I 9 8 



ELEMENTS OF SCIENCE 



cells. Many unicellular organisms also divide them- 

 selves into two equal halves, which each grow as large 

 as was the previously undivided cell. Thus new 

 individuals are generated in the simplest fashion 

 imaginable. 



Other forms send forth more or less delicate pro- 

 longations of their substance, at the ends of which 

 minute cells, termed spores, are produced, each of which, 

 under favourable circumstances, will grow up into the 

 form of the parent which produced it. 



Minute water-weeds, which may consist of but a 



FIG. 29. 



AMCEBA SHOWN IN TWO OF THE MANY IRREGULAR 



SHAPES IT ASSUMES. (After Howes,} 



The clear space within it is a contractite vesicle. The dark body is 

 the nucleus. In the right-hand figure there is shown a particle 

 of food, passing through the external surface. 



thread-like single series of cells (confervce) will, when 

 two such threads are adjacent, produce spores by con- 

 jugation. For this purpose, processes from opposite cells 

 of two such thread-like plants, will grow forth, meet, 

 and then blend together their protoplasmic contents. 

 The result of this process is the production of a spore, 

 which will afterwards grow into another conferva 

 thread. Those multicellular fungi known as ."pun- 

 balls," give forth, when ripe, such a multitude of 

 minute "spores" as to resemble a puff of smoke 



