PRINCIPAL GROUPS OF PLANTS. 11 



arrange themselves in a radiating sphere, with the cilia at the 

 periphery. Each cell of the colony contains a more or less spiral, 

 yellowish chromatophore, bearing a reddish spot at one end, a 

 nucleus at the centre, and a few vacuoles. The cells secrete a 

 large quantity of oil, which is of an unpleasant, fish-like odor, and 

 is due either to the decay or breaking up of the cells by mechanical 

 means. This breaking up of the cells is the cause of the disagree- 

 able odor occasionally found in public water supplies. Uroglena 

 is found in New England and has been reported as far west as 

 Indiana, and is probably rather widely distributed in the United 

 States. It seems to thrive best in cold temperatures, usually 

 occurring in greatest numbers when the water is frozen over. 

 It multiplies by cell division, which takes place when the colony 

 becomes rather large. It also produces resting spores which 

 enable the organism to survive conditions which would otherwise 

 exterminate it. 



A closely related organism, Synura, is responsible for the ripe 

 cucumber odor which was formerly thought to be caused by 

 fresh-water sponges. 



Nostoc, a form occurring in filaments like a string of pearls, is 

 made up of spherical or elliptical cells, the whole being surrounded 

 by a thick, mucilaginous membrane (Fig. 6). Usually one finds a 

 number of these filaments growing together in a mass which can 

 be seen by the naked eye floating in the water. These masses 

 vary from globular to sub-globular, are irregularly divided or 

 occur in definitely expanded groups. These forms are marked by 

 having two kinds of cells, the one filled with a granular proto- 

 plasmic content, the other being free from protoplasm and some- 

 what larger than the other cells. These latter are fewer in number 

 and are called " heterocysts," which means simply " other cells." 

 At these latter cells the filaments separate, and thus many new 

 colonies are formed. Nostoc is mostly of an olive-green color, 

 but may be dark bluish-green, dark brown, or light yellow or even 

 colorless. It occurs mostly in fresh- water ponds, seldom in 

 brackish water, being found on damp rocks, on mosses and more 

 or less submerged plants, and variously in limestone springs or wet 

 calcareous rocks or on aluminous soil. The colonies vary greatly 

 in size and color, and while some of them may be of microscopic 



