PARASITISM AMONG THE RED ALG^. 



By WILLIAM ALBERT SETCHELL. 

 (Read April ig, 191S) 



The question as to when a particular plant is, or is not, a parasite 

 is often difficult to answer, although in many cases the parasitic re- 

 lation is readily to be inferred because of certain morphological 

 pecuharities and also because of the apparent dependence of the 

 one (parasite) upon another (host) in the matter of nourishment. 

 The fundamental conception is, of course, that the parasite draws 

 upon the host for materials of greater or less metabolic value, 

 but, as to amount and extent, is difficult of demonstration and may 

 be inferred largely from various indications of a morphological 

 nature. 



There are also to be considered in connection with parasites, 

 especially among the thallophytes, epiphytes and endophytes. A 

 true epiphyte uses the plant upon which it grows only for mechan- 

 ical support. Its metabolism is independent of that of the plant on 

 which it grows. It is conceivable that even those epiphytes which 

 penetrate the tissues of the supporting plant do so only in a mechan- 

 ical way, although it seems probable that penetration is usually as- 

 sociated with the establishment of metabolic relations. 



In case of the endophyte, a variety of relations seems to exist 

 between it and the plant it inhabits. Epiphytes exist both among 

 cormophytes and thallophytes. Endophytes are always thallophytes 

 and they may grow entirely within the body of another plant or only 

 partially so. Some algal endophytes only penetrate between the 

 layers of the outer walls while others descend deep among the cells 

 of the plants they inhabit. It seems very possible that there may 

 be parasitic relations between many, or most, of the endophytes and 

 their " hosts." 



Epiphytes are numerous among the red algae and, while no exact 

 enumeration has been made, it seems safe to say that, at least, half 



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