FOBCE BXEKTED DURING GROWTH. 395 



bilatoral symmetry ; by Vochting ' upon the modification of 

 foliar and axial organs. 



1034. The amount of force which is exerted by certain organs 

 during their growth has been accurately measured for only a few 

 cases. Thus Darwin ^ found that the transverse growth of the 

 radicle of a germinating bean was able to displace a weight of 

 1,500 grams, or 3 lbs. 4 oz., and in another instance, 8 lbs. 8 oz. 

 " With these facts before ns, there seems little difficulty in under- 

 standing how a radicle penetrates the ground. The apex is 

 pointed, and is protected by the root-cap ; the terminal growing 

 point is rigid, and increases in length with a force equal, as far 

 as our observations can be trusted, to the pressure of at least a 

 quarter of a pound, probabl}' with a much greater force when 

 prevented from bending to any side by the surrounding earth. 

 Whilst thus increasing in length it increases in thickness, push- 

 ing 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 does not therefore act like a nail when 

 hammered into a board, but more like a wedge of wood, which, 

 whilst slowlj^ driven into a crevice, continually expands at the 

 same time hy the absorption of water ; and a wedge thus acting 

 will split even a mass of rock." 



By means of a framework placed around the fruit of a vigor- 

 ous squash kept under conditions most favorable to its rapid 

 development, Clark ' estimated the force exerted by growth to 

 be about 5,000 pounds. 



1035. That external pressure can retard growth is well shown 

 by the experiments of De Vries ' upon the formation of autumn 

 wood (see page 138). By increasing the external pressure ex- 

 erted by the bark he was able to diminish the calibre of the 

 wood-cells and ducts ; whereas, bj' diminishing the pressure (by 

 making longitudinal incisions into the bark) he was able to 



1 Botanische Zeitung, 1880, p. 693. 



2 The Power of Movement in Plants, 1881, y. 76. 



' For a full account of this experiment, see Report of the Secretai-y of the 

 Massachusetts Board of Agriculture for 1874. 



The great force exerted by the increase in size of the stems and roots of 

 woody plants is sometimes demonstrated in an extraordinary manner by the 

 development of seedlings in crevices. Thus, at the Marien Cemetery in 

 Hanover, Germany, the base of a tree has dislodged the heavy stones of a 

 strongly built tomb. One of the stones, which measures 23 X 28 X 56 inches, 

 has been lifted upon one side to the height of five inches. The tree measures 

 just above its base from ten to fourteen inches in diameter. 



4 Flora, 1872, p. 241. 



