GROUPS OF PLANTS. 15 



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

 The girdle is provided with a series of pores connecting 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 series 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 

 wide (Fig. 10, 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 associated 

 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 

 the old one. In each case the valve formed fits into the old one 

 and hence in the case of the smaller valve the new cell or plant 

 becomes smaller than the parent plant, the walls not being able 

 to expand on account of the siliceous composition. In this way 

 the cells of one series gradually become smaller and smaller until 

 a certain minimum is reached, when the plant rejuvenates itself 

 by the production of spores (auxospores). These are formed in 

 two ways: In one case the valves separate from each other, the 

 protoplast escapes, grows larger and develops a new wall ; in the 

 other case, of which there are several types, two individuals come 

 together, and envelop themselves in a mucilaginous covering. 

 They then throw off their siliceous walls and the protoplasts unite 

 to form a zygospore which grows until it is three times the orig- 

 inal size, after which it develops a new wall, the larger valve 

 forming first (Fig. 10, H, I). 



Economic Uses of Algae. — Diatomaceous earth, which is 



• The micron (/i) is the xTrTnr of a millimeter. 



