LINEAR AND SUPERFICIAL AGGREGATES- 21 



green color, and slippery " feel ' ' between the fingers.* Under 

 the microscope, they are at once distinguished from other 

 filamentous algae by the shape of their 

 chloroplasts. In Spirogyra these form 

 one or more flattish, spirally wound rib- 

 bons, notched on the edges, and embedded 

 in the protoplasm near the cell-wall {ch> 

 fig. 24). In Zygnema there are generally 

 two irregularly star-shaped chloroplasts 

 (figs. 25, 26) ; while in Mesocarpus a 

 single flat, plate-like chloroplast, nearly 

 as wide as the cell, traverses its center 



(fig. 27). f 



Embedded in the chloroplasts of these 

 and other algae are usually seen one or 

 more angular, colorless bodies, often sur- 

 rounded by a jacket of starch. These are 

 crystals of reserve proteid, known as pyre •- 

 noids (p, figs. 24, 27). Their size depends FlG . 27 ._ A ceil from fiia- 

 upon the amount of reserve food possessed 

 by the plant. 



In these plants there is little or no dif- 

 ference between the parts of the filaments. 

 If broken into two, each part may continue 

 growing with no damage to any part 

 except the cells which were ruptured in 

 severing the plant. 



26. Ulothrix, etc. — But other filamentous algae show a 

 distinction between base and apex. In Ulothrix (fig. 301) 



ment of Mesocarpus. 

 The darker body nearly 

 filling cell is the chloro- 

 plast (face view) in 

 which are pyrenoids, /, 

 and tannin vesicles, g. 

 If seen from a direction 

 at right angles it would 

 appear as a narrow 

 stripe in the center of 

 the cell, z, the nucleus. 

 Magnified about 200 

 diam.— After Zimmer- 



* This slipperiness is due to the gelatinous outer part of the cell-wall 

 (fig. 26), which is only visible after special treatment or on examining the 

 filaments in a thin mechanical solution of Chinese ink. 



f See also Ulothrix (fig. 301), which has in each cell a single chloro- 

 plast in the form of a thick ring. 



