LIGNIFIED TISSUES 251 



revealing the fundamental structure. If further pressure were care- 

 fully applied, splitting of the fibres was brought about, and by 

 careful manipulation the material could be laid flat on the slide. 

 Additional hydrolysis and continued pressure made it possible to 

 peel apart the different layers of the structure, and a study could 

 then be made of their structure under higher magnification than 

 in the case of untreated fibre. 



Cells that are to differentiate into fibres are developed in the 

 apical meristem. They lie just within the endodermis, and are 

 distinguished soon after the primary xylem elements have been 

 formed. The middle lamella between the young cells is composed 

 of pectose. The secondary wall, i.e., additions to the primary wall 

 during the period of active enlargement of the cell, consists of 

 cellulose containing pectose in varying amounts, especially near 

 the middle lamella. The walls of the young fibre are seen in cross 

 section to be slightly wrinkled and rather irregular. 



The enlargement phase of fibre development involves two 

 types of increase, an early rapid and extensive increase in dia- 

 meter and length, and a slow but perceptible increase in diameter 

 which is limited to certain cells and which continues throughout 

 the life of the plant. 



The first evidence of tertiary wall formation appears in the 

 lower part of the fibre, at the close of the rapid period of elonga- 

 tion. This is not due to any gradual and uniform thickening of 

 the secondary wall, but is the result of the periodic addition of 

 new layers of material. Each successive layer of material is 

 secreted from the protoplast, and appears in cross -section much 

 infolded and wrinkled. It is in contact with the secondary wall 

 at only a few points, or even not at all. The cellulose when first 

 deposited is highly hydrated, and seems almost gelatinous. The 

 material of the infolded deposit is pure cellulose. The first 

 tertiary layer is pushed to the secondary wall, where it fits closely. 



It does not adhere to the secondary wall, nor is any cementing 

 material present between them. When pressed against the 

 secondary wall by cell turgor it loses its infolded and wrinkled 

 appearance and becomes firmer and more rigid. After the first 

 infolded layer has been pushed against the secondary wall, other 



