2 6 THE MECHANICS OF GROWTH 



Whatever the precise mode of growth may be, it is always directed 

 and regulated according to the specific structure of the organism, and to 

 the special character of its vital activity. It is by the latter that 

 permanent organs such as the nucleus are reproduced and maintained, 

 and that new structures such as the cell-walls are formed which sub- 

 serve special functions. The production of the cell-wall is closely 

 analogous to the addition of new lamellae to a growing crystal intro- 

 duced into a saturated solution l , or to the production of a growing 

 precipitation film around a drop of sulphate of copper when introduced 

 into a solution of potassium ferrocyanide. From this point of view it is 

 immaterial whether the cell-wall is produced from dead materials or by 

 the transformation of living substance. The growth of an organism is, 

 however, the result of a variety of interacting factors, whereas that of a 

 crystal is a single simple reaction. 



Owing to the power of regulation possessed by the organism, the 

 same general forms of energy may be used to bring about the most 

 varied formative changes. The latter are in the last resort based upon 

 the molecular processes occurring in the protoplasm, i.e. upon chemical 

 arBnity and dissociation, upon physical polarity and molecular pressure, 

 upon surface-tension energy, and the like. The molecular or osmotic 

 pressure of solutions, though of great importance for the stretching-growth 

 of dermatoplasts, and also for overcoming external resistance to growth, 

 plays little or no part in the growth of gymnoplasts, or in that of the 

 nucleus, plastids, and cytoplasma. Indeed the growth of the latter, and also 

 the growth in thickness of the cell-wall, takes place against the osmotic 

 pressure of the cell. The latter usually amounts to from 5 to 15 atmo- 

 spheres, and hence is able to afford a considerable source of energy, though 

 not so much as the other forms of energy mentioned. For example, 

 a pressure of 2,500 atmospheres is necessary to prevent dry starch from 

 absorbing water and swelling, while to prevent the freezing of water at 

 -20 C. a pressure of 13,000 atmospheres would be required 2 . The energy 

 of crystallization and of chemical reaction may attain equally high 

 values 3 . By the aid of such forces as these very great external work 

 may be done, and even rocks split asunder. Hence it is hardly surprising 

 to find that the cohesion of cellulose is not sufficiently great to prevent 

 the formation and growth of crystals of calcium, oxalate in the cell-wall. 

 The cohesion of the protoplasm is so small as to offer only a trifling 

 mechanical resistance to the growth of a body in its substance. 



1 In the absence of any such stimulus, a supersaturated solution can be produced. Cf. Ostwald, 

 Zeitschr. f. physikal. Chem., 1897, Bd. xxil, p. 289. On periodic crystallization cf. Liesegang, I.e., 

 Bd. xxin, p. 365. 



2 Clausius, Die mechanische Warmetheorie, 1876, Bd. I, p. 174. 



3 Lehmann, Molekularphysik, 1888, Bd. I, p. 349. 



