GENERAL BIOLOGY. 



11 



kind, and in this instance by two methods : gemmation grnug 

 rise to the bead-like aggregations alluded to above; and in- 

 ternal division of the protoplasm {endogenous division). 



From the circumstances under which growth and reproduc- 

 tion take place, it will be seen that the original protoplasm of 

 the cells may increase its bulk or grow when supplied with 

 suitable food, which is not, as will be learned later, the same in 

 aU respects as that on which green plants thrive ; and that this 

 may occur in darkness. But it is to be especially noted that the 

 protoplasm resulting from the action of the living cells is 

 wholly different from any of the substances used as food. This 

 power to construct protoplasm from inanimate and unorgan- 

 ized materials, reproduction, and fermentation are all proper- 

 ties characteristic of living organisms alone. 



It will be further observed that these changes all take place 

 within, narrow limits of temperature; or, to put the matter 

 more generally, that the life-history of this humble organism 

 can only be unfolded under certain well-defined conditions. 



Peotococcus {Protococcus pluvialis). 



The study of this one-celled plant will afford instructive 

 comparison between the ordinary green plant and the colorless 

 plants or f xuigi. 



Fia.5. 



Pio.6. 



Pig. 7. 



Pigs. 5 to 7 represent successive stages observed in the lite-history of Protococci 



scraped from the bark of a tree. 

 Fig. B.— a group in the dried state, illustrating method of division. 

 Fig. 6. — One of the above after two days' immersion in water. 

 Fig. 7. — Various phases in the later motile stage assumed by the above specimens. 



The nucleus is denoted by nc ,* the cell wall by c.w ; and the coloring-matter by 



the dark spot. On the left of Fig. 7 an individual may be seen that is devoid of a 



cell wall. 



