14 W. J. Robbiyu 



ledons (coleoptiles) of Plialaris prevents the lower part, though fully 

 exposed to a lateral light, from becoming curved. . . . We must, there- 

 fore, conclude that when seedlings are freely exposed to a lateral 

 light some influence is transmitted from the upper to the lower part, 

 causing the latter to bend. . . . These results seem to imply the pres- 

 ence of some matter in the upper part which is acted upon by light 

 and which transmits its effects to the lower part." 



He concluded also that stimuli were perceived by the root tips 

 and transmitted to the adjoining upper part where the bending oc- 

 curred. To quote from his own words: "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 toward the center of the earth. That 

 there is transmission of this kind was proved in an interesting manner 

 when horizontally extended radicles of the bean were exposed to the 

 attraction of gravity for 1 or li/o hours and their tips were then am- 

 putated. Within this time no trace of curvature was exhibited and 

 the radicles were now placed pointing vertically downwards; but an 

 influence had already been transmitted from the tip to the adjoining 

 part, for it soon became bent to one side, in the same manner as 

 would have occurred had the radicle remained horizontal and been 

 still acted on by geotropism. . . . To see anything of the above kind 

 in the animal kingdom, we should have to suppose that an animal 

 whilst lying down determined to rise up in some particular direc- 

 tion; and that after its head had been cut off, an impulse continued 

 to travel very slowly along the nerves to the proper muscles; so that 

 after several hours the headless animal rose up in the predetermined 

 direction .... We believe that there is no structure in plants more 

 wonderful, so far as its functions are concerned, than the tip of the 

 radicle." 



If you have not recently read this volume on The Poxver of Move- 

 ment in Plants, and especially the two chapters on light and gravity, 

 I recommend them to you. You will feel as though you yourself were 

 participating in the experiments as you note how he covered soil 

 around the seedlings with black paper to prevent upward reflection 

 of light, records the number of plants in many of the experiments 

 which reacted or failed to do so, and tliscusses alternate explanations 

 for his results. For example, he says, "When the upper halves of the 

 cotyledons of Phalaris and Avena were enclosed in little pipes of 

 tinfoil or blackened glass. . . . the lower and unenclosed part did not 

 bend when exposed to lateral liglit, and it occurred to us that this 

 fact might be due not to the exclusion of the light from the upper 

 part, but to some necessity of the bending gradually travelling down 



