ALG.E 



179 



be used. The thread is divided into long cells by cross-walls which, 

 according to the species, are either straight or curiously folded (Fig. 

 314). 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. 313, by delicate strands 

 of protoplasm radiating toward the wall and terminat- 

 ing 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 (146) they become clear. 



Spirogyrais propagated vegetatively by the break- 

 ing off of parts of the threads, which continue to grow 

 as new plants. Eesting-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. 314). The projections or processes from 

 opposite cells grow toward each other, meet and fuse, 

 forming a connecting tube between the cells. The 

 protoplasm, nucleus, and chlorophyll band of one cell 



314. Conjugation, 

 of spirogyra. 

 Ripe zygospores 

 on the left; a, 

 con nee ting 

 tubes. 



now pass through this tube, and unite with the contents of the other 

 cell. The entire mass then becomes surrounded by a thick cellulose 

 wall, thus completing the resting -spore, or zygospore (Fig. 314, 0). 



Vaucheria is another alga common in shallow water and on damp 

 soil. The thallus is much branched, but the threads are not divided 

 by cross-walls as in spirogyra. The plants are attached by means of 

 colorless root-like organs which are much like the root-hairs of the 

 higher plants : these are rliizoids. The chlorophyll is in the form of 

 grains scattered through the thread. 



Vaucheria has a special mode of vegetative reproduction by means 

 of swimming spores or swarm-spores. These are formed singly in a 

 short, enlarged lateral branch known as the sporangium. When the 

 sporangium bursts the entire contents escape, forming a single large 

 swarm-spore, which swims about by means of numerous lashes or cilia 

 on its surface. The swarm-spores are so large that they can be seen 

 with the naked eye. After swimming about for some time they come 

 to rest and germinate, producing a new plant. 



The formation of resting- spores of Vaucheria is accomplished 



