BOT.-VOL. II.] PEIRCE— ROOT-TUBERCLES. 309 



plants are studied with a view to ascertaining the effects of 

 each factor in the environment, it will become more and 

 more evident that many of the effects which are now 

 attributed to internal causes, lumped together under the 

 name inheritance or distributed among the various func- 

 tions of the organs of the body, are the reactions of the 

 parts to stimuli exerted upon them from outside. If the 

 formation of lateral roots by the division of the cells of 

 the pericambium should prove to be the result of external 

 stimuli, it will be found that these stimuli operate upon 

 the cells immediately concerned. 



The diverse development of tubercle and lateral root, the 

 result of the persistence of the different stimuH which called 

 them into existence, obscures the common morphology of 

 the two organs so that it is only natural that Frank should 

 have called the tubercles new organs. If the tubercles 

 were the result of hypertrophy indiscriminately among the 

 cells of the cortex of the root, as may be the case in several 

 of the species which Frank studied and described, this 

 would be no evidence that the tubercles are morphologically 

 roots. But in Bur Clover the case is clear. Doubtless the 

 same can be made out in other leguminous plants when 

 the origin of the tubercles is studied in properly stained 

 microtome sections of young roots. I have not examined 

 Frank's Lupinus, bean, pea, etc., for it is a matter costing 

 much time and patience to find just the right stages, and I 

 preferred to study my own plants and to leave a review of 

 the origin and structure of the tubercles of the plants stud- 

 ied by Frank to a later time or to others interested. 



In Bur Clover, at least, I think I have advanced strong 

 evidence against Frank's contention that the tubercles are 

 new organs. As to his hypothesis, involving inheritance as 

 one of the factors in their origin and development, that 

 lupines grown in sterilized and uninoculated soil may form 

 small enlargements like young tubercles, I think my demon- 

 stration of the common origin of tubercles and lateral roots 

 also has some significance. Assuming that the soil remained 

 sterile, which is not wholly probable, the plant might begin 



