The Response of Plants 145 



of these simple plants, that without semi-circular canals 

 without complicated spirit-levels or any apparatus at 

 all that we have yet been able to discover, are yet able 

 to perceive the direction of the earth 's center, absolutely, 

 as if guided by lines of force unseen? Nor is this all. 

 Eoots are not absolutely bound to the earth 's radii, if for 

 some special reason at a given time another direction 

 would seem preferable in presence of some other over- 

 mastering but misguiding stimulus. I once planted cer- 

 tain seeds in wet sawdust in a sieve, intending to clip 

 the advancing root-tips as they came through below. 

 Judge of my surprise, a few days later, on turning the 

 sieve to find that the roots had indeed made their way 

 down and out as I had surmised but, had immediately 

 turned about and were now growing upward, lost in the 

 sawdust again overhead! They had followed the evi- 

 dently irritating wires. Truly the plant responds to the 

 law of gravity ; but relentless, unceasing in its operation 

 as is that unknown force, nevertheless the plant in its 

 response is in a measure free ; uses this cosmic force even 

 when most by it controlled. 



In the same way it might be shown how plants respond 

 to heat, to currents of water, to currents of air, or to 

 other varying or constant phases of environment. The 

 response is real, and often surprising both in promptness 

 and extent. Nothing in this field of experiment is more 

 astonishing than the response made, in form of leaf, by 

 certain semi-aquatic plants to the presence or absence of 

 water. 



The response of plants to moving currents whether of 

 wind or water is too familiar to require more than men- 



