168 PLANT BIOLOGY 



The thread or filament continues to increase in length by th 

 growth and division of certain individual cells that compose it. 

 At the close of the season most of the filaments perish, but some of 

 them undergo peculiar changes. The bands of chlorophyll lose their 

 definiteness, the protoplasm becomes massed, tiny outgrowths from 

 the sides of the cells occur, and these continue to extend till they 

 meet similar outgrowths from a neighboring filament (Fig. 85, ^1). 

 These outgrowths unite, and thus a tube from one cell to the other 

 is formed. The contents of one cell pass through to another, and 

 the two masses fuse. A thick wall forms about the united mass and 

 the old cell walls decay and fall away, leaving these thick-walled 

 zygospores on the bottom of the pond. In the spring the protoplasm 

 within each of these zygospores begins to grow, breaks through the 

 thick wall, and proceeds to form a new filament by cell division. 

 The formation of the zygospores is known as conjugation; it is a 

 kind of sexual reproduction, though the two cells taking part in the 

 process are the same in appearance. 



If one observes pond scum on a sunny day, bubbles will be seen 

 escaping from the mass. A test of this gas proves it to be oxygen, 

 and as we should expect, it occurs in connection with the process of 

 carbohydrate manufacture the same as in other green plants. In 

 fact it has been proved that these simple plants manufacture foods, 

 digest, assimilate, respire, and reproduce as do the higher plants 

 we have studied. The differences, then, between a simple plant 

 like Spirogyra and a bean plant or an oak tree are mainly those of 

 structure and adaptations for the performance of functions which 

 are largely common to both. Indeed, it is evident that every cell 

 of the Spirogyra is in contact with the water, from which all the sub- 

 stances needed are obtained by absorption. Hence, any special 

 adaptations for securing food materials or of giving off wastes, such 

 as are found in higher plants, are unnecessary. 



187. Suggestions for the study of Spirogyra. No. 67. It is 

 desirable that pupils should see the " pond scum " in its habitat, 

 even if they do not collect material for work. The escape of bubbles 

 may be noticed at this time or in the laboratory. The mass should be 

 described as to color and " feel," and the fine threads noted by 



