74 HEREDITY AND EVOLUTION IN PLANTS 



and molecules is it made possible that the peculiar tone 

 of one's voice, the color of a rose, the odor of a carnation, 

 the evenness (or otherwise) of one's disposition, may be 

 transmitted from one generation to another? How may 

 it be transmitted through one generation, without causing 

 any external expression, and reappear in the second gen- 

 eration removed? Is the cytoplasm the carrier, or the 

 chromatin, or both combined, or neither? Is the transfer 

 accomplished by little particles (pangens), as de Vries 

 contends, or by chondriosomes, or otherwise? We do 

 not definitely know, but many careful investigations point 

 to the chromatin as the bearer of the hereditary factors. 

 64. Weismannism. It was a botanist, Nageli, who first 

 recognized and clearly stated that inheritance must depend 

 upon a least quantity of matter, and numerous experi- 

 ments by both botanists and zoologists soon made it 

 evident that the hereditary substance is in the cell- 

 nucleus, rather than in the cytoplasm surrounding the 

 nucleus. Nageli called the hereditary substance idio- 

 plasm. Observations of the germ-cells of plants by Stras-- 

 burger, and of the germ-cells of animals by O. Hertwig, 

 led them to conclude that the chromosomes of the dividing 

 nucleus (Fig. 30 )are the locus of the hereditary substance. 

 The subsequent evidence upon which this conclusion rests 

 is too voluminous, and some of it too technical, to be pre- 

 sented here in any detail. * As an illustration there may 

 be cited the experiment of Boveri who removed the 

 nucleus from the egg-cell of one species of sea-urchin, and 

 then caused the remaining cytoplasm to be fertilized with 

 a sperm-cell of another species of sea-urchin; the result- 

 ing larva possessed only paternal characters. 



1 See Morgan, T. H. The physical basis of heredity. Philadelphia, 

 1919. 



