436 JAMES BURTON ON ALGAE COLLECTED IN 1911. 



in 1911 was on our excursion to Totteridge, on June 24tli. In 

 one of the ponds there the water was a deep transparent green, 

 but was not thick or opaque ; a bottlef ul had about the same 

 appearance, but nothing could be detected with an ordinary low- 

 power hand-lens. Examination under the microscope showed the 

 colour to be due partly to the presence of various small organ- 

 isms motile gonidia and others of an indefinite character, with 

 a few Eitglena viridis, but chiefly to the presence in large 

 numbers of a species of Anabaena which was new to me. A 

 closely related genus (Nostoc) is well known to most microscopists ; 

 they consist usually of very much twisted and convoluted strings 

 of small bead-like cells embedded in a gelatinous matrix ; there 

 -are both aquatic and terrestrial species, and are fairly common. 

 The species of Anabaena resemble them very closely, the chief 

 difference being that in these the jelly is quite or almost absent 

 and the filaments instead of being convoluted are more often 

 straight or slightly curved; they are found as aquatic organ- 

 isms only. The specimen from Totteridge was formed of the 

 usual bead-like cells, but the filaments were coiled into a fairly 

 regular spiral, or rather helix. The cells were about 5 /x in 

 diameter, filled with bluish-green protoplasm, with numbers of 

 darker granules in it. Most of the coils had in them a cell 

 known as a heterocyst paler in colour and without granules. 

 The function of these is uncertain, but the filaments often 

 multiply by dividing where they occur. Sometimes the direction 

 of the spiral changed at the heterocyst, and usually the turn 

 containing one was smaller than the normal, giving rise often 

 to a peculiar M -shaped figure. Spores were not infrequent; 

 they are foi-med from the ordinary cells, but become oval, larger, 

 and have denser contents. The coils were short, having from 

 two to four or five turns. Later in the year what was obviously 

 the same organism, but in a somewhat different condition, was 

 met with at Barnet, on our excursion to Hadleigh Common, in 

 September, In this case spores were not present, and heterocysts 



