170 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



protoplasm of the pleurococcus is immortal, since it 

 passes from cell to cell by means of cell division. 



Spirogyra is one of the multicellular green algae. 

 It is a slimy thread, called "pond scum," found near 

 the surface of a pond, often buoyed up by bubbles of 

 gas which it forms. The filamentous plant body 

 consists of several cells joined end to end, each with a 

 characteristic spirally-banded chloroplast. 



Examination of a single cell shows a colorless cell 

 wall, the cytoplasm of the cell mostly adhering to its 

 inner surface. Strands of cytoplasm radiate from a 

 central colorless nucleus, which is suspended in a large 

 vacuole or sap cavity. The most characteristic fea- 

 ture is the large twisted chloroplast, on which are 

 scattered many pyrenoids, bodies which contain some 

 of the starch manufactured by the chloroplast. In- 

 dividuals grow in size by forming, through transverse 

 division of the cells, longer or shorter filaments, de- 



A Spirogyra pending upon the environmental conditions. 



cell showing the 

 spiral chloro- 

 plast containing 

 pyrenoids, and 

 the nucleus. 



At certain times in the year, the plants form resting 

 spores called zygospores. Two adjoining filaments 

 come to lie parallel, the cells opposite to each other 

 sending out bulging outgrowths which meet to form a 

 connecting tube. Meantime, owing to the dissolving of the cell wall 

 at the end of the outgrowths, water gets inside of the cells, so that 

 they show signs of plasmolysis, rounding up into ovoid masses. 

 Curiously, however, the cells of one filament remain stationary, 

 while the cell contents from the other filament move over through 



Conjugation of Spirogyra. Explain what happens. (After Coulter.) 



