44 



VEGETABLE PHYSIOLOGY 



On either hypothesis the quantity of water is capable of 

 considerable increase or diminution, and the wall can be 

 made to swell up by causing it to imbibe more fluid. This 

 can be brought about by exposing it to the action of strong 

 mineral acids, such as sulphuric acid. The water is held, 

 however, by the solid particles with very great tenacity. 



A different view of the composition of cell-wall has been 

 advanced in recent years by Wiesner. He holds that the 

 substance of cell-wall as it is first formed consists of rows 

 of granular bodies which he terms dermatosomes ; these are 

 connected together by protoplasm which surrounds them. 



On this hypothesis the cell-wall 

 is living while young and grow- 

 ing. The protoplasm thus exists 

 between particles of solid matter 

 and holds water in its substance. 

 The thickening which always 

 supervenes to a greater or less 

 extent upon the first formation 

 of the cell -wall is brought about 

 by the protoplasm in a way 

 similar to that of its original 

 construction. Layers composed 

 like the original one are continually secreted by the proto- 

 plast and are deposited upon its exterior in apposition with 

 the wall already there. Hence walls which have a perceptible 

 thickness show a certain stratification, which is most easily 

 seen in transverse sections (fig. 42). When several such 

 layers can be distinguished it has been found that pectic 

 bodies are prominent in the layers furthest from the proto- 

 plasm and cellulose in those nearest the interior of the cell. 

 The action of the protoplast is frequently irregular, so that 

 the thickening layers are often seen as bands of various 

 form, giving the surface of the membrane particular patterns, 

 thin and thick places alternating in various ways (fig. 43) . 

 These are seen most conspicuously in the walls of the 

 vessels of the wood. 



FIG. 42. THICKENED CELLS OF 

 WOOD, SHOWING STBATIFICA- 

 TION. (After Sachs.) 



