272 



ELEMENTS OF GENERAL SCIENCE 



The resting cell is called a spore, which means a special 

 reproductive cell which can form a new plant like those that 



formed the spore. In 

 forming the spore the 

 mass of protoplasm 



'i, bd in one cell is fertilized 



Mb? 



by having the proto- 

 plasm from the other 

 cell unite with it. 

 Reproduction that is 

 brought about by 

 having two cells unite 

 to form the reproduc- 

 tive cell is called 

 sexual reproduction. 

 The spore thus formed 

 may be called the sex 

 cell or sex spore. 

 In pond scum many 

 paired cells of two 

 adjacent plants may 

 produce sex spores, 

 and many new plants 

 may thus be pro- 

 vided for. 



This one sex cell 

 begins the growth 

 of a new pond-scum 

 plant, and by succes- 

 sive divisions of the 

 cells thus formed a 

 many-celled plant may 

 soon be grown. Whether a plant or animal in its adult form con- 

 sists of one or of many cells, it consists of one cell at first. This 

 is not true usually of plants or animals formed vegetatively, 



FIG. 126. A pond scum 



A, two vegetative cells, showing the form of the 

 cell and of the spiral chlorophyll band (ch), also 

 the nuclei of the cells (//,), and the granular part 

 of the protoplasm (cy) ; B, cells showing the be- 

 ginning of the union of the tubes (a and 6), 

 and tubes which have failed to unite (c and d) ; 

 C, completed spores (z) 



