7G ACTIOK OF THE KADICLE. Chap H. 



pincers rested on glass-plales to lessen the friction from 

 the sand. The holes were a little larger (yiz. -14 inch) 

 and considerably deeper (viz. -6 inch) than in the 

 trials with the sticks ; so that a greater length of a 

 rather thicker radicle exerted a transverse strain. 

 After 13 days they were taken up. The distance of 

 two dots (see the figure) on the longer ends of the 

 pincers was now carefully measured ; the radicles were 

 tlien extracted from the holes, and the pincers of 

 course closed. They were then suspended horizontally 

 in the same manner as were the bits of sticks, and a 

 weight of 1500 grams (or 3 lbs. 4 ozs.) was necessary 

 with one of the pincers to open them to the same 

 extent as had been effected by the transverse growth 

 of the radicle. As soon as this radicle had slightly 

 opened the pincers, it had grown into a flattened form 

 and had escaped a little beyond the hole ; its diameter 

 in one direction being 4*2 mm., and at right angles 

 3'5 mm. If this escape and flattening could have 

 been prevented, the radicle would probably have 

 exerted a greater strain than the 3 lbs. 4 ozs. With 

 the other pincers the radicle escaped still further 

 out of the hole; and the weight required to open 

 them to the same extent as had been effected by the 

 radicle, was only 600 grams. 



With these facts before us, there seems little diffi- 

 culty in understanding how a radicle penetrates the 

 ground. The apex is pointed and is protected by 

 the root-cap ; the terminal growing part is rigid, and 

 increases in length with a force equal, as far as oui- 

 observations can be trusted, to the pressure of at least 

 a quarter of a jwund, probably with a mucli greater 

 force ^\hen prevented from bending to any side by the 

 surrounding earth. Whilst thus increasing in length 

 it increases in thickness, pushing away the damp 



