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468 



MR. G. MASSEE ON THE LIFE- HISTORY 















f. 6 ; Bab. Alg. Eur. sect. iii. p. 47, f. 20 (sinistra) ; Wolle, 

 Alg. U. States, p. 186, pi. clvi. f. 28 ; Cooke, Brit. Alg. p. 21, 

 pi. ix. f. 2. Exsicc. Bab. Alg. no. 789. 







Wolle 







pl. elfin, f. 12. 























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to 



Dictyospitjeuium pulchellttm, Wood, Alg. N. Amer. p. 84, 

 pl. x. f. 4. Exsicc. Wittrock et Nordstedt, Alg. Eocs. no. 239. 



Two species are common to Europe and North America, 

 whereas D. HitcTicocTcii and D. pulchellum have hitherto been 

 observed in the last-named country only. 



The species are all very similar in general appearance, and 

 under a low magnifying-power resemble minute hyaline spheres 

 studded with brilliant green spots. 



During the past summer a Dictyosphcerium, which appears 

 agree with the spherical-celled variety of D. Ehreribergianum 

 figured by Cooke *, has been fairly common in the open-air tank 

 at Kew, mixed with Spirogyra and Cladophora. Although the 

 tank has been regularly searched for Alga) for some years, no 

 trace of a Dictyosph cerium was seen before the present season. 

 Last year the same tank was covered for some time with a dense 

 orange-coloured scum consisting of the rare alga Sphceroplea 

 annulina, Ag. ; whereas this year not a trace of this alga has been 

 seen, although myriads of its oospores must have sunk into the 



mud. 



In connection with the last-named plant, it may 



be 



mentioned that just before its appearance the tank had been dry 

 for some time for the purpose of cleaning and repairing, 

 stocked two jars with Sphceroplea obtained from the tank, one of 

 which, owing to neglect, became dry during the summer, and 

 remained in that condition for a week. The second has always 

 contained water, and up to the present time no trace of 

 Sphceroplea has appeared in it ; whereas in the jar that became 

 dry, the Sphceroplea appeared in quantity about ten days after it 

 had been filled up with water. From the above statement it 

 would seem that the oospores of Sphceroplea benefit by a period 

 of desiccation : as to whether this condition is absolutely necessary 

 remains yet to be proved, and possibly similar conditions may to 

 some extent influence the sudden appearance or disappearance of 

 other forms of freshwater Algae. 



* Brit. Alg. pl. ix. f. 1, c. 











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