168 SETCHELL— PARASITISM AMONG RED ALG^. 



(1892, p. 66 and 1895, p. 317), Schmitz (1893, p. 390) and Olt- 

 manns (1905, p. 334) have all spoken emphatically of the fact that 

 so many of the parasitic red algse are restricted to near relatives as 

 hosts. Oltmanns further remarks (loc. cit.) that no satisfactory ex- 

 planation of this can be brought forward. The suspicion has been 

 produced, especially earlier in the progress of our knowledge, that 

 some of these parasites, especially some species of Actinococciis, are 

 really parasitic tetrasporangial generations of the hosts they inhabit. 

 Darbishire (1899, p. 264) has voiced this suspicion and has stated 

 his opinion that while this is not impossible, it is not very probable. 

 The probability, as it seems to the writer, is, however, that the various 

 parasites, or some of them, may have originated in close connection 

 with their hosts by some mutation decreasing the chlorophyll con- 

 tent or power in one or other of the different forms of spore. Such 

 an inducement to increase the power of penetration and possible 

 protoplasmic connection between a spore (tetraspore or carpospore) 

 germinating in position might, it would seem probable, initiate para- 

 sitism on the parent plant, and this parasitic tendency increasing 

 penetration and dwarfing, might, therefore, be inheritable. 



There is one case known which seems to be such a case or in 

 line with such action. This is the condition found in Agardhiella 

 tenera (J. Ag.) Schmitz (" Rliabdonia tcncra" J. Ag.) by Oster- 

 hout (1896). It was noticed that the tetrasporangial plants have, 

 in many cases, numerous short bristle-like branches or proliferations 

 projecting at right angles to the main stem and branches, but that 

 the antheridial and cystocarpic plants are always destitute of them. 

 Examination showed that these peculiar branchlets are usually an- 

 theridial, while full-sized antheridial plants are rare. Sometimes, 

 however, the bristly short branchlets have tetrasporangia or cysto- 

 carps and the three kinds of reproductive bodies are sometimes 

 borne on branchlets side by side. Some of the branchlets remain 

 sterile "but in the majority of cases they bear reproductive organs 

 before they are more than a quarter of an inch in length." (Oster- 

 hout, loc. cit, p. 420.) 



It was found by Ostcrhout that the zonate tetrasporangia divide 

 in regular fashion forming what seem to be four tetraspores each, 

 arranged serially. These spores then sometimes divide further by 



