50 



VEGETABLE PHYSIOLOGY 



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). 



FIG. 43. LONGITUDINAL SECTION OF VASCULAR BUNDLE OF SUNFLOWER 

 STEM. (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 



