SPIROGYRA. 39 



CHAPTER V. 

 SPIROGYRA. 



This plant belongs to the third class of the Thallophyte 

 series, which is known as Algce. The class includes nearly all 

 the Thallophyte plants which contain chlorophyll. 



The algae are an assemblage of quite simple plants, none of 

 the members attaining any great degree of complexity. For 

 the most part, the plant body consists of an elongated filament 

 composed of united cells; sometimes, however, they form sur- 

 faces, 'and in other cases the plants are unicellular or aggre- 

 gated into communities. In these plants we find the first 

 examples of undoubted sexuality, and throughout the group 

 the organs and methods of fertilization are nearly enough uni- 

 form to enable us to use them as distinguishing characters. 



The algae are for the most part aquatic plants and inhabit 

 either fresh or salt water. They abound in ponds and slow- 

 running streams. 



Spirogyra will illustrate the characteristics of the class 

 (Fig. 29). Its position in the system of plants is given in the 

 table (Chapter IV). It belongs to the order Conjugatse. This 

 order differs from all other algae in the peculiarly complex 

 structure of the chlorophyll bodies and the mode of sexual 

 reproduction (except some of the Diatomaceae) which consists 

 in the direct conjugation or union of two ordinary vegetative 

 cells, hence the name Conjugate. Spirogyra is a filamentous 

 plant, very common in ponds and ditches as a green scum com- 

 posed of silky, green threads, which sometimes attain a length 

 of six or eight inches. The filaments are unbranched and com- 

 posed of a row of cylindrical cells all alike and independent of 

 each other and loosely joined together. The name is given in 

 allusion to the fact that the chlorophyll bodies, i. e., the bodies 

 bearing the green coloring matter, form spiral bands winding 

 around the cell on the interior of the cell-wall. Sometimes the 

 bands are single, at other times double or treble (Zygnemas 

 have stellate chlorophyll bodies, two in each cell, arranged 

 axially). At intervals along each band are to be seen highly 

 refractive lenticular bodies called pyrenoids. When exposed 

 to the light for some time the pyrenoids would be found, on 

 appropriate treatment, to be surrounded by starch grains. 



The cells are bounded bv well-marked, refractive cellulose 



