164 ORGANISMS ILLUSTRATING RIOLOGICAL PRINCIPLES 



One of the most common diatoms found in pond water is Navicula. 

 In this form the cell wall consists of two valves, one of which fits into 

 the other. The part that fits over the inner valve is called the girdle. 

 The cell appears quite different in structure when seen from the valve 

 side or the girdle edge. In the latter view, a bridgelike mass of pro- 

 toplasm containing a nucleus appears, while in a valve view a line 

 running down the center, called the raphe, is seen, that shows three 

 tiny spots, one in the middle and one at each end. A mucilaginous 

 material exudes through a series of pores which form the base of the 

 raphe. Navicula has two chloroplasts, colored yellowish-brown by a 

 pigment called carotin. These can be seen best when the cell is 

 viewed from the flat side. At the time of cell division, the chloro- 

 plasts first increase in size, pushing the two valves apart so that they 

 barely touch. Then the nucleus, chloroplasts, and cytoplasm of the 

 cell divide, an inner valve forming for each cell. Each of the new 

 cells thus formed is much smaller than the parent cell. 



Desmids 



Another one-celled form common in fresh water is the bright 

 green desmid, Closterium. Like diatoms, desmids are of various 

 shapes and sizes. They are beautiful symmetrical structures with 



large, bright green chloro- 

 plasts, which may be lobed, 

 starshaped, or platelike. The 

 cell wall is thin and transpar- 

 ent, the granular protoplasm 

 within being obscured by 

 chlorophyll, but the nucleus, 

 in the center of the cell, may 

 be easily recognized. 



Desmids divide by a simple 

 transverse splitting, forming 

 two cells, each new desmid 

 consisting of half of an old 

 cell from which an entire cell 

 is formed. In addition, a process of conjugation takes place, in 

 which two cells come together, each sending out a protoplasmic 

 protuberance that forms a connecting canal. The contents of the 

 two cells meet in this tube, fuse, and form a single cell which grows a 

 thick wall, whereupon it remains as a dormant spore or zygote until 



Closterium. 



Two cells undergoing conjuga- 

 . tion. 



