THE GROWTH OF THE CELL-WALL 31 



the cell- walls continue to increase in surface extent for a time, although the 

 stretching by the internal osmotic pressure steadily decreases, and is ultimately 

 entirely antagonized by the external pressure 1 . The cohesion of the cell-wall, how- 

 ever, does not decrease, nor is the limit of elasticity exceeded when the same or 

 an increased osmotic pressure is allowed suddenly to act upon and stretch the cell- 

 wall by removing the original external resistance. If the pressure is only partly 

 released, growth in surface extent may continue in the cell-walls, although they 

 are little if at all stretched. Rapid growth may indeed take place under such 

 conditions, as, for example, in the cell-walls of the pith of certain stems, which 

 are hardly at all stretched during their period of most active increase in size 2 . 

 This fact is hardly surprising, for we have already seen that growth by intus- 

 susception may take place without the aid of, or even against, the osmotic 

 pressure. Indeed, the foldings produced in certain cell-walls are probably the 

 result of the continuance of growth in walls which are no longer stretched 3 . 

 Whenever growth in surface extent takes place in cell-walls of constant elasticity, 

 although they are not under tension, this may be taken as a sure indication of 

 growth by intussusception, for to produce growth by plastic stretching in walls 

 under minimal tension would require the assumption of very special powers on 

 the part of the protoplast. Sufficient has, however, been said to show that the 

 matter is not quite so simple as Noll 4 supposes it to be, for he inclines to 

 regard all growth as the result of apposition. 



Growth in surface extent by plastic stretching takes place in Oedogonium, and 

 probably is a common phenomenon. Thus in many cases the thickness of 

 the cell-wall or of certain layers of it decreases as the cell increases in size 5 . 



It is not, however, always possible to say precisely which of these modes of 

 growth are active, and it is, moreover, fairly certain that both intussusception and 

 apposition are possible not only in growth by cell-division, but also during the 

 subsequent increase in size of the individual cells. 



SECTION 9. The Growth of the Cell-wall (continued). 



We are not concerned with the visible changes of shape during the 

 growth of the cell-wall, for however important these may be, they throw 

 little or no light upon the physiological processes concerned in growth. 

 The protoplast may produce a cell-wall either by the direct conversion 

 of the peripheral particles of protoplasm into chitinous or more commonly 



1 Pfeffer, Druck- u. Arbeitsleistungen, 1893, p. 429. 



a Kolkwitz, Fiinfstiick's Beitrage z. wiss. Bot., 1897, 1, p. 246. 



8 Strasburger, I.e., p. 586 ; Kny, Ber. d. Bot. Ges., 1893, p. 377 ; Zimraermann, Beitr. z. Morph. u. 

 Physiol., 1893, pp. 167, 198. 



4 Noll, Flora, 1895, Erg.-bd., p. 66. 



6 Noll, Exp. Unters. ii. d. Wachsthum d. Zellhaute, 1887, p. 132 ; Flora, 1895, Erg.-bd., p. 73 ; 

 Strasburger, Jahrb. f. wiss. Bot., 1898, Bd. xxxi, p. 586 ; also the works of Schmitz, Kleb?, &c., 

 quoted in these publications. Frequently the cuticle becomes detached owing to the growth of the 

 inner layers of the epidermal cell-walls, but this says nothing as to the causes of growth. 



