WALL STRUCTURE IN THICK CELL WALLS 145 



vessels; if one element in a vessel is heterogeneous, then all the elements 

 in the vessel are similarly heterogenous; if one element is homogeneous, 

 then so are all the rest. 



Collenchyma cells 



These cells, occurring in the outer reaches of the cortex in many 

 plants, are often similar in shape to the fibrous cells already dealt with. 

 They differ from these, however, in two respects at least. Firstly, the 

 wall is not uniformly thick, and this is the feature by which these cells 

 may be recognized. In the differentiation of the cells, the walls first 

 begin to thicken in the neighbourhood of the intercellular spaces 

 lying between three or more neighbouring cells. This causes the 

 cells in transverse section to appear thickened at the corners; the 

 thickening may then spread to cover the whole of the walls lying 

 tangentially in the stem — but never to the radial walls — and collenchyma 

 cells have been classified somewhat arbitrarily into four main types 

 according to the pattern of this thickening. In longitudinal view 

 the thickenings can be seen as "bars" running down the length of the 

 cells. Secondly, the wall does not contain lignin, but has a high per- 

 centage of pectic substances. It is apparently to this high pectin content 

 that the walls owe their extensibility, and it is certainly the presence of 

 pectin which confers upon the wall its relatively enormous swelling in 

 water. CoUenchymatous tissue dehydrated and mounted in balsam is 

 often difficult to distinguish because the thickening bars have shrunk to 

 about the same thickness as the rest of the (unthickened) wall. When, 

 however, the material is replaced in water, the bars swell again to the 

 dimensions observed in fresh material, the swelling being of the order 

 of 150% or more (Table XI). 



This swelling occurs, however, only in the transverse plane and, in 

 fact, only in the radial direction. Swelling in length is negligible 

 (Table XIa), a fact which, as first pointed out by Haberlandt (before 

 the structure of cellulose was known), would indicate that the units of 

 structure in the wall lie longitudinally. 



The wall structure of the three main types of collenchyma have been 

 investigated (36), and the relevant results for two of them are presented 

 in Table XII. It will be clear that in the main the cellulose chain orienta- 

 tion resembles that in the central layer of very long tracheids or fibres, 

 confirming the suggestion of Haberlandt. In transverse section the walls 

 do show some differences from these, however, when viewed between 

 crossed Nicols. The thickened regions of the walls are usually very 

 clearly lamellated with lamellae alternately dark and bright, but this 



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