172 



REPRODUCTION OF DIATOMS 



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 

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

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

 duction are called asexual, since but one cell is utilized in the 

 process. In sexual reproduction a cell is formed by the union 

 of the contents of two cells. The nuclei and protoplasmic con- 

 tents of each unite so completely that a cell with but one nucleus 

 and protoplasmic contents results. These sexually produced 

 cells are called gametospores because they are formed by the 



^■p^^^ 



it 



B 



Fig. 99. Sexual reproduction of Pinnularia: A two diatoms, enveloped 

 in a mass of jelly. The valves have been thrown off and the content of 

 each diatom has divided into two sexual cells or gametes. B, the fusion of 

 the gametes.— After Karsten. 



union of two gametes and are capable of forming a new plant 

 in the same way as do spores. We may distinguish the spores 

 derived from single cells as asexually formed spores or we may 

 call them simply spores, while the spores formed by the fusion 

 of the two cells or gametes may be termed sexually formed spores 

 or gametospores. Both kinds of spores are devices to enable 

 the plant to increase in number or to meet some other problem 



