30 THE MECHANICS OF GROWTH 



who has also given a detailed account of the preparatory and stimulatory 

 processes involved in growth, as well as of the necessary transformations of 

 energy. In addition, it may be mentioned that Sachs attached primary impor- 

 tance to the stretching of the cell-wall by the internal osmotic pressure, which he 

 supposed to aid in the interstitial deposition of cellulose particles, and thus to 

 allow growth in surface extent of the cell-wall by intussusception 1 . The upholders 

 of the theory that the growth of the cell-wall is due to apposition and plastic 

 stretching assume, with the exception of Wortmann, that a softening action is 

 exercised upon the cell-wall by the protoplasm, and that this precedes or 

 accompanies the increase in surface extent by stretching. It is, however, not 

 necessary to discuss the erroneous assumption of Wortmann and de Vries that 

 a general parallelism exists between the activity of growth and the amount of 

 stretching of the cell- wall 2 . 



A growth of the cell-wall by apposition has been proved in many cases by 

 means of natural or artificial marks. Thus crystals of calcium oxalate adhere 

 to the inner surface of the cell-wall in Citrus and Pandanus, and are then 

 slowly imbedded by the internal apposition of new layers of cellulose 3 . Klebs 

 and Noll have observed dead protoplasmic particles become imbedded in a similar 

 manner, while Noll (I.e., p. 124) has demonstrated the deposition of new layers 

 of colourless cellulose on cell-walls of Caulerpa, which had been stained with 

 Prussian blue. 



Intussusception is most readily observed when foreign bodies such as silica, 

 calcium oxalate, and carbonate are deposited in the wall in the form of minute 

 particles or crystals. It is, however, always difficult and often impossible to 

 observe any interpolation of particles of the same material as the ground sub- 

 stance of the cell-wall. According to Correns (1. c.), however, the latter does 

 actually occur in the walls of Glaeocapsa and Apiocystis, while Cramer (1. c.) has 

 observed an increase in the mass and volume of the walls of Neomeris Kelleri 

 in the absence of any possibility of growth by apposition 4 . Growth by intus- 

 susception also takes place in the spore-membranes of Hepaticae (Leitgeb, 1. c.), 

 and also in the membranes of certain pollen-grains (Wille, I.e.; Strasburger, 1898, 

 I.e., p. 574). According to C. Miiller 5 , visible needle-shaped masses of cellulose 

 are deposited in the cell-walls of the root-endodermis of Spiraea filipendula. 



Growth by intussusception seems also to be indicated by the fact that 

 when the external increase in size of a growing organ is mechanically prevented, 



1 Sachs, Lehrb. d. Bot., 1873, 3. Aufl., p. 699. Marriotte's suggestion (CEuvres de Mariotte, 

 1 71 7, p. 132) that the pressure of the sap aids in growth by stretching the branches and leaves is only 

 of historic interest. 



a De Vries, Mechan. Ursachen d. Zellstreckung, 1877, p. 107; Wortmann, I.e., p. 234. Cf. also 

 Pfeffer, Studien z. Energetik, 1892, p. 234; Druck-u. Arbeitsleistungen, 1896, p. 306. Schwendener 

 and Krabbe, Jahrb. f. wiss. Bot., 1893, Bd. xxv, p. 323. 



3 Pfitzer, Flora, 1872, p. 130; H. C. Miiller, Entstehung von Kalkoxalat in Zellmembranen, 

 Leipziger Dissert., 1890, p. 45. 



* Arguments of this kind were first put forward by Nageli, Starkekorner, 1858, p. 281. See 

 also Strasburger, Jahrb. f. wiss. Bot., 1898, Bd. xxxi, p. 572 ; H. Fitting, Bot. Ztg., 1900, p. 151. 



5 Ueber die Einlagerung von Cellulose, 1897 (repr. from Ber. d. Deutsch. Pharm. Ges., 

 Jahrg. vu, Heft i). 



