36 A TEXT-BOOK OF BOTANY. 



ring in both fresh and salt waters. They form the plankton or 

 floating microscopic life found in oceans and lakes, which is the 

 source of food of small animal forms inhabiting these waters. 

 The mud at the mouths of many rivers, the sediment of ponds, 

 ditches, and even rain troughs may contain great numbers of 

 these minute organisms. They have been found in the polar 

 ice, and have been detected in the dust evolved from volcanoes. 

 One of the distinguishing characters of the group is that the cell 

 wall is incrusted with silica. For this reason they are practically 

 indestructible and form marls and strata in the earth. They occur 

 either singly or grouped in bands or chains. They are very 

 variable in shape, being boat-shaped, ellipsoidal, spherical, or 

 peculiarly curved in some forms. They are either free or attached 

 to a substratum, as stones, water plants, etc., those which are 

 free having an active movement (Fig. 19). 



The cell wall of Diatoms practically consists of two halves, one 

 fitting over the other like the lid of a box. These are known 

 as " valves " or " theca." The manner in which the two valves 

 are joined results in the formation of a " girdle " or " pleura." 

 The girdle is provided with a series of pores conecting with 

 canals at either end and in the middle, through which food from 

 without is supplied to the protoplast. The valves are very often 

 beautifully marked by a serie^ of parallel cross lines, dots, cir- 

 cles, or polygons, which are characteristic of the different groups. 

 Some forms are used in testing the definition of objectives, as 

 Pleurosigma angulatum, in which the lines are one-half micron 

 (0.0x305 mm.) wide (Fig. 19, A). 



In the Diatoms the protoplasm lies as a thin layer close to the 

 wall surrounding a large central vacuole. The nucleus is sur- 

 rounded by a relatively dense mass of cytoplasm, and occurs in 

 definite positions according to the species. The chromatophores 

 frequently occur in plates which are typical for certain species. 

 They are sometimes greenish-yellow, the color being generally 

 masked by the presence of a brown substance known as diatomin. 

 They frequently contain pyrenoids, which are sometimes asso- 

 ciated with granules of starch. 



Reproduction takes place by simple division or fission, the two 

 valves separating and a new valve forming on each half to replace 



