DEVELOPMENT OF PLANTS 



153 



production 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 

 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 



B 



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

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

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

 of the gametes. After Karsten. 



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 

 in life, such as conditions unfavorable for growth. The spores 

 are especially to be looked upon as devices to bring about a 

 rapid increase in the number of individuals while the gameto- 

 spores more usually serve to carry the plant over periods of 

 drought or extremes of temperature which would prove fatal to 

 the plant. For this reason the gametospores are often provided 

 with thick walls and dense cell contents which enable them to 



