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A TEXTBOOK OF BOTANY 



[Ch. X 



forms the contents of both cells move into the projections, 

 and conjugate midway between the filaments; and in other 

 kinds two adjoining cells of the same filament conjugate. 

 These conjugation tubes reappear in higher Algae, and are 

 common in the Fungi; and they anticipate the pollen 

 tube of the flowering plants. The Pond Scums spread 

 by various incidents of water movement, without special 

 adaptations, aside from formation of resting spores. Some 

 300 species are known, but none have economic importance. 



?TZ1% 



Fig. 295. — Desmids, of various forms ; X 200. (From Kerner.) 



Desmidiacece : the Desmids. These are chiefly unicellular 

 floating plants of fresh water, notable for their beautifully 

 symmetrical, diversely shaped, and highly sculptured 

 cellulose walls (Fig. 295). They live mostly around the 

 margins and bottoms of ponds, sometimes attached by the 

 abundant jelly secreted from their walls; and they are 

 familiar in all material drawn from such sources for exami- 

 nation by the microscope. A characteristic feature is their 

 duplicate symmetry, for the cell consists of two similar 

 halves, related like an object and its mirror image, spread- 

 ing from a mid-zone, usually constricted, in which lies 



