92 FOUNDATIONS OF BIOLOGY 



phase of metabolism. The single cell, whether a whole 

 organism or a single unit of a complex body, increases in 

 volume up to a certain limit and then divides. In the former 

 case two"Tie^wnMividuals replace the parent cell; in the 

 latter, liie^complelTbody has been increased to the extent of 

 one celL- In both "cases cell division has resulted in cell 

 reproduction. Thus cell division is always reproduction, 

 though it is customary and convenient to : restrict the term 

 reproduction to cell divisions which result in the formation of 

 new individuals single cells or groups of cells which sooner 

 or later separate from the parent organism. 



It will be recalled that during the life cycle of Sphaerella 

 there is associated with the reproductive act of cell division, 

 the formation of cell individuals which exhibit in simple form 

 the fundamental characteristics of spores and gametes. We 

 shall now see that the development and specialization of 

 these is at the basis of the elaborate reproductive processes of 

 the higher plants. 



A. SPORE FORMATION 



As already emphasized, cell division among unicellular 

 plants results in the formation of new individuals, and, among 

 multicellular plants, in the growth of the single individual. 

 This is well illustrated by the familiar pond scums in which 

 the plant body consists of a series of similar cells placed end 

 to end to form a long thread-like body. In such cases, cell 

 division results merely in an increase in the length of the fila- 

 ment constituting the plant body, unless the newly formed 

 cell becomes detached from the parent plant. As a matter 

 of fact, however, under certain conditions the protoplasmic 

 content actually does make its escape from the cell wall and 

 swims about in the surrounding water. This independent 

 PROTOPLAST is a spore. 



