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 
