SECT. xrv. SCLEROGEN. 421 



potatoes, &c., being in a state of change, imparts that 

 state to the starch, and converts it into a sweet so- 

 luble matter known as dextrine or starch-gum which is 

 capable of being carried throughout the plant with the 

 sap, and which is itself ultimately changed into sugar. 

 Dextrine is an ingredient in the primordial cell. Starch, 

 dextrine, and cellulose are isomeric : consisting of the 

 same elements with different characters. 



The woody part of trees and shrubs, the fibres of hemp, 

 flax, of the Agave, and many other plants, are formed of 

 cellulose, the purest form of that substance being bleached 

 flax and linen. During the progress of vegetation, the 

 cells of the ligneous tissue of trees, also those of woody 

 and fibrous plants, which are transparent and colourless 

 when young, become internally coated or filled with 

 sclerogen, the colouring matter of wood, a substance of 

 various hues. In extra-tropical countries it is generally 

 some shade of brown, sometimes dark, sometimes so 

 pale as to be almost while with a yellowish or reddish 

 tinge ; and occasionally it is beautifully marked as in the 

 wood of the olive. In tropical countries the colours are 

 more vivid and varied, deeper and even black, as in ebony. 

 This colouring matter has the same quantity of oxygen 

 as cellulose, but it contains hydrogen and more carbon, 

 hence wood is combustible in proportion to the quantity 

 of sclerogen it contains. In beech it forms half of the 

 wood, in oak two thirds, and in ebony nine tenths, so it 

 is the most, highly combustible of the three. The ad- 

 ditional carbon is obtained by increased respiration, the 

 hydrogen by decomposition of water in the sap. 



Sugar is almost as universal a constituent of the 

 higher classes of plants as cellulose and starch, for be- 

 sides the saccharine juice of innumerable plants, starch, 

 the acids of unripe plants, and even the acrid juice 

 of the fig and other plants, is turned into sugar as the 

 plant advances to maturity, and the fruits ripen. Manna 

 and other saccharine exudations from the leaves or stems 



