HISTORICAL REVIEW 1 9 



servers apparently believe that light acts directly on the 

 part which bends, but we have seen with the above described 

 seedlings^ that this is not the case. Their lower halves were 

 brightly illuminated for hours, and yet did not bend in the 

 least towards the light, though this is the part which under 

 ordinary circumstances bends the most." (p. 566), " We 

 believe that this case [referring to an experiment of Wies- 

 ner], as well as our own, may be explained by the excite- 

 ment from light being due not so much to its actual amount, 

 as to the difference in amount from that previously re- 

 ceived; and in our case there were repeated alternations 

 from complete darkness to light. In this, and in several 

 of the above specified respects, light seems to act on the 

 tissues of plants, almost in the same manner as it does on 

 the nervous system of animals." (p. 567), " It is an inter- 

 esting experiment to place caps over the tips of the cotyle- 

 dons of Phalaris, and to allow a very little light to enter 

 through minute orifices on one side of the caps, for the lower 

 part of the cotyledons will then bend to this side, and not 

 to the side which has been brightly illuminated during the 

 whole time." (pp. 568-569), " In the case of the radicles 

 of several, probably of all seedling plants, sensitiveness to 

 gravitation is confined to the tip, which transmits an influ- 

 ence to the adjoining upper part, causing it to bend towards 

 the center of the earth. That there is transmission of this 

 kind was proved in an interesting manner when horizon- 

 tally extended radicles of the bean were exposed to the 

 attraction of gravity for i or i\ h., and their tips were 

 then amputated. 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. Radicles thus treated con- 

 tinued to grow out horizontally for two or three days, until 

 ^ The tips of these were covered with opaque caps. 



