14 Joint Bulletin 3 



A second subdivision of the blue-green algae which invites especial 

 attention is represented by spherical or flattened colonies of plants. 

 These develop from single microscopic resting cells 1/2500 of an inch 

 or so in diameter, and form colonies as much as an inch in diameter 

 in some species. These are species of the genus Nostoc, and abound 

 in almost every habitat affording quiet fresh water, though many 

 species do not form colonies of a noticeable size. They will be found 

 growing with moss on dripping cliffs, or in quiet pools, and it is in the 

 latter habitat that they obtain their greatest size. The Nostocs vary 

 to some extent in color, but are mostly a rich olive green. Some 

 species require a continuous water habitat while others may be found in 

 moss or mud at the water's edge. 



It will be observed that the dividing plants of the Nostoc remain 

 for a time adjacent to each other in the gelatinous mass of their partly 

 soluble cell-walls, so that the microscope reveals them to us as count- 

 less little chains of a dozen or more cells each, with occasional thicker- 

 walled resting cells. The next subdivision leads us to plants which 

 form individual filaments each having a gelatinous sheath but not 

 united in any definite mass with other filaments. These plants are best 

 represented in the genus Scytonema, and are found on moist cliffs, 

 where they often entwine with moss and form the basis of lime in- 

 crustations. Some of the rocks at the Winooski gorge are covered with 

 a red scab-like growth which consists almost entirely of these plants. 



We have in the Scytonemas a growth which is a filament. In the 

 next subdivision we find a differentiation of the filament into a base 

 and apex, and the plants are attached to rocks under water, usually in 

 quiet pools or streams. The filaments divide and subdivide, and we 

 find strands radiating upward from a central point to form little 

 cushions up to a half inch in diameter. These are most frequently 

 represented in the genus Rivularia. 



Another interesting subdivision of the blue-green algae is found 

 to be characterized by an unbranched filament without a noticeable 

 gelatinous sheath, the common genus of which is Oscillatoria. These 

 plants move about clockwise and sidewise. They are perhaps the most 

 common of the blue-green group — no pond or stream or swamp is 

 without them, and they occur frequently as brownish or greenish 

 "slime" on moist earth. 



The Chlorophyceae. or simple green algae, are not so simple as we 

 might wish. In the blue-greens we traced a structural development 

 which began with a single free-floating sphere and ended in a plant 

 which had a filamentous form and a base and apex. But the struc- 



