REPRODUCTION BY BINARY FISSION 



247 



takes in food from the outside, grows to its maximum size, and 

 then divides again. In the simpler animals and plants, after repro- 

 duction, the new cells may separate and go about as single individ- 

 uals or they may cling together. If they cling together, each acts 

 as a separate individual independent of the other. 



Reproduction by binary fission. The simplest of all plant 

 organisms belong to the Thallophyta. As previously men- 

 tioned, they are distinguished by the character of having no 

 division into roots, stems, and leaves. The thallophytes are 

 divided into two sub-groups : the fungi — thallophytes lacking 

 chlorophyll, such as bacteria, yeasts, and molds ; and the algae — 

 those having chlorophyll, such as Pleurococcus and Spirogyra. 



Bacteria show little diflFerentiation of structure. Thev consist 

 of a mass of protoplasm with nuclear material scattered through 

 the cytoplasm. They are surrounded b}^ a thin cell wall. Bac- 

 teria reproduce by splitting in half, thus forming two new indi- 

 viduals. During reproduction, the nucleus elongates and divides. 

 A cell wall is formed between the two nuclei, cutting the cell in 

 half. Probably the division of the nucleus is always a mitotic 

 division. Sometimes the newly formed cells remain attached ; 

 often they separate. This type of 

 reproduction is binary fission. 



Many of the algae reproduce 

 much like bacteria. Pleurococcus 

 is found on the shadv, moist sides 

 of trees, rocks, and stumps every- 

 where. Each plant is a single cell 

 consisting of a cell wall, cytoplasm 

 and nucleus. It makes its own 

 food, reaches a maximum size, and 

 divides. The resultin^^ cells either 

 separate or remain in clusters. 



n r-( • T • 1 1 Germs multiply rapidly when conditions 



The cells of Spirogyra divide by are favorable. 



