GROWTH OF THE CELL-WALL. 219 



which are interpolated between the old. This is the intuseus- 

 ception theory. It has gradually displaced an older theor3-, 

 nainelj-, that of growth by apposition. As llie older theory was 

 usually held, it presented two niodificatious/ — one that the 

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

 the wall, so that in a stratified wall all tiie 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 attra<;ted much attention 

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

 bj' Strasburger.^ As now held by these authois, the ^•iew is this : 

 stratified and other cell-walls grow in thickness b^' the deposi- 

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

 crj-stal 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. 



An}' 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), maj- now be added that property' wliich is possessed 

 also by man}' other organized substances ; nameh", that of swell- 

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

 cell is of course moist, and its increase in size on tlic 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 tiie apphcation of water 

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

 amount of water may take place with great rapidit}' upon 

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

 amount of water in the protoplasm in contact is correspondingly 

 modified. 



597. Historical note regarding protoplasm. The word proto- 

 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 Linnsea, 1846, and Mohl's, 

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

 presented in Mulder's Physiological Chemistry. 



2 Strasburger : Ban und Waehsthum der Zellhaiite, 1882. 



