264 



ALG^. 



Order 2. — Zygnemace^. 



The individual consists, as in the Mesocarpaceae, of a filament of 

 cells placed end to end, which is almost always simple and unbranched. 

 The filaments are cylindrical, and the cells of which it is composed often 

 of comparatively large size ; those of Spirogyra crassa (Ktz.) as much as 

 -^\^ of an inch (125 yu.) in diameter, and twice as long as broad. The 

 chlorophyll is arranged in one or more straight (Siro- 

 gonium, Ktz.) or more commonly spiral (Spirogyra, Lk.) 

 bands, or in stars placed in pairs in each cell (Zygnema, 

 Ktz.), or occasionally in an axile plate (Mougeotia, 

 deBy.), and encloses large starch-grains and a nucleus, 

 often very large and easily discernible even without the 

 use of staining reagents, connected with the parietal 

 protoplasm by radiating threads. In some species of 

 Zygnema the characteristic appearance of the endo- 

 chrome is assumed only when the cells are about to 

 conjugate. The ease with which some members 

 of this family, especially species of Spirogyra and 

 Zygnema, are cultivated in fresh-water aquaria, and 

 the beautiful arrangement of the endochrome, not only 

 make these algae extremely striking objects under the 

 microscope, but afford especially good opportunities for 

 observing, in their details, the processes of division 

 of the nucleus and of the cell.' It is also in this 

 family that the interesting process of conjugation is 

 most easily followed. 



The filaments increase in length by ordinary cell- 

 division ; and single cells, which become very easily 

 detached, are able to develop in this way into new 

 individuals, corresponding, therefore, functionally to 

 the non-sexual spores of fungi. Like the Desmidiacese 

 and other floating algae, they can obtain their nourish- 

 ment entirely from the water, and increase without 

 any attachment to the substratum. The formation of 

 resting-cells and of cysts has already been mentioned. The only other 

 known mode of reproduction is by conjugation ; but the reproductive 

 organs of other algae or fungi, which are sometimes parasitic upon or 



Fig. 236. — Spirogyra. 

 porticalis Vauch. ; 

 stages in the forma- 

 tion of zygosperm 

 ( X 100). (From 

 nature.) 



' See Strasburger, Ueber Zellbildmtg und Zetttheilung ; also Sachs, Text-book of 

 Botany, 2nd English cd. 1882, p. 16. 



