INTRODUCTIOxN TO PLANT HISTOLOGY 



461 



CELLULOSE UNITS 



LIGN 

 UNIT 



ADSORBED 

 WATER 



unit molecular weight between 800 and 900 for Spruce lignin and about twice 

 as much for angiospermic lignin. The lignin of the middle lamella is denser 

 and has a lower oxygen content. There is a considerable evidence for the 

 existence of an aromatic or hydro-aromatic nucleus in the lignin molecule, 

 perhaps related to coniferyl alcohol, and the colour reactions by which 

 lignin is microscopically distinguished are attributed to this group. Similar 

 colour reactions are given by many of 

 the aromatic terpenes or "essential oils." 

 No formula for lignin is yet possible as there 

 is no agreement as to the molecular groupings 

 or linkages present. 



Lignification involves the impregnation 

 of the cellulose wall with this mixture of 

 substances, partially combined with the 

 cellulose, but largely free, and usuallv 

 results in thickening of the wall, with 

 increased density and higher refractive index, 

 i.e., " brightness " under the microscope. 

 It greatly strengthens and stiffens the cell 

 wall, and in the case of fibres, at any 

 rate, this is its primary value, as their 

 function is entirely mechanical or skeletal. 

 They are naturally therefore most de- 

 veloped in young organs, and particularly 

 in Monocotyledons, where there is little 

 wood. The strength of the wall is further increased by the presence of 

 fine longitudinal rods of silica (Fig. 448). 



The tensile strength of lignin is remarkably high, in some cases nearly as 

 high as that of steel, but it differs from metals in two mechanical respects, 

 namely that its elastic extension is greater and that there is a much smaller 

 interval between its limit of elasticity and its breaking point. A tissue which 

 has been strained beyond its elastic limit is obviously permanently damaged 

 and any further resistance to breakage would be valueless. 



Cellulose is more elastic than either lignin or steel and therefore collen- 

 chyma allows for even greater stretching without damage than does lignified 

 fibre ; hence it is especially valuable in flexible or ganslike petioles and young 

 stems. 



Occasionally fibres are found as isolated cells, and very striking examples 

 may be seen in some leaves, as in Came/lia or Hakea. They are not always 

 pointed but may be columnar in form, and are called sclereids (Fig. 449). 



Fibre cells are, however, usually found in bundles or masses called 

 sclerenchyma, forming longitudinal strands, which are the material of many 

 commercial fibres, such as jute, sisal and hemp. This solid, mechanical tissue 

 is distributed in such a way as to give the maximum rigidity to the organs in 

 which it occurs, and collectively it forms the stereome or supporting tissue 

 of the plant (see Chapter XXI). 



Fig. 448. — Diagram of sub- 

 microscopic structure of the 

 lignified wall of a tracheid. 

 {After Maby.) 



