CHYTRIDIALES 113 



subjected to inoculation experiments must necessarily be taken from 

 their natural environment and placed under more or less restricted 

 and controlled conditions, where the results can be observed. Hence 

 such experiments are handicapped from the beginning, and the sig- 

 nificance of the results is definitely limited. It is difficult to judge in 

 successfully induced infection under laboratory conditions whether 

 the virulence of the parasite or the weakened condition of the host 

 made the invasion possible. As has been previously mentioned, changes 

 of environment in all probability bring about changes in the suscep- 

 tibility of the host. This was early pointed out by Dangeard (1889b), 

 who emphasized that anything which caused impoverishment of the 

 host plant, such as alterations in the properties of the cellulose wall, 

 known to occur under conditions of culture, favored the development 

 of parasites. It should be recognized in situations where conflicting 

 evidence regarding the parasitism or saprophytism of a species arises 

 that almost never would two investigators be observing the same strain 

 of host and fungus under exactly the same environmental conditions. 

 In this connection, the results of inoculation studies by Johnson (1955a), 

 discussed below, are extremely pertinent. 



Host Specificity 



It is commonly found in a mixed collection of algae that only one 

 species of host has been attacked by a particular chytrid. In spite 

 of this, there is little direct evidence to support the view that many 

 of the aquatic chytrids are confined to a single host species or genus. 

 Generalizations in this respect have, however, but slight significance, 

 since relatively little data have thus far been accumulated. A few 

 well-marked instances of apparent host specificity may be cited: 

 Species of Micromyces have thus far been found only on conjugates; 

 Chytridium olla, with one exception, has been noted only on the oogonia 

 and eggs of Oedogonium; Polyphagus euglenae primarily attacks quies- 

 cent or encysted Euglena, although it has also been reported on Chlam- 

 ydomonas; P. parasiticus is confined to Tribonema bombycina; Rhi- 

 zophydium laterale, to Ulothrix; Blyttiomyces spinulosus, to the zygotes 

 and zygospores of Spirogyra; Podochytrium clavatum, R. fusus, and 

 Chytridium versatile to various diatoms. These are all species which 



