DEVELOPMENT OF PLANTS 



i6i 



substance the valves are pushed apart and the cell contents di- 

 vides, forming two diatoms with but one valve each. A new 

 valve that fits into the old valve is soon developed on the un- 

 covered side of each diatom and two complete diatoms are thus 

 formed (Fig. 98) which may become free at once or remain at- 



3n^K 



Fig. 98. Diagram illustrating the divisions of a diatom and the resulting 

 reduction in size. The brackets connect the two diatoms that were formed 

 by each successive division. — After Pfitzer. 



tached and by further division form the odd groupings or colo- 

 nies shown in Fig. 94. Since the valves are of unequal size, i. e., 

 one fitting into the other, the two diatoms formed by the division 

 must vary in size and it will be seen that the majority of the off- 

 spring will become greatly reduced in size as a result of the 

 rapid and repeated divisions. That this reduction may not go 

 on too far, when a certain minimum size has been reached the 

 cell contents, with or without dividing, throws off the valves 

 entirely and grows to the full size of the diatom, when new 

 valves are formed and the diatom is ready to repeat the dividing 

 process. In some cases we find quite a different method of re- 

 production. Two diatoms become enclosed in a jelly-like mass, 

 into which the cell contents, with or without dividing, is dis- 

 charged. These naked cells now unite in pairs and the bodies 

 thus formed finally grow into diatoms (Fig. 99). This latter 

 method of reproduction is termed sexual reproduction. The two 

 fusing cells are looked upon as male and female cells or gametes 

 although in these simple plants there is no indication of sex that 

 would enable us to recognize them as such. In the preceding 

 groups, multiplication in numbers or reproduction has been ef- 

 fected by the division of the cells or by the formation of more 

 or less modified cells, termed spores. All such methods of re- 



