GROWTH. 53 



the whole region at the same time. Supposing the fibril 

 to be made up of cells piled up one upon another in lon- 

 gitudinal rows, then the greatest energy of growth, 

 marked by the turgescence of the cells, occurs at one time 

 in one row, to shift at another time into the next row, 

 and so on in succession all round the root. The effect of 

 this greater turgescence and intensity of growth now in 

 one place, now at another is to move the tip of the root, 

 not" in a circle, because growth is going on behind the 

 tip as it moves, but in an advancing spiral coil, so that 

 the tip is forced to enter the soil and to penetrate 

 between its particles, just as the point of a corkscrew is 

 made by the pressure of the hand to penetrate the cork, 

 the pressure of 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, cal- 

 culates that the terminal growing part of the radicle (or 

 primary root produced from the seedling plant) "in- 

 creases 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 eight pounds in 

 one case, of three pounds 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 



