28 EVOLUTION OF PLANTS 



While in the lower groups the sexual cells are borne 

 in cells differing but little from the vegetative ones, 

 the higher algse, mosses, and ferns have them con- 

 tained in multicellular structures of very characteristic 

 form which may properly be considered as true sexual 

 organs. 



A marked degeneration of the sexual cells is observed 

 in many fungi where, when present at all, they are 

 usually reduced in structure and sometimes apparently 

 functionless, while in very many of them no traces of 

 sexual organs have as yet been discovered. 



Among the flowering plants there are produced spe- 

 cial accessory structures connected with reproduction 

 but not to be considered strictly themselves as repro- 

 ductive. The various parts of the flower are of this 

 nature, the true reproductive organs being special 

 minute structures within the pollen-grain and ovule. 

 The development of brightly colored and sweet-scented 

 flowers is doubtless connected with the fertilization of 

 the germ-cells within the ovule, and the same is true of 

 the various mechanical devices for insuring pollination 

 through insect agency. The correlation of structures 

 in flowers and insects is often extraordinary, and is 

 sometimes so great that a single species of flower and 

 insect are absolutely dependent on each other for their 

 existence. We shall, however, consider this question 

 more at length in a later chapter. 



A high degree of specialization is also seen in the 

 subsidiary reproductive parts of plants other than the 

 seed plants, although less marked than in those. Thus 

 in the mosses and ferns there are very perfect me- 

 chanical devices for distributing the ripe spores. The. 



