78 Chapters in Modern Botany chap. 



they do in structure, have the water-tension of their cells, 

 and secondarily, their growth differently affected by the 

 force of gravity ; but other explanations are in the field. 

 Some botanists go so far as to suppose a distinct stem- 

 protoplasm and a distinct root -protoplasm reacting in 

 opposite ways to given stimuli. Hence we see that after 

 all the research and discussion which has taken place the 

 subject is still far from being cleared up. Here, as in the 

 allied or, at any rate, intermingled problem of explaining 

 the mechanism of light -seeking and light -avoiding, the 

 student must guard against the too common habit of find- 

 ing an explanation in what is but a technical nomenclature, 

 and not cease to ask himself the child's puzzling questions 

 of why does this stem grow up and that root grow down 

 merely because he is told to call them in longer words 

 "negatively"' or "positively geotropic," For though we 

 may all laugh with Moliere at old-world medicine, it is 

 no easy matter to leave its virtus dorniitiva out of our 

 minds. 



Darwin laid emphasis on the very tip of the radicle. 

 " In the case of the radicles of several, probably of all 

 seedling plants, sensitiveness to gravitation is confined to 

 the tip, which transmits an influence to the adjoining upper 

 part, causing it to bend towards the centre of the earth." 

 When the tips were amputated the power was lost until a 

 new tip was formed. Subsequent experiments do not, 

 however, confirm this opinion, if indeed they have not 

 disproved it. 



Darwin also made a large number of experiments show- 

 ing how a radicle on whose tip some minute object was 

 fastened, or some slight injury inflicted, bent towards the 

 free or uninjured side. But when the stimulus is applied 

 not to the side of the tip, but to the growing region a little 

 above the tip, the bending takes place towards the stimulus. 



