i62 CHEMICAL COMPOSITION OF PLANTS 



and (r) certain aromatic compounds not yet isolated in a pure 

 state : the substances b and c are together generally spoken of as 

 Ugnin or lignone. Lignocelluloses are primary constituents of 

 plant tissues, and are not celluloses on which ' lignin ' is encrusted 

 or deposited as the result of secondary chemical changes. 



Lignified walls become pink when treated with phloroglucin 

 and hydrochloric acid, and stain a yellow colour in solutions of 

 aniline chloride ; with chlor-zinc-iodine the walls become yellow. 



The cell-walls of lignified tissues in the heart-wood of trees, 

 and other parts of plants, frequently become stained by tannin 

 and various colouring matters. 



Paper of all kinds consists chiefly of cellulose obtained from 

 linen rags, cotton, wood and straw. 



Ex. 87. — To prepare ' chlor-zinc-iodine,' dissolve 25 parts of zinc chloride 

 and 8 parts of potassium iodide in %\ parts of water, and add as much iodine 

 as will make the solution a dark sherry colour. 



Cut sections of the stems and other parts of plants, and mount them in the 

 solution ; note the blue colour of the unlignified and uncuticularised walls. 

 Notice the effect ofthe solution upon ' cotton-wool,' and upon sections of wood. 



Ex. 88. — Cut sections of the seeds with a dry razor ; mount and examine 

 some of the sections in water, and some in pure glycerine. Soak some white 

 mustard and flax-seeds-(linseed) in water ; note the slimy mucilaginous nature 

 of the surface of the seeds when wetted, 



Ex. 89. — Cut sections ofthe stems of various plants, and mount them in a 

 saturated solution of aniline chloride, to which a few drops of hydrochloric 

 acid have been added ; the lignified walls stain a golden yellow colour. 



d. Pentosans. — Associated with the cellulose of plant tissues 

 are carbohydrates termed pentosans (CsHgO^). When heated 

 with dilute acids they are hydrolysed and converted into the 

 pentose sugars (CjHjqGj) arabinose or xylose. 



Pentosans are produced during the early stage of growth, and 

 the amount generally increases with the age of the plant. 

 These carbohydrates appear to be of little use in the nutritive 

 processes of plants, but are partially digested and assimilated by 

 herbivorous animals. They are common in all plajnt tissues and 

 are especially abundant in grasses and cereal straw. 



