from the original sheath. Further, their thalli remain 

 uninucleate until the beginning of fructification so that the nucleus may 

 attain a diameter of 25/x. 



Synchytrium, chiefly parasitic on phanerogams, is divided into 

 three subgenera according to the method of spore formation: Pycno- 

 chytrium where the summer spores (if they are produced) are as thick walled 

 as hypnospores and germinate with a protruding sorus; Mesochytrium 

 whose summer sori are thin walled but germinate as in Pycnochytrium; 

 and Eusynchytrium which agrees morphologically with Mesochytrium, but 

 forms zoospores inside the membrane of the initial cell. How far these 

 differences are fundamental is still obscure; the protrusion of the sorus 

 may be only the result of a brittleness of the exospore which cannot 

 expand as rapidly as the sorus. In the subgenera Eusynchytrium and 

 Mesochytrium, the protoplast is yellow or reddish yellow ; in Pycnochytrium, 

 it varies according to the species and age, from reddish yellow to yellow 

 or hyaline. 



In the subgenus Pycnochytrium, S. endobioticum causes the potato 

 wart on Solanum tuberosum, also attacking S. nigrum and S. Dulcamara 

 (Curtis, 1921). The zoospore swarms especially between 12 and 19°. 

 After it comes to rest on the epidermis of the host, it withdraws its 

 flagellum, throws it off and penetrates the host cell where its body is 

 carried to the bottom of the cell by the streaming of protoplasm (Fig. 

 11, 1 to 4). The host cell swells under the influence of the parasite and 

 becomes pyriform; repeated division of the neighboring tissue cells forms 

 a tumor; the surrounding epidermal cells divide similarly and become 

 woody so that a rosette is formed with the infected cell in the middle 

 (Fig. 11, 5 and 6). 



The zoospore develops to a summer spore, also called prosorus or 

 initial cell, and surrounds itself with a double wall, a thick golden-yellow 

 exospore and a thin hyaline endospore (Fig. 11, 7). Its nucleus reaches 



