50 VEGETABLE PHYSIOLOGY 
thickness show a certain stratification, which is most easily 
geen 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. 48). 
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Fia. 43.—Lonarrupman Srorion or Vascutar BunpuE or SUNFLOWER 
Srem. (After Prantl.) 
a, Vessels of the wood thickened in various ways. 
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. 
In cell-walls which have undergone considerable thickening 
the membrane shows a marked differentiation. The centre 
of the wall is found to possess a chemical composition 
