THE MECHANICS OF GROWTH 25 



susception forms a fundamental property of both unicellular and multi- 

 cellular organisms. The growth of a leaf or petiole takes place not by 

 external deposition (apposition), but by the increase in size and numbers 

 of the constituent cells. Similarly intussusception takes place when 

 a crystal penetrates the protoplast, or when micellae, molecules, atoms, 

 or electrons enter and unite with the organized substance of the nucleus 

 or of other plasmatic organs. The distribution of the male fertilizing 

 cell throughout the ovum affords another instance of intussusception. 



Both intussusception and apposition are possible in bodies capable 

 of swelling, whereas in impenetrable bodies such as crystals only the 

 latter mode of growth is possible 1 . The growth of the protoplasm, and 

 also that of the cell-wall, takes place in both of these ways. The mode 

 of growth is however often difficult to decide when similar material is 

 added, although little doubt is possible when crystals of calcium oxalate 

 or of other foreign substances are secondarily deposited within the 

 substance of the protoplasm, or of the cell- wall. 



Intussusception was recognized by Lamarck 2 as an essential property of the 

 organism, and the term may be taken to include not only the introduction of 

 formed particles from without, but also their production from other substances 

 within the mass of growing material. In the same broad sense growth by apposition 

 takes place when new cells, peripheral membranes, molecules, or atoms are superposed 

 upon existing parts. There is no reason to restrict the term intussusception to the 

 molecular processes involved in growth, or to cases in which similar substances 

 are added to the growing materials. This error has been made by certain 

 authors whose attention was concentrated upon a special form of growth 3 . In 

 the ultimate resort all growth must take place by apposition, that is, by the 

 addition of indivisible and impenetrable atoms 4 to the chemical or physiological 

 units, that is, to molecules or micellae. The latter may also possibly grow tem- 

 porarily or permanently by intussusception, and since the two modes of growth 

 may pass more or less gradually into one another, it is impossible to make a sharp 

 distinction between them. Such a transition occurs when the segments formed from 

 a meristem cell subsequently fuse together, and also when the mercury placed 

 upon the surface of a zinc or lead plate gradually penetrates into it. Owing 

 to the possible extreme complexity of growth, little permanent value attaches 

 to the purely artificial distinction between cellular, lamellar, and molecular 

 intussusception 5 . 



1 Albuminoid crystalloids which are capable of swelling also grow by apposition. Pfeffer, 

 Jahrb. f. wiss. Bot, 1872, Bd. VIII, p. 516. 



2 Lamarck, Phil, zoolog., nouv. e"d., 1830, I, p. 382 (i e d. 1808). 



3 Cf. Krabbe, Jahrb. f. wiss. Bot., 1887, Bd. xvm, p. 412 ; Noll, Flora, 1895, p. 80; Biitschli, 

 Unters. u. Structuren, 1898, p. 223. For a more general view see Pfeffer, Stud. z. Energetik, 1892, 

 p. 250; Wiesner, Elementarstructur, 1892, p. 193. 



* Pfeffer, I.e., p. 254. [If, however, an atom is subdivided into several hundred or thousand 

 electrons, growth by intussusception might be possible under special conditions inside it.] 

 5 Wiesner, I.e., p. 223. 



