342 The Living Plant 



sions take place as a secondary architectural arrangement. It is 

 easy to follow the method whereby the individual cells grow from 

 the tiny food-packed condition to the large protoplasm-lined and 

 water-filled state that distinguishes them when adult; and the 

 matter is well illustrated in the accompanying figure 129. First 

 of all, inside the dense protoplasm there appear little rifts which 

 contain a sugar-rich sap. Into these little sap-cavities water is 

 absorbed osmotically, making them swell and exert pressure 

 which pushes the protoplasm against the walls and stretches them 

 tensely. But this pressure is relieved by the deposition of new 

 substance all through the innermost texture of the stretched wall ; 

 and this allows a still further stretching, and so on until the cell is 



FIG. 129. Generalized drawings, in optical section, of a cell during enlargement from the 

 newly developed to the fully-adult condition. 



full grown. The sap-cavities, meanwhile, are not only enlarging 

 but are merging together; and the food substance originally 

 stored in the cell is being transformed into new cell-wall, proto- 

 plasm, and materials dissolved in the sap. The final product is a 

 fully-grown cell, many times larger than its embryonic original 

 and provided with a tightly-stretched wall against which lies 

 a thin lining of protoplasm, enclosing a single sap-cavity well- 

 nigh as big as the cell itself. The exact direction of expansion of 

 the cell, and therefore its final shape, are of course by no means 

 accidental, but are under control of the living protoplasm, which 

 thus simply makes use of osmotic pressure as the mechanical 

 power for forcing cell enlargement. And the degree to which 

 that enlargement may proceed, from the newly-developed to the 



