70 A PRIMER OF BIOLOGY 



matter what its original position, bent towards the 

 sky. If some germinating peas be pinned to the rim 

 of a vertically revolving wheel, so that their roots and 

 shoots form all possible angles with the horizon, it 

 will be found that both roots and shoots grow in the 

 directions in which they have been originally placed, 

 because, owing to the slow revolution of the wheel, 

 the stimulus does not affect the same part continuously 

 in the same direction. What stimulus is given during 

 one half revolution woi^ld appear to be neutralised 

 during the second half ; at all events, even though 

 the stimulus be appreciated there is no visible response 

 so long as the wheel is revolving. Gravity has, we 

 might say, been put on both sides of the equation and 

 may, as the mathematician puts it, be ignored. 



In botanical terminology the normal root is said 

 to be geotropic, and the normal shoot a- or apo- 

 geotropic. A very simple and instructive experiment 

 is to take some moistened mustard seed and throw 

 them against the inside of a damp empty flower-pot. 

 The seeds \vill adhere to its surface, and will germinate 

 in situ. The pot is then turned upside down over 

 damp blotting-paper or wet sawdust, &c., the pot 

 being at the same time covered over by a wet cloth. 

 If the pot be examined after a couple of days, it will 

 be found that all the young roots have grown down- 

 wards along the wet wall of the pot, and the shoots 

 have grown upwards, but without touching the wall. 

 If the pot be now placed in its normal position, so 

 that the roots point upwards and the shoot down- 

 wards, and if the mouth of the pot be covered with 

 a black cloth and be left for forty-eight hours, the 

 roots and shoots will then be seen to have bent through 

 an angle of 180 and regained their originally selected 

 positions. 



