Darwinism stated by Darwin himself · Charles Darwin
INFLUENCE OF GRAVITATION UPON PLANTS.
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 an hour or an hour and a half, and their tips were then amputated. Within this time no trace of curvature was exhibited, and the radicles were now placed pointing vertically downward; 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 continued to grow out horizontally for two or three days, until a new tip was reformed; and this was then acted on by geotropism, and the radicle became curved perpendicularly downward.
Text from Project Gutenberg, public domain in the United States.