GROWTH OF THE CELL-WALL 49 



sequent growth are to a large extent dependent upon the activity 

 of the living protoplasm. Generally speaking, in fact, the cell-wall 

 may be regarded as a secretory product of the protoplast. The mode 

 of origin of cell-membranes is, however, very imperfectly understood, 

 and can only be discussed in the most general manner in the present 

 treatise. 



Theoretically there are three distinct methods of growth whereby 

 a cell- wall may add to its thickness. Where growth takes place by 

 apposition (in the strict sense) new particles of cell-wall substance or 

 perhaps new molecules of cellulose are deposited separately and suc- 

 cessively upon the pre-existent wall-surface, in a manner resembling 

 the precipitation of metallic particles in the electrotype process. In 

 the second type of growth termed growth by intussusception by 

 Nageli the new particles are deposited in the interior of the old 

 membrane, where they become incorporated in the molecular or micellar 

 framework of the wall under the influence of forces which reside in the 

 latter. Finally, the increase in the thickness of a cell-wall may be due 

 not to the deposition of separate particles but to the addition of 

 successive entire lamellae of new cell-wall substance, which are super- 

 imposed one upon the other like the leaves of a book. This last- 

 mentioned process is included under the traditional comprehensive 

 definition of growth by apposition. [It might be termed growth 

 by superposition.] Of these three conceivable modes of growth 

 in thickness, the last-mentioned is the only one that lends itself 

 to direct observation ; as a matter of fact a whole series of 

 investigators, including Schmitz, Strasburger, Klebs, Noll, Krabbe, etc., 

 have succeeded in demonstrating the deposition of successive entire 

 lamellae of cell-wall substance in a number of cases. A particularly 

 clear and convincing illustration of the process is furnished by the 

 growth in thickness of the walls of bast-fibres. It is, of course, 

 possible, though by no means certain, that lamellae primarily laid down 

 by apposition may subsequently add to their thickness by intus- 

 susception. In any case nothing but intussusception can explain 

 either the occurrence of secondary differentiation within a cell-wall or 

 the development of centrifugal thickenings ; it is conceivable that these 

 processes are in certain cases facilitated by the penetration of living 

 protoplasm into the substance of the wall. 



Growth in surface on the part of a cell-wall often depends 

 especially among Algae upon a passive extension of the pre-existent 

 lamellae and a subsequent deposition of new layers, the area of which 

 is naturally adapted to the increased surface available for deposition. 

 In this case the increase in surface is, strictly speaking, not effected by 

 growth at all, but is due purely to passive extension. Genuine growth 



D 



