•211 



firmly attached to one spot; roaming about is impossible, and to see and 

 smell would be useless. If they were in bodily danger, no acuteness of 

 sight or hearing would avail them in the least. Were the aspen quaking 

 through lear of some horrible calamity, it could not move an inch out of 

 the path of destruction. Again, plants take no solid food, and have no 

 use for a sense of taste. In short, animals are endowed with a set of senses 

 which would be practically useless to plants, from the fact that the latter 

 are, with very few exceptions, fixed instead of being locomotive organisms. 



But are there no movements within the power of a fixed organism that 

 can be brought about by the action of stimuli, which may aid in self-pres- 

 ervation or improving the conditions of existence? I think that a little 

 reflection will show that there are; and if we can find that plants have de- 

 veloped special mechanism in connection with a superior localized sensi- 

 tiveness to enable them to take advantage of the conditions of their exis- 

 tence, we shall have demonstrated the possession of special senses. 



There is no requirement for plants more universal or more necessary 

 than that their roots should penetrate the soil and their foliage be spread 

 to the air. Yet the root or shoot has no more power to deviate from ex- 

 tension in a straight line unless acted on by some external force, than a 

 cannon ball or other moving body has to vary its course from a straight 

 line. If a seed in germinating should lie in such a position that the roots 

 point upward and the stem downward, some device is needed by which 

 the plantlet may readjust itself, by either turning over bodily, or chang- 

 ing the direction of its growing parts. As everyone knows the latter al- 

 ternative is adopted, and the roots bend down and penetrate the earth, 

 while the stem bends up and lifts its foliage into the air. It is so appar- 

 ently a matter of course that stems grow up and roots grow down, that 

 we may never have given a thought to an explanation of the process. 

 Even botanists have only recently felt the full necessity for accounting for 

 the fact, as it has been only a decade since Vochting announced his theory 

 of rectipetality, or the inherent tendency of growing organs to extend in a 

 straight line unless acted upon by outside forces. 



There is only one force known that acts uniformly in the direction of 

 the center of the earth, that is gravity ; and it was the genius of Andrew 

 Knight, an Englishman, to demonstrate as long ago as 1806, that this force 

 does furnish the directive influence in securing verticality to plants. He 

 grew plants on revolving wheels, and found that they responded to cen- 

 trifugal force, and that when the wheel was placed horizontally and re- 



