GROWTH OF THE CELL-WALL. 219 



which are interpolated between the old. This is the intiM-r-us- 

 ception theory. It has gradually displaced an older theoiy, 

 namely, that of growth by apposition. As the older theory was 

 usually held, it presented two modifications, 1 - one that the 

 growth of a cell-wall in thickness takes place on the exterior of 

 the wall, so that in a stratified wall all the outermost portions 

 are the newer ; the other, that all the new matter is laid down 

 upon the interior of the old. 



The apposition theory has recently attracted much attention 

 from the studies of Schmitz, and from its adoption and advocacy 

 by Strasburger. 2 As now held by these authors, the view is this : 

 stratified and other cell- walls grow in thickness by the deposi- 

 tion of new particles upon the inner face of the cell, much as a 

 crystal adds new particles to itself; growth in surface is the result 

 of a simple stretching of the wall by the pressure of the con- 

 tents upon it. 



Anv solution which causes a shrinking of the contents of the 



/ 



cell, and thus diminishes the pressure on the wall, may cause 

 a diminution of the size of the cell itself. The bearing of this 

 upon the turgescence of the cell will be again adverted to under 

 " Properties of New Cells and Tissues." 



To the physical characters of cellulose already mentioned 

 (see 129), may now be added that property which is possessed 

 also by many other organized substances ; namely, that of swell- 

 ing greatly when placed in water. The wall of a living and active 

 cell is of course moist, and its increase in size on the addition of 

 more water is seldom marked ; but under certain circumstances 

 the amount of water in the cell-wall even of an active cell may 

 fall below its usual amount, and then the application of water 

 will cause an appreciable change of bulk. Such change in the 

 amount of water may take place with great rapidity upon 

 slight external disturbances, such as shock : in these cases, the 

 amount of water in the protoplasm in contact is correspondingly 

 modi iled. 



597. Historical note regarding protoplasm. The wordjoroto- 

 plasm appears first in a memoir by Mohl, in 1846, ; 'On the 

 Movement of Sap in the Interior of Cells," which deals, however, 



1 For an account of the two modifications of the apposition theory, the 

 student is referred to Harting's paper, translated in Linnaga, 1846, and Mohl's, 

 in Botanische Zeitung, 1846. A fair statement of the first modification is 

 presented in Mulder's Physiological Chemistry. 



2 Strasburger : Bau und Wachsthum der Zellhaiite, 1882. 



