STUDIES IN CRYPTOGAMS 



185 



plant often forms the greater part of the floating green mass (or 



" frog spittle ") on ponds. The threadlike character of the thallus 



can be seen with the naked eye or with a hand 



lens, but to study it carefully a microscope 



magnifying two hundred diameters or more 



must be used. The thread is divided into long" 



cells by cross walls which, according to the 



species, are either straight or curiously folded 



(Fig. 266). The chlorophyll is arranged in 



beautiful spiral bands near the wall of each cell. 



From the character of these bands the plant 



takes its name. Each cell is provided with a 



nucleus and other protoplasm. The nucleus is 



suspended near the center of the cell (a, Fig. 



265) by delicate strands of protoplasm radiat- 

 ing toward the wall and terminating at certain 



points in the chlorophyll band. The remainder 



of the protoplasm forms a thin layer lining the 



wall. The interior of the cell is filled with 



cell-sap. The protoplasm and nucleus cannot 



be easily seen, but if the plant is stained with 



a dilute alcoholic solution of eosin they become 



clear. 



Spirogyra is propagated vegetatively by the 



breaking off of parts of the threads, which con- 

 tinue to grow as new plants. Resting-spores, 



which may remain dormant for a time, are formed by a process 

 known as conjugation. Two threads lying side 

 by side send out short projections, usually from 

 all the cells of a long series (Fig. 266). The 

 projections or processes from opposite cells 

 grow toward each other, meet, and fuse, form- 

 ing a connecting tube between the cells. The 

 protoplasm, nucleus, and chlorophyll band of 

 one cell now pass through this tube, and unite 

 with the contents of the other cell. The en- 

 tire mass then becomes surrounded by a thick 

 cellulose wall, thus completing the resting- 

 spore, or zygospore (z, Fig. 266). 



FIG. 266. CON- 

 JUGATION OF 

 SPIROGYRA. 

 Ripe zygospores 

 on the left; a, 

 connecting 

 tubes. 



FIG. 267. STRAND, 

 OR FILAMENT OF 

 ZYGNEMA, freed 



from its gelatinous 

 covering. 



Zygnema is an alga closely related to spiro- 

 gyra and found in similar places. Its life 

 history is practically the same, but it differs 

 from spirogyra in having two star-shaped 

 chlorophyll bodies (Fig. 267) in each cell, in- 

 stead of a chlorophyll-bearing spiral band. 



