54 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



root hairs of the legume, certain cells are stimulated to 

 growth and modification into nodule cells. Growth of these 

 cells continues up to a certain point, when not only the cell 

 ceases to grow, but bacterial growth also ceases. When the 

 cells have reached this stage, they are no longer capable 

 of resisting other microorganisms. When one examines an 

 old nodule, microscopically, one usually finds the cells 

 loaded with a great variety of microorganisms, and very 

 few if any Bacillus radicicola. It appears evident that the 

 root nodule cells control the growth of the contained bacteria. 

 It further appears probable that the microorganisms in 

 turn exercise an influence upon cell reproduction or division. 

 Eventually, in the growth of the tissue, a state of equilib- 

 rium is attained in which the cell and its symbiotic organ- 

 isms come to a state of rest. Obviously, all hving tissues 

 may not develop the same kind of growth mechanism. 

 Many cells in the body stop growing at a certain period in 

 adult hfe, others, the epithehal cells of the skin, for example, 

 continue to reproduce so long as life is maintained. 



The complex changes that chromatin material undergoes 

 in mitosis or cell division, does not appear, on first analysis, 

 to be dependent on influences emanating from the cyto- 

 plasm. Obviously, this remarkable series of progressive 

 changes in the chromatin must be dependent upon some 

 stimulus, extrinsic to its own substance. It is conceivable 

 that mitochondria produce chemical substances, or perhaps, 

 physical influences which ultimately become sufficiently 

 strong to inaugurate mitosis. The observations that mito- 

 chondria are more numerous in young, growing cells than 

 in old or senile cells, appear to support the hypothesis that 

 mitochondria may be the controUing influence in cell di- 

 vision. However, it would be difficult at present to attempt 

 to definitely correlate the number of mitochondria with cell 

 division. While many observations have been made in 

 which mitochondria appear to be more numerous in yoimg, 



