CHAPTER V. THE THALLOPHYTA. 



2 95 



The Spirogyras are filamentous algae, very common in ponds 

 and ditches, and they occur in masses of silky, green threads 

 which sometimes attain a length of six or eight inches. The 

 name Spirogyra was given in allusion to the fact that the chlo- 

 rophyll-bodies form spiral bands winding around the cell adja- 

 cent to its interior wall. In some species the bands are single, 

 in others there may be two or more. Their modes of fission and 

 conjugation have already been described in Part II. See, also, 

 Fig. 491. 



The Zygnemas differ from the Spirogyras in having the chloro- 

 phvll-bodies stellate in form and arranged axillary, a pair of 

 them in each cell. 



In Mesocarpus and some other related forms the zygospore is 



Fig. 492. — Cells of two filaments of Mesocarpus scalaris with zygospore formed between 

 them. Magnified about 350 diameters. 



not formed within either of the conjugating cells but in the 

 space between them, as shown in Fig. 492. 



The Desmids are found in great abundance and variety in 

 clear, fresh water. They are mostly unicellular, but in some 

 cases are loosely united into filaments. They have firm but not 

 flinty cell-walls, and the cells are usually more or less constricted 

 in the middle ; when this is not the case the symmetrical arrange- 

 ment of the cell contents into two halves is more or less evident. 

 The species are exceedingly numerous, and the semi-cells of many 

 are lobed, spinose, delicately striated or otherwise ornamented. 



Asexual reproduction takes place by division along the plane 

 of symmetry between the semi-cells. The new wall thus formed 

 is double, and on each side of it is formed a new semi-cell. 

 When these reach a size about as great as that of the old semi- 

 cells, separation takes place. Each of the new plants thus 



