460 



A TEXTBOOK OF THEORETICAL BOTANY 



Cutinized wall 



Epidermis 



Phellem 



Phellogen 



Phelloderm 



Cortex 



^^Jrt ^f 



Fig. 447. — Acer campestre. Formation of cork in stem. 

 For explanation of terms see p. 863. 



Elementary Tissues : Prosenchymatous Types 



These cells differ from the parenchymatous forms in being markedly 

 elongated and having more or less pointed ends. The distinction is by no 

 means clear-cut, but it has some value in designating at least two classes 

 of cells, namely, fibre cells and conducting cells, which differ strikingly from 

 any parenchymatous cell. 



Fibre Cells 



The typical fibre cell is narrow, very elongated and has long tapering 

 points. The wall is highly thickened with concentric layers of cellulose 

 usually impregnated with lignin, so that little internal space is left. In 

 this wall are small pits, with pit canals running through the thickness of the 

 wall to the cell lumen. The pits are circular in outline, but the canals are 

 often expanded into narrow slits, which lie at an acute and constant angle 

 to the cell axis. The slope of these slits in adjacent fibres is opposite, so that 

 in longitudinal view they appear crossed within the circle of the pit outline. 



Lignin is so important a substance that it is unfortunate that its chemistry 

 is not yet fully known. It is the characteristic material of wood, and is hard, 

 dense and relatively insoluble. 



In spite of the efforts of many workers, the constitution of lignin is far 

 from being satisfactorily settled. Indeed there is no certainty that lignin 

 is really a homogeneous substance. Certainly a considerable chemical 

 difference exists between the lignin in the cell wall and that in the middle 

 lamella. The lignin of the cell wall contains 61 to 64 per cent, of Carbon and 

 only 30 per cent, of Oxygen, so that it is not wholly carbohydrate, although 

 it probably includes the sugar arabinose. Several workers have suggested a 



