6o THE PLANT WORLD. 



There are four principal groups of Algae : The Blue-green, the 

 Bright-green, the Brown and the Red. This classification on the basis 

 of color is an old one, but at the present time when plants are set off 

 in groups according to their methods of reproduction, it still holds 

 good, for it is found that nearly all the Red Algae develop their fruits 

 in a similar manner, and so with the Brown, the Green and the Blue- 

 green. 



CYANOPHYCEiE, or Blue-green Algae.— Plants unicellular 

 or filamentous; heterocysts generally present ; filaments branched or 

 unbranched; reproduction by hormogones or spores, or both. 



These are the lowest of the Algae. In looking for them take por- 

 tions of the slimy, bluish-green or nearly black sheets which often 

 cover the bottoms of pools, ditches, wooden troughs and wet stone- 

 work. Sometimes they form variously colored crusts on the sides of 

 dripping cliffs. Two common forms may be described. 



Oscillatoria. (Latin oscillare^ to swing, to oscillate). 



The plants belonging to this genus flourish best in stagnant waters 

 containing decaying organic matter, though they occur to some extent 

 in the purest spring water. They generally form a dark, bluish-green 

 or nearly black slimy scum. A small quantity placed in a vial of water 

 will be enough for a class to examine. Upon the return home it should 

 be taken from the bottle and placed in a shallow dish of water until 

 wanted. Placed under the lens, a bit of this scum appears as hundreds 

 of cylindrical, bluish-green, oscillating threads. A single thread or 

 plant is made up of a number of similar disc-shaped segments, as a 

 row of checkers placed face to face would form an elongated cylinder. 

 The two end segments are hemispherical. The plant reproduces by 

 breaking into several short filaments (hormogones), each consisting of 

 a few segments. These short filaments have the power of moving 

 about in the water for a time, when they develop new plants by the 

 growth and division of their segments. 



Nostoc. (Etymology unknown). 



At the edges of pools and shallow lakes one often finds floating 

 brownish-green balls up to an inch in diameter. These are Nostoc 

 colonies and are made up of a somewhat firm jelly in which are em- 

 bedded thousands of filaments or plants. The filaments magnified 

 look like strings of beads of two sizes. The larger "beads" or seg- 

 ments are placed at intervals in the filament. They are somewhat 

 spherical, nearly colorless and have distinct walls Their use to the 

 plant is not known. The smaller vegetative segments have brownish- 

 green contents and are more or less spherical in form. Nostoc repro- 



