CHAPTER XVIII. 



SEXUAL REPRODUCTION. 



304. Cell union. — All methods of sexual reproduction 

 consist in the formation of a single cell by the union of two 

 specialized cells, known respectively as the male cell, or 

 sperm, and the female cell, or egg, neither of them capable 

 of growing further without such union. 



The organs and processes of sexual reproduction in plants are scarcely 

 visible except with the microscope, and therefore will not be further dis- 

 cussed here. (See the author's Plant Life.) 



The cell formed by sexual union is capable of developing 

 into a new plant under suitable conditions. It may grow at 

 once into a new plant, or it may remain dormant for a longer 

 or shorter time. If it remains dormant it forms a resting spore. 

 To protect itself, it thickens its wall, often very greatly.* It 

 may then escape from the parent, but more commonly re- 

 mains enclosed until set free by the death and decay of the 

 parent. In the other case, the spore develops at once. 

 Except in the brown seaweeds, whose eggs are ejected into 

 the water before union of the sperms with them, the spore 

 remains enclosed in the parent, within which it begins to 

 form a young plant, the embryo. 



305. Seed. — In all but the seed plants the development of 

 the embryo is uninterrupted until a mature plant is formed. 



* Thick-walled resting spores are also formed vegetatively. 



2i8 



