150 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



wall. As in these other elongated cells, one of these is the primary wall 

 whose structure will be discussed in a later chapter. In cotton it is 

 particularly easy to characterize this primary wall since it is the only 

 part of the wall which stains intensely with ruthenium red corresponding, 

 it would therefore seem, to the restriction of pectin to this primary 

 wall region. Below this layer, however, there can be distinguished a 

 lamella which Hock, Ramsay and Harris have called the "winding" and 

 which Rollins has interpreted as the inner lamella of the primary wall. 

 The more recent evidence brought forward by Kerr (51), however, makes 

 it rather certain that this is the outermost layer of the secondary wall, 

 an interpretation which is most satisfactory in the light of the known 

 structure of other elongated cells. Thus, the windings can be distin- 

 guished even after the primary wall has been removed mechanically; its 

 component cellulose matrix is well oriented (unlike the primary wall, 

 see Chapter IX) in a spiral rather flatter than in the rest of the secondary 

 wall; further the spiral reverses regularly along the length of the hair 

 and, though the reversal points do not correspond with those in the 

 rest of the secondary wall, the absence of reversals in the primary wall 

 would seem to distinguish it rather completely. There seems therefore 

 little doubt but that this "winding" corresponds both in development 

 and structure to the outer layer of the secondary wall as found in, for 

 example, the tracheids. An innermost layer with similar structure is not, 

 however, found in any variety of cotton. 



In terms of this similarity of structure between cotton and other 

 elongated cells, and in view of the effect of environment on wall 

 structure in the algae (p. 107) it is very interesting indeed to note the 

 pronounced effect of light on the structure of cotton hairs. As men- 

 tioned above, cotton hairs are characteristically finely lamellated when 

 viewed in transverse section under appropriate conditions. Now it was 

 obvious from the earlier work of Balls that there could be a rough 

 correlation between the number of these lamellae and the number of 

 days over which the secondary wall continued to increase in thickness. 

 Such a correlation has been fully confirmed by later workers, notably 

 by Kerr and his collaborators, and it is therefore certain that the lamella- 

 tion here is associated, directly or indirectly, with some factor in the 

 environment showing regular daily fluctuation. That this factor is in 

 fact light was shown by Anderson and Moore (44) by growing cotton 

 under constant light conditions. The hairs developed under these 

 conditions showed no traces of lamellation and the observation that 

 other ceU types in the stem of the plant, removed therefore from the 

 direct influence of external factors of this kind, continue to show wall 



