86 



BOTANY 



Classification of Peridineae (Schiitt) 



A. Cells naked, or with continuous membrane. Ord. I. Gymnodi- 



niacece. 



B. Membrane composed of two pieces. Ord. II. Prorocentracece. 



C. Membrane composed of several plates, arranged in two groups, 



or valves. Ord. III. Peridiniacece. 



Coccospheres and Rhabdospheres (Murray, 13, 15) 



In the surface water of the ocean, especially in the Tropics, there have been 

 found great numbers of excessively minute organisms to which the above names 

 have been given. Our knowledge of their structure is due mainly to the investi- 

 gations of Murray and Blackman. They are globular cells, having a yellow pig- 

 ment, and, imbedded in the cell-wall, calcareous plates of peculiar form. Fission 

 has been observed in some of them, but our knowledge of their life-history is 

 still incomplete, and their affinities are very obscure. 



The Diatoms are among the most widespread of plants, being 

 exceedingly abundant in both fresh and salt water, or even upon 

 the ground where it is damp. It is estimated that there are about 



FIG. 60. Diatomacese. A, Navicula sp., showing the two chromatophores and the 

 nucleus, n (x 500). B, two cells of Melosira varians, showing the chromato- 

 phores, cr (x 500). C, Cymbella lanceolata (X500). D, Nitschia sigmoidea, 

 showing the two valves ( X 250) . 



ten thousand existing species, and they may occur in enormous 

 masses, at times discoloring large bodies of water in which they are 

 suspended. While they exhibit great variety of form and size, the 

 structure is essentially the same in all of them, and they constitute 



