A POND SCUM 45 



quantities as a result of the tearing-down process of respira- 

 tion is carbon dioxid. In respiring, therefore, Spirogyra, 

 like nearly all plants and animals, takes in oxygen and gives 

 off carbon dioxid ; at the same time it may be taking in car- 

 bon dioxid as a food and giving off oxygen as a waste product 

 of carbohydrate formation. The net result of these two 

 opposite processes upon the supply of oxygen and carbon 

 dioxid in the water and the air about the plant depends 

 upon which process goes on more rapidly. During the day, 

 carbohydrate formation ordinarily goes on so actively that 

 much more carbon dioxid is taken in for this purpose than is 

 given off in respiration, and much more oxygen is given off 

 than is absorbed. At night, respiration continues but carbo- 

 hydrate formation is stopped, and so only oxygen is taken in 

 and only carbon dioxid is given off. 



69. Reproduction by Division. As in the other plants that we 

 have studied, the number of cells of Spirogyra is increased by the 



division of one cell into two, 



each of these into two, and so 

 on. The cells are so much 

 larger than those of the bac- 

 teria, Protococcus, or yeasts 

 that the method of division 

 can be more easily studied. 

 Unfortunately, division al- 

 most always goes on in Spiro- FIG. 15. A stage in the division of 

 gyra during the night. The a Spirogyra cell (as seen in a lengthwise 

 first step in preparation for section). After Strasburger. 

 the division of the cell is a 



division of the nucleus into two. This division of the nucleus is brought 

 about by a long and complicated process. The two daughter nuclei 

 then move apart toward the ends of the cell, and about halfway be- 

 tween them the outer layer of the cytoplasm (just within the wall) 

 becomes furrowed inward (Fig. 15), much as though some one had 

 wound a thread about the middle of the cytoplasm and then had begun 

 to tighten the thread. The furrow, which extends entirely around 

 the cell, becomes deeper and deeper, until the cell is cut into two cells, 

 the chloroplast also being cut in two in the middle. As the furrow 

 cuts inward, a new wall is formed within the furrow, connected with 



