240 



BIOLOGY 



spore formation is unusual, except in cases where it alternates 

 with a cell union, as in Plasmodium. Among bacteria there is 

 a spore formation of a peculiar kind. 

 Here, as shown in Figure 33 E, each 

 bacterium produces a single spore only, 

 instead of several, and the spore forma- 

 tion is really not a form of multiplica- 

 tion. The cells formed are, however, 

 called spores, although their function 

 seems to be to resist adverse condi- 

 tions rather than to reproduce the or- 

 ganism. They have resisting walls and 

 are capable of developing into new in- 

 dividuals, thus agreeing with other 

 spores except in the fact that one only 

 is produced in a cell. 



Reproduction by Cell Union among 

 Unicellular Organisms. The process 

 of cell division among single-celled or- 

 ganisms may continue for a long time, 

 producing an indefinite series of off- 

 spring. Whether in any case this kind of division can really 

 go on indefinitely we do not positively know. There are some 

 organisms like yeast and bacteria, in which we have reason 

 for suspecting that cell division may go on indefinitely if proper 

 conditions can be maintained, and in which, up to the present, 

 no trace of any other kind of reproduction has been found. 

 It is believed by some that even animals like Paramecium, 

 which conjugate occasionally, may, if proper conditions be 

 maintained, go on dividing indefinitely. Whether this is true 

 or not, it is certain that under ordinary conditions cell division 

 in time becomes slower, and in Paramecium it has a tendency 

 to come to an end, unless it is reinvigorated in some way. 

 In nature such an invigoration is brought about by a fusion 

 of cells with each other as already described; see Fig. 23, page 64. 



FIG. 111. GONIUM^ AN 

 ORGANISM FORMED OP 

 SIXTEEN CELLS UNITED 



BY JELLY 



A, view from the side; B, 

 view from above and showing 

 the method of reproduction by 

 division of each cell into sixteen 

 parts, which separate to form 

 new colonies. 



