DESMIDS, DIATOMS, AND FRESH-WATER AhGM. 105 



spirals winding within the cell-wall, the number help- 

 ing to determine the species, of which there are many. 

 The plants usually grow in masses, and especially form . 



Fig. 85. — Spirogyra. 



those soft green clouds apparently floating just under 

 the surface of the water. They are often ' entangled 

 among submerged objects, but almost as often free. 

 Their manner of producing spores is remarkable, 

 but not confined to them, as other Algse have a simi- 

 lar method. Each of the cells of two filaments lying 

 side by side begin, usually at the same time, to protrude 

 from those sides nearest each other a narrow tube. 

 These tubes meet and grow together, 

 so that the two plants soon resemble a 

 ladder, the original filaments forming 

 the sides, the tubes being the rounds. 

 The coloring matter then falls away 

 from the cell-walls, and the entire 

 contents of the cells, of one filament 

 pass through the rungs of this living 

 ladder into the opposite cells, where 

 the contents of both mingle. From 

 this commingling the spore is formed, 

 one in each cell, and is, when ripe, 

 ovoid and dark brawn. This conju- 

 gation, as it is called, and the result- 

 ing spores are shown in Fig. 86. The 

 plants are often found in this condition Fig. se.— Spirogyra in 



, , X 1 • in conjugation: 



in June and July, sometimes even with spores, 

 earlier than June. 



