60 THE SMALLEST LIVING THINGS 



of a band of protoplasm in the isthmus formed by the constric- 

 tion. With its formation the two semicells are pushed apart. 

 A division plane passes through the center of the band of proto- 

 plasm and two daughter desmids result, each with one old semi- 

 cell and a new bud. The latter soon grows to the size of the 

 old semicell and the division is completed. In some types the 

 daughter cells remain united, thus forming chains of cells or 

 filaments, as in the genus Desmidium. 



In many desmids no sexual processes are known, but in some 

 forms a peculiar type of conjugation* occurs which resembles the 

 fertilization processes of many kinds of filamentous algae and 

 justifies the inclusion of desmids with the latter in the group 

 Conjugatae. In this process two individuals come together and 

 are enveloped in a common gelatinous matrix. A tube-like out- 

 growth emerges from the isthmus of each desmid and in these 

 tubes the protoplasmic bodies meet and fuse to form the fertilized 

 cell, or zygote. The zygotes develop heavy coatings for pro- 

 tection, and their germination occurs after a prolonged resting 

 period. 



Class Chrysophyceae 



In botanical textbooks the order "Chrysomonadida" of pro- 

 tozoa is termed the Chrysophyceae. Here the green chlorophyll, 

 which in normal plants is composed of four pigments (two yel- 

 low, two green), becomes yellow by the preponderance of one 

 of the yellow pigments (phytochrysin) . Products of nutrition 

 are stored up in each cell in the form of oil and of highly refrac- 

 tile, colorless rounded bodies composed of a substance known 

 as leucosin, the chemical composition of which is still uncertain. 

 These leucosin granules are characteristic of the class. The oils 

 frequently cause disagreeable odors and taste in drinking waters 

 — Uroglenopsis (Fig. 22, page 51) and Synura (Fig. 32) being 

 particularly noteworthy in this respect. 



In the cells of this group the flagella, one or two in number, 

 are delicate whip-like structures. They are inserted at the for- 

 ward end (the apex) of the cell, and are very difficult to see 

 without the use of reagents. The cell bodies are comparatively 

 simple in structure and are usually covered by a delicate, color- 



* See "The Plant World," pages 14 and 25, in this Series. 



