REPRODUCTION. 653 



scarcely a plant which has long been cultivated and pro- 

 pagated by seed, which is not highly variable. 



Now that we have become acquainted with the main 

 facts of reproduction in plants, we will endeavour to eluci- 

 date their physiological significance, to form some general 

 theory of reproduction. Many such theories have been pro- 

 pounded at different times, but we will confine our attention 

 to some of the more recent. Beginning with Darwin's theory 

 of Pangenesis, we find its main assumption to be this, that 

 each separate part or unit of the body throws off minute 

 gemmules, not only in the adult state, but during all stages 

 of development of the organism. These gemmules are capable 

 of multiplying by division, and they may either develope 

 immediately on their formation, or they may remain dormant 

 for a longer or shorter period, and so be transmitted from 

 generation to generation. When the gemmules are especially 

 aggregated in certain parts of the organism, these parts 

 constitute the reproductive organs. In organisms of high 

 organisation the gemmules are confined to the specialised 

 reproductive organs; but in organisms of lower organisation 

 they are not thus confined, but are dispersed throughout the 

 body, so that almost any member which may be isolated can 

 develope into a new individual. 



The theory of Pangenesis certainly facilitates a reason- 

 able apprehension of the main facts of reproduction. We 

 can account for the great capacity of plants for vegetative 

 reproduction, by attributing it to a dispersion of the gemmules 

 throughout the body. For instance, a cutting, when planted, 

 produces roots and thus constitutes a new individual ; and it 

 is able to produce roots because at the time of separation 

 from the parent-plant it contained root-gemmules. Similarly, 

 it eventually produces reproductive organs, because it con- 

 tained reproductive gemmules. Again, on this theory, spores 

 are reproductive cells which are so rich in gemmules, that 

 they can develope into a complete individual; whereas 

 gametes are reproductive cells which do not individually 

 contain a sufficient number of gemmules for independent 

 development. Hence the significance of the sexual process 



