MODIFICATIONS OF WOODY FIBRE. 



65 



of the wood of the stem and roots; it partly composes the leaf- 

 stalk, midrib, and veins of the leaves, and may even be traced 

 in flowers ; to many fruits, also, it imparts firmness and consist- 

 ence. When no longer required for the conveyance of fluid, 

 additional firmness and toughness are given to it, as to cellular 

 tissue, by the deposition of various secretions within its tubes ; 

 and it is in the presence or absence of these, that the difference 

 exists between the heart-wood and sap-wood of a trunk (. 131). 

 The woody tubes of the former are entirely choked up with the 

 hard matter deposited in their cavities ; and the sap rises through 

 the latter only. This hardened tissue may be in some degree 

 compared to the cartilage or gristle of animal bodies. 



78. A peculiar form of woody fibre is found in the stems of 

 resinous woods, especially the Pine and Fir tribe. The diameter 

 of its tubes is much greater than that of any other woody tissue ; 

 and they alone perform the office of transmitting the sap upwards 

 through the stem ; the wood of these trees being destitute of the 

 ducts or canals (presently to be described), which in other kinds 

 of trees assist in this function. But it is by a peculiar set of dots 

 seen along their course, that these woody 

 tubes may be readily distinguished from 

 all others. These dots appear to be 

 produced, by the formation of certain 

 little bodies between the adjacent tubes ; 

 and as the tubes are closely pressed toge- 



ther, these bodies (the nature of which FlG - 2 & GLANDULAR WOODY 

 is not certainly ascertained) project into * A DKAL SHAVINS * 



the cavities of the tubes between which they lie. "Whatever be 

 their character, they are of great interest, as aiding to establish 

 the true nature of Coal. 



79. That this substance had a vegetable origin, has long been 

 generally admitted ; but from the comparative frequency and per- 

 fection with which the remains of Ferns occur in it, it has been 

 supposed to have been produced by the decay of vast forests of 

 this tribe of plants. As Ferns do not form resins, however, this 

 hypothesis would not account for the large quantity of bituminous 

 matter which coal contains ; and hence it was supposed that coal 



