44 VEGETABLE PHYSIOLOGY 
no definite aggregation of them into micelle. 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 py. 49-TarckennD CELLS oF 
stratification, whieh is imost  Woom% saowie Srmarieres- 
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. 48). 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. 
