CHYTRIDIALES 117 



lated substances. The algal cytoplasm is ultimately consumed save for 

 some residual material, which appears as a series of equidistant broad 

 bands around the absorbing organ of the chytrid. 



Wager (1913) described the sequence of changes occurring in Euglena 

 parasitized by Polyphagus euglenae. He observed that discoloration 

 of the chloroplasts occurs soon after infection, the bright green changing 

 to yellow or yellow green. The chloroplast gradually disappears and 

 is replaced by a clump of rusty-red granules. Coincidently with these 

 changes the protoplasm is absorbed, the paramylum grains are broken 

 up, and the granules of the red eye spot are disassociated. The nucleus 

 and cell membrane persist for a time, but the former is gradually ab- 

 sorbed, and after several days the wall disappears, releasing the reddish 

 residual material into the water. 



Chytrids are often found parasitizing other aquatic Phycomycetes 

 in old water cultures. If endobiotic, as are species of Rozel/a, the hyphae 

 of the host are commonly stimulated to form conspicuous ellipsoidal, 

 spherical, or saclike enlargements. These are frequently isolated from 

 the rest of the hyphae by cross walls apparently produced by the host. 

 Reproductive organs, sporangia, for example, when attacked by such 

 endobiotic parasites retain their general configuration but are usually 

 somewhat enlarged. The protoplasm of infected regions is often strongly 

 vacuolate, or it may contain numerous bodies of fatty material. 



Resistance of Algae to Infection by Chytrids 

 There is no marked evidence to show that healthy algae attacked 

 by chytrids possess varying degrees of immunity. It is frequently ob- 

 served, however, both in nature and in gross cultures, that during 

 the progress of an epidemic certain individuals of a species remain 

 uninvaded. Whether this is due to inherent factors within the alga 

 (general health, vigor, state of the cellulose wall, and so on) or to ex- 

 ternal conditions is not known. 



The attempt of some algae to ward off attack by chytrids by what 

 might be termed "mechanical means," that is, by "protective plugs" 

 of wall material, has been observed in numerous instances. This 

 curious method of defense was early noted by Braun (1856a) in Ulo- 

 thrix zonata attacked by Phlyctidium laterale and has since been found 



