52 PLANT LIFE ON THE FARM. 



the hand being replaced, in the case of the root, by the 

 superincumbent weight of soil. 



Darwin, who has done so much to illustrate and make 

 known the movements of roots and of other organs, calcu- 

 lates that the terminal growing part of the radicle (or 

 primary root produced from the seedling plant) "increases 

 in length with a force equal to ... the pressure of at 

 least a quarter of a pound probably with a much greater 

 force when prevented from bending to any side by the 

 surrounding earth. While thus increasing in length, it 

 increases in thickness, pushing away the damp earth on 

 all sides with a force of above 8 Ib. in one case, of 3 Ib. in 

 another case. . . . The growing part, therefore, does not 

 act like a nail when hammered into a board, but more 

 like a wedge of wood, which, whilst slowly driven into a 

 crevice, continually expands at the same time by the 

 absorption of water; and a wedge thus acting will split 

 even a mass of rock." 



Movement of Stems The circumnutation of stems as 

 a result, or at least as a concomitant of active growth, is 

 most easily seen in the case of climbing plants like the 

 hop, the free ends of whose growing shoots sweep round in 

 wide curves till they come in contact with a support round 

 which to twine,* and thus remove their leaves from the 

 surface, where they would be overshadowed, to a point of 

 vantage where they would be exposed to light, and this 

 with the least expenditure of material. Very similar are 

 the movements executed by stolons and runners, as of the 

 strawberry, and probably, though the cases have not been 



* t>ee Darwin, The Movements and Habits of Climbing Plants. 



