LAGENIDIALES 995 



sporum, Graff(1928: 169), Spirogyra sp., Sparrow (1932b: 289, fig. 4d), 

 Mougeotia sp., Spirogyra sp., Oedogonium sp., Karling (1942e: 77), 

 Sparrow (1943: 666), United States; Spirogyra sp., Cook (1932a: 142, 

 figs. 39-45; 1935: 75, pi. 1, figs. 1-14, pi. 4, figs. 1-8), Great Britain; 

 Zygnema sp., Linder (F. 2704) (1947: 244, pi. 13, figs. A-B), Canadian 

 Eastern Arctic; Spirogyra sp., Shen and Siang (1948:201, fig. 12), 

 China. 



This species, well known from the excellent researches of Zopf, has, 

 with two exceptions, been reported only in Conjugatae. In early spring, 

 it occurs usually in the sporangial stage. By midsummer, according to 

 Zopf, the thalli are mostly converted into gametangia. 



Both Atkinson and Cook have noted the lack of a vesicle during zoo- 

 spore formation, and it is probable that under certain conditions this 

 structure deliquesces soon after the emergence of the protoplasm. The 

 antheridium, particularly on monoecious individuals, may be strikingly 

 differentiated from the ordinary segment of the thallus and may assume, 

 as Zopf (1884: pi. 12, fig. 26) and Sparrow (1932b: fig. 4d) have shown, 

 a shape reminiscent of the crooknecked type found in Pythium. 



One-celled thalli, transformed either into sporangia or gametangia, 

 have been described by Zopf, but are atypical. 



Germination of the oospore in this species (the only instance known 

 in the genus) has been reported by Cook (1935) to be as follows: Soon 

 (twenty-four hours) after formation of the resting-spore wall it breaks 

 down and a single nearly spherical biflagellate zoospore 8 u. in diameter 

 is liberated, which after a short period of activity comes to rest on a 

 healthy host cell and infects it. This spore gains access to the outside 

 medium by the breaking down of the algal wall, presumed by Cook to 

 be achieved by natural agencies such as wind and water currents. 

 Although the quick germination of resting structures is recorded in 

 other fungi, the breaking down of the wall and, in particular, the highly 

 inefficient process of liberating only a single zoospore from a sexually 

 formed structure are rare, if not unknown, among these plants and need 

 confirmation. 



Very often, Lagenidium rabenhorstii will wipe out a population of 

 Spirogyra in the vegetative stage, particularly in hot, sunny localities. 



