WALL STRUCTURE IN THICK CELL WALLS 147 



lamellation is here not due, in the main at least, to an alternation of 

 cellulose chain direction; it resembles much more closely that already 

 reported in algae (p. 95). If the material is subjected to a preliminary 

 swelling and then stained in ruthenium red, then alternate lamellae take 

 up the stain much more intensely than the rest, and it seems very clear, 

 as first pointed out by Anderson, that the pectin is largely, though not, 

 of course, entirely, confined to separate lamellae. 



Alternatively, the wall may be stained with iodine and 70 % sulphuric 

 acid in order to demonstrate the distribution of cellulose. Here again, 

 in the collenchyma of Solarium lycopersicwn{36a) and of Heracleum 

 sphondylium (36(Z))) fine lamellation can be observed so that there is no 

 doubt here but that the lamellation is of the cellulose-rich pectin-poor, 

 cellulose-poor pectin-rich, type. With Petasites vulgaris (36(c)) (showing 

 collenchyma of the tubular type where the thickening of the wall round 

 each intercellular space gives the impression of a uniformly thickened 

 intercellular space) lamellation has never been observed in cellulose 

 stains. In this type, therefore, it seems that the cellulose is uniformly 

 distributed through the wall while the pectin occurs mostly in separate 

 lamellae within the cellulose matrix. Since the walls of these collenchyma 

 cells shrink on drying and swell on wetting much as do those of other 

 types (Table XI), it seems unsafe to attempt any close connection 

 between pectin content and sweUing. The suggestion has been put for- 

 ward that the presence of pectin is in some way associated with the 

 presence of large intermicellar spaces throughout the cellulose of these 

 cells, and therefore presumably with a high ratio of non-crystalline to 

 crystaUine cellulose fraction, and that this accounts for the high degree 

 of swelling. 



The other notable difference between the appearance in transverse 

 section of collenchyma cells and the lignified elongated elements is that 

 the outer bright layer is missing. In its place we find a layer which is 

 isotropic both in transverse and longitudinal view, which does not stain 

 in ruthenium red, iodine and sulphuric acid or in Sudan III. This layer, 

 or cuticle as it may perhaps be called, is in fact of unknown composition. 

 The thick central layer within it is, however, faintly birefringent in 

 transverse section and strongly so in longitudinal view, very much like 

 the central layer in tracheids and fibres. The figures given for micelle 

 direction in Table XII undoubtedly refer to this layer, and this is also 

 the layer which is responsible for the X-ray diagram (which strongly 

 resembles that presented in Plate II, Fig. 2 though the arcs are much 

 more diffuse, in harmony with the general picture of wall structure). 

 A tenuous innermost layer is bright in transverse section between 



