42 HEREDITY AND EUGENICS 



that the division rate of protozoa is not affected 

 by the extract. The continued growth of the sala- 

 mander is evidently due to continued cell multipli- 

 cation rather than increase in the size of cells, the 

 hormone effect being probably not directly on the cells 

 of the body, but through the intermediary^ of some 

 other substance which stimulates cell growth and 

 division in all the tissues. 



In plants it has been shown by Bottomley (191 7) 

 that auximones or growth-promoting substances, 

 bearing certain resemblances to the vitamins,* may 

 be obtained from the water extract of bacterised 

 peat. These substances are probably organic de- 

 composition products obtained in peat which has 

 partially decomposed under anaerobic conditions, 

 and is then acted upon by aerobic bacteria. When 

 368 parts per million of organic matter from the 

 water extract was added to a culture of Lemna minor, 

 grown in nutrient solution, the effect was remarkable. 

 In six weeks the increase compared with that of con- 

 trol plants was sixt^^-two times in weight and twenty 

 times in number of plants. The increase in size was 

 striking, not onty as regards the individual plants, 

 but also in the cells, nuclei, and chloroplasts. 



That gigantism of body and of cells in plants is 

 also often associated with tetraploidy or doubling in 

 the chromosome number has been shown by Gates 

 (1909) and by Tupper and Bartlett (191 6), with de- 

 tailed measurements of cells and nuclei in various 

 tissues. This is another example of the same mor- 

 phological difference being produced by an external 

 stimulus, in which case it is not inherited, or by a 

 germinal change, when it is inherited. 



* Vitamins are substances of vegetable origin whose presence 

 in minute quantities is necessary for the proper development of 

 the higher animals and man. In their absence such diseases as 

 scurvy, beri-beri, and rickets develop. 



