44 VEGETABLE PHYSIOLOGY 



no definite aggregation of them into micellae. He pictures, 

 therefore, a linkage of the atoms into a molecular network, 

 the meshes of which are occupied by water. 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. 



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 

 protoplast and are deposited 

 upon its exterior in apposition 

 with the wall already there. 

 Hence walls which have a per- 

 ceptible thickness show a certain FlG 42 .-nncmED CELLS OF 



Stratification, which is most WOOD SHOWING STBATIFICA- 



TION. (After Sachs.) 



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 protoplasm and cellulose in those 

 nearest the interior of the cell. The action of the proto- 

 plast 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. 



In some cases the thickening is caused or materially 

 aided by the intercalation of fresh molecules of cellulose 

 into the substance of the existing wall. This process is 

 known as intussusception. It appears to be not so general 

 as was formerly supposed. 



