ORDER CHYTRIDIALES 47 



They are more frequently parasitic but may occur as saprophytes within 

 the dead host cells. The posteriorly uniflagellate zoospore settles on the 

 exterior of the host and the flagellum disappears, probably in most cases 

 being withdrawn into the body of the cell which then becomes covered 

 with a thin wall. A slender infection tube grows through the host cell wall 

 and the contents of the encysted zoospore pass into the host. The empty 

 cyst soon disappears. Within the host cell the uninucleate fungus enlarges 

 and usually early produces a cell wall. In some genera, e.g., Olpidium, the 

 fungus cell does not completely fill that of the host, but in Rozella, as it 

 expands, the fungus eventually completely fills the host cell, the walls 

 of the fungus and host coming into close apposition. The most important 

 genus of the family is Olpidium which is parasitic in algae in fresh or salt 

 water, in pollen grains or fungus spores that have fallen into the water, 

 in small aquatic animals or their eggs, as well as in the roots of various 

 land plants. 0. hrassicae (Wor.) Dang, is parasitic in the roots of cabbage 

 and other plants of the genus Brassica, and 0. viciae Kus. inhabits the 

 roots of vetch (Vicia). In these the posteriorly uniflagellate zoospore 

 becomes attached to the root of the host plant and encysts there. The 

 contents of the cyst dissolve a small hole through the host cell wall and 

 enter the cell. It forms at first a naked uninucleate mass near the center 

 of the host cell. It may remain there or may dissolve its way into the next 

 underlying cell or even further. Eventually the fungus cell encysts and 

 begins to grow. Alore than one fungus cell may infect the same host cell, 

 up to 15 or 20 in 0. viciae. In this case they do not attain so great a size 

 as when single and become flattened where they press against one another. 

 As the fungus cell enlarges the nucleus divides repeatedly. Eventually the 

 organism consists at maturity of a smooth, thin-walled, more or less 

 round or ellipsoid zoosporangium, within which cleavage into uniflagellate 

 zoospores occurs. These escape through one (rarely more) exit tube which 

 grows to the outside of the host and then permits the zoospores to escape 

 by the softening and giving way of the apex of the tube. Sexual repro- 

 duction in 0. viciae and in 0. trifolii Schroet. occurs, according to Kusano 

 (1912, 1929), by the union of two zoospores (in this case functioning as 

 gametes) outside the host, the resulting biflagellate zygote encysting and 

 infecting the host in the manner described above. The resultant cell 

 becomes thick-walled and is often more or less angular. It is usually 

 smaller than the asexually produced sporangia. These thick-walled resting 

 spores may live for a period of several months until favorable conditions 

 occur, when they swell, cracking open the outer thick wall and permitting 

 the emergence of an exit tube from which the zoospores escape. The 

 union of the two nuclei does not occur until the following spring shortly 

 before the germination of the resting spores. It is probable that meiosis 

 occurs early in the series of nuclear divisions within the resting spore. 



