MECHANICAL ACTIONS 



125 



gradually decreases as the tension of the walls is antagonized by the 

 external pressure, and it may cease before the full osmotic pressure comes into 

 play against the external resistance. When the reaction involves a rise of 

 turgor, however, the maximum pressure appears to be attained in a few days. 



The same principles hold good when the resistance to growth is 

 offered by living or dead plant-tissues, as during the production of tissue- 

 strains by the action of compressed tissues against stretched ones. In 

 a plaster-cast the tissue-strains decrease and finally disappear when the 

 maximal external pressure is reached. On removing from the cast, the 

 original tissue-strains and also the original osmotic tensions in the walls 

 of the individual cells return. This happens very suddenly when a root 

 bursts the cast, and hence it is of importance that the cell-wall retains 

 during its imprisonment the power of resisting the original osmotic tension. 



When a root pushes an object in front of it, although it performs more 

 work, its rapidity of growth is not perceptibly affected unless the 

 resistance offered is very great. The rate of growth is, for example, 

 practically the same in tenacious soil as in water or moist air, if the other 

 factors are constant. The radicle of Faba can exert a maximal apical 

 pressure of 300 to 400 grammes, and grows nearly as rapidly in a clay soil, 

 offering a resistance of 100 to 120 grammes, as in water 1 . Krabbe 2 has 

 found that the growth in thickness of our forest trees is not affected by 

 an increased pressure equal to between 2 and 4 atmospheres, but is 

 retarded by one of 10 to 15 atmospheres. 



A soft growing organ will accommodate itself to a certain extent to 

 the shape of the space in which It is confined, and in this way the Chinese 

 have long been accustomed to give gourds and cucumbers all kinds of 

 remarkable shapes by confining them in moulds when young 3 . The 

 cessation of external growth as the result of external pressure is naturally 

 followed by a cessation of cell-division, and hence in plaster-casts the 

 primary meristem and cambium remain in an inactive condition, while 

 segment-cells previously formed differentiate somewhat abnormally and 

 in part do not attain the usual size 4 . 



Methods. The fact that plants can overcome considerable external resistances 

 has long been known, but Pfeffer was the first to correctly explain this phenomenon 5 . 



1 Pfeffer, 1893, 1. c., pp. 328, 422. See also Wakker, Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 90. 

 The supposition that the root-apex grows more rapidly when the growth of the elongating zone is 

 mechanically retarded is erroneous. Cf. Pfeffer, Jahrb. f. wiss. Bot., 1895, Bd. xxvn, p. 481. 



2 Krabbe, Wachsthum d. Verdickungsringes u. d. Holzzellen, 1884, pp. 55, 60; Friedrich, Bot. 

 Ztg., 1897, p. 371. 



3 Cf. Pfeffer, 1893, 1. c., p. 267. 



4 Pfeffer, 1893, 1. c., p. 356 ; Newcombe, Effect of Mechanical Resistance on Growth, Leipziger 

 Dissert., 1893; Annals of Botany, 1894, Vol. vin, p. 403 ; Bot. Gazette, 1894, Vol. XIX, p. 149; 

 Krabbe, Wachsthum d. Verdickungsringes u. d. Holzzellen, 1884, pp. 50, 60. 



5 Pfeffer, Druck- u. Arbeitsleistungen, 1893; Studien znr Energetik d. Pflanze, 1892. 



