THE STUDY OF THE ALG^. 149 



alga called Protococcus. The same plant occurs on flower-pots in 

 hot-houses. The rocks in the stone quarries which are kept wet by 

 the spray of water-falls are covered with tufts of Cladophora, which 

 is also attached to the larger pebbles in running streams. Pond-scum 

 floats on the surface of sluggish waters. Vaucheria or "green felt" 

 forms a velvet-like layer on wet ground. Diatoms, though individu- 

 ally microscopic, by gradual accumulation on the surfaces of dripping 

 rocks form a quite noticeable brownish coating. In pure waters — 

 and therefore in most cold springs — Draparnaudia is found. On the 

 other hand, poisonous forms, such as Nostocs and Gloeotrichias, are 

 inhabitants of sluggish waters — for the most part those that are in 

 communication with stagnant marshes. Some algae are parasitic, 

 dwelling within the tissues of succulent plants. An odd place of hab- 

 itation is the human mouth, for a lively little organism of the algal 

 tribe lives on the teeth and is the direct cause of their decay. 



The life history of one 

 plant may be described, 

 and will serve as an ex- 

 ample of the rest. At- 

 tached to the sides of 

 wooden troughs, which 

 are used to conduct cold 

 spring waters, one often 

 finds a minute, delicate 

 fringe of green filaments 

 which upon examination 

 under the microscope is 

 found to be a form known i-ig. i. 



as Ulotlu-ix zonata. The vegetative portion consists of a row of 

 cylindrical cells each about as long as broad. The protoplasm with 

 the chlorophyll granules is evenh^ distributed throughout the cell, 

 giving it a uniform green color. The surrounding wall is delicate and 

 colorless. This filament comprises the young plant. After it has 

 become mature, the cells begin to swell, while the contents di- 

 vide into spherical or somewhat elongated bodies. In certain 

 cells these rounded masses are four in number {Fig. i), and as 

 they enlarge they press upon each other, giving much the same ap- 

 pearance as if four unbaked loaves of bread should be squeezed into a 

 glass cylinder. If at this stage one of these cells be observed some- 

 time during the early forenoon, the wall of the cell will be seen to 

 burst and the four bodies pass slowly out. After pausing an instant 

 as if to get their bearings they suddenly dart off in different directions 

 with a motion very much like that of a spinning top. During this 



