252 BOTANY 



layers are subsequently added, in the same manner, on the inside 

 of those already present. These layers of tertiary deposition are 

 not cemented together, nor is there any attachment between 

 them. 



There are two phases of tertiary deposition that have to be 

 considered : a division of all tertiary deposits into from two to 

 five distinct layers and the appearance in these larger divisions of 

 many minute lamellae. 



The mature fibres when separated from the stem have many 

 marked characteristics. A definite banding is particularly evident 

 on staining with iodine and dilute sulphuric acid, as well as the 

 presence of local displacements or nodes of local enlargement, and 

 minute spiral striations are also present as well as isolated proto- 

 plasts surrounded by definite cell walls in the centre of mature 

 fibres. The presence of bands of material surrounding the fibre 

 can be seen by swelling the fibre rapidly with strong sulphuric 

 acid. With this treatment, the swelling is limited to certain 

 places by the presence of these constricting bands and the swollen 

 fibre has the appearance of a chain of tiny sausages. These bands 

 are the remnants of the secondary wall that still adhere after 

 retting of the fibre. 



The minute spiral striations that can be made out on the 

 surface of the fibre are fibrils which build up the fibre wall. These 

 are arranged as aggregates of anisotropic fibrils in distinct layers, 

 which are spirally wound alternately right and left in the separate 

 layers. The separate fibrils show a high bi-refringence and 

 parallel extinction under crossed nicols and retain their parallel 

 extinction when rotated about an axis parallel to their elongation, 

 indicating that they are homogeneous units. Pectic compounds 

 under crossed nicols reveal their colloidal nature very sharply, and 

 can therefore be readily distinguished from the adjacent cellulose, 

 so that this method of using polarised light offers a ready means 

 of determining the extent of pectic material in the cell wall. 



Lignification of the flax fibre begins in the middle lamella by 

 the conversion of pectose already present in that position. The 

 lignification continues into the secondary wall first by the trans- 

 formation of pectic material in that region, and finally the whole 



