436 



SYSTEMATIC BOTANY. 



sion of cells or by the formation of " zoospores." Keprocluction 

 or formation of spores effected by the antherozoids emitted from 

 the antheridia, either on the same plant (monoecious) or on different 

 ones (dioecious). Spores motionless, solitary, or in groups of four 

 in a single sporange. 



General Remarks. The most familiar examples of this Class are the 

 Seaweeds ; but it also includes a great number of plants found in fresh 

 water and in damp situations, many of which are altogether of micro- 

 scopic dimensions, and invisible, except in quantity, to the naked eye. 



Fig. 503. 



Algffi. A. Oscillatoria autumnalis : a, filament escaped from the sheath, b (magn. 300 dia- 

 meters). B. Desmidiece : a, Pediastrum Boryanum; b, Cosmarium margariiiferum (200 

 diam.) ; c, Closterium Lunula (30 diam.) ; d, Closterium acerosum in conjugation, with the 

 resulting spore (200 diam.). C. Diatomacece: a, Diatoma vulgare; b, end view of a cell 

 (200 diam.). D. Volvocinea: Pandorina Morum (100 diam.). E. Botrydium granulatum 

 (15 diam.). 



The lowest forms, the Palmellece, consist of simple cells, of most varied 

 shapes, usually found connected together in definite or indefinite masses 

 by gelatinous excretion or products of the decomposition of the older 

 cells (figs. 504, 512, E, a). The individual cells, each often representing a 

 distinct plant, are characterized by a wonderful diversity and, in certain 

 families, beauty of form, as in the Desmidees and Diatomacece (fig. 503, B, 

 C) ; sometimes, a& in the Volvocineae, they are provided w r ith vibratile 

 cilia, and exhibit an active spontaneous motion (fig. 50o, D). As a rule 



