CHAPTER IV. REPRODUCTION. 259 



for believing that the development of seedless apples, and other 

 seedless fruits, is due, in many cases at least, to the stimulant 

 effects of the pollen on the other tissues of the ovary when the 

 fertilizing material has failed to reach the ovules. 



But, however this may be, there cannot be the least doubt 

 that fertilization makes a profound impression on the entire 

 economy of the plant. There are not only a priori reasons for 

 believing this would be the case, but there are well authenticated 

 instances where the character of the flowers and fruits produced 

 by plants, have been more or less permanently modified as the 

 result of crossing with a plant which produced a flower of a 

 different color, or a fruit of a different quality. 



The question naturally arises, why two modes of reproduction, 

 the asexual and the sexual, should exist among plants. 



Comparing the two processes, we find that the asexual mode 

 is simple, and involves little expenditure of energy on the part 

 of the plant, while the other often requires for its consummation 

 complicated machinery, and is an expensive process, a heavy 

 draft on the vitality of the plant. Why, then, does the sexual- 

 mode exist? Science cannot yet give a complete answer to 

 this question, but it may be partly answered by observing the 

 difference between the offspring produced by the two processes. 

 That of the asexual presents very little variation from the paren- 

 tal form. If we wish to perpetuate a fine variety of fruit, we do 

 not sow the seeds, but rather multiply the plant by grafting, bud- 

 ding, layering, or by some other process of division, imitating 

 nature's modes of asexual multiplication. Should we plant the 

 seeds, we would probably obtain a variety of fruits, those from 

 different seeds differing more or less from each other and from 

 the parent form, and all, very likely, inferior in excellence to the 

 fruit we wish to perpetuate. Now, variation, which in this in- 

 stance we wish to avoid, is of immense advantage to plants in 

 their struggle for existence. The physical conditions of the 

 earth's surface are slowly but constantly changing, and by varia- 

 tion, plants are constantly adapting themselves to these ever- 

 changing conditions. Those varieties best adapted to the exist- 

 ing conditions are the ones to survive ; those unfitted for them 

 must perish. 



The reason of the greater variation in sexually generated off- 



