215 



Gunnera has an additional interest for the botanist because of 



of the presence of a species of Nostoc, a blue-green alga, that 

 lives in its stem. The stem is tuberous and largely subterranean. 

 In cross section numerous areas of a deep-green or blue-black 

 color are seen distributed through the tissue. On microscopic 

 examination these areas are seen to be colonies of the blue- 

 green alga. 



Symbiosis, or cohabitation with other plants, js not uncommon 

 in the genus Nostoc. In the case of some lichens, which are 

 compound plants, formed by the association of an alga with a 

 species of fungus, the algal component is a Nostoc. In such a 

 case the commensalism is probably of mutual benefit to both 

 organisms. The chlorophyll-bearing cells of the alga manu- 

 facture the carbohydrates necessary for the nourishment of both 

 itself and of the fungus, while the fungus supplies water and 

 protection to the alga, and is possibly of service in other ways. 

 Both the alga and the fungus, however, may grow independently. 



A similar relationship between a Nostoc and the liverwort 

 Anthoceros is also well known. This association has recently 

 been found by Pierce (Bot. Gaz, 42 : 55. 1906) not to be obligate, 

 for either plant can be grown in cultures without the presence of 

 the other. 



In the present instance it is difficult to recognize any mutual 

 advantage. The tissues of the host plant furnish a substratum 

 for the alga, and supply it with water and other nourishment, but ' 

 if the presence of the alga is an advantage to its host, we are 

 unable to say how. If the advantage is to both plants, then this 

 is a case, not of true parasitism, but of mutualism. 



Reinke, in 1873, was one of the first investigators to observe 

 the Nostoc colonies in Gunnera, and Merker, in 1889, refers to 

 it as a case of symbiosis. 



It is worthy of note that, although the alga is probably in com- 

 plete darkness within the stem of its host, its cells develop the 

 green coloring matter, chlorophyll, usually formed only in sun- 

 light. 



It will be of interest to ascertain at what time the alga enters 



