CARBOHYDRATES l6i 



None of the above-mentioned celluloses are ever met with in 

 a pure state in plants ; they are always combined or mixed with 

 other substances forming three main types of what may be termed 

 compound celluloses as indicated below. 



(i) Pectocelluloses. — Tlieseare compounds or intimate mixtures 

 of typical celluloses with pectose ; the latter when hydrolysed with 

 dilute acids or alkaUes fitXds, pectin, a substance whose solutions 

 gelatinise easily. The cell-walls of raw cotton, flax-fibres and 

 other unlignified fibres, as well as most parenchymatous tissues 

 and especially those of fleshy roots and fruits, such as carrots, 

 mangel, turnips, apples, pears and currants, consist chiefly of 

 this form of compound cellulose. 



Mangin asserts that the first walls produced during cell-division, 

 consist mainly of pectose, the secondary thickening-layers of 

 most unlignified cell-walls being formed of cellulose and pectose 

 combined. 



Closely allied to pectocelluloses are the mucocelluloses composed 

 of cellulose and substances which yield mucilaginous solutions with 

 water : they are chiefly met with in certain seeds and fruits. 



(ii) Adipocelluloses. — The cell-walls of cork-tissue appear to be 

 composed chiefly of a fatty or waxy substance termed suberin 

 combined with a very small amount of cellulose. Allied to these 

 are the cutocelluloses forming the cell-walls of the epidermis of 

 plants : the substance cutin closely resembles suberin in its 

 composition and properties. Both suberised and cutinised cell- 

 walls turn brownish-yellow when treated with ' chlor-zinc iodine ' ; 

 they are impermeable to water and successfully prevent the loss 

 of water from tissues covered by them. Whether cutin and 

 suberin are products of the direct conversion of cellulose is a 

 question at present unsolved. 



(iii) Lignocelluloses. — The cell-walls of the woody tissues of 

 plants consist oi lignocelluloses which are homogeneous compounds 

 of (a) cellulose or oxycellulose, 



((5) a pentosan known as wood-gum, 



1. 



