394 EFFECT OF GRAVITY UPOX GROWTH [Cii. XV 



how the mere weight of the tip of the plant, supported as it 

 is by the firm soil, should enable it to find its way down. 

 Experiments, also, have shown that the radicle will curve 

 downwards against considerable resistance such as is afforded 

 by the surface of a cup of mercury, by a weight which is lifted 

 by means of a thread passing over a pulley (JOHNSON, '29), 

 or by a delicate spring which is compressed by the down-curv- 

 ing radicle (WACHTEL, '95). By these various methods it has 

 been demonstrated that the down-curving of the geotropic 

 radicle is an active process capable of overcoming resistance 

 amounting, in one case, to 150 milligrammes. Consequently 

 gravity does not act in a grossly mechanical way, but as a 

 stimulus inciting a growth response. 



If now the down-curving of the root is a response the 

 question arises, where is the stimulus received ; at the region 

 of curvature or at some other point ? This is a question which 

 has excited much discussion, owing to apparent contradictions 

 in the experimental evidence. The first attempt to answer 

 this question was made by CIESIELSKI * ('72), who cut off the 

 terminal one-half millimeter of the radicle, placed the rootlet 

 in a horizontal position, and found that, although growth con- 

 tinued, no down-curving occurred until, after several days, the 

 root tip had regenerated. He concluded that the root tip 

 is useful or necessary in geotropism. SACHS ('73, p. 433) 

 repeated CIESIELSKI'S experiments and likewise found an 

 incomplete exhibition of geotropism. But this he attributed 

 to the excessive irritation of the cutting, which led to exagger- 

 ated movements of nutation, tending to obscure geotropism. 

 DARWIN ('80) confirmed CIESIELSKI'S results after both 

 cutting off the root apex and cauterizing it, and explained 

 SACHS' partial failure to get like results on the ground that he 

 did not amputate the radicles in a strictly transverse direction. 

 DARWIN concluded that the "tip of the radicle is alone sensi- 

 tive to geotropism, and that when thus excited it causes the ad- 

 joining parts to bend." The exact length of the sensitive part 

 seems to vary with certain conditions, but it is generally less 



* HARTIG ('66, p. 63) had already stated that the decapitated root was not 

 geotropic. 



