OF VEGETABLE TISSUE. 15 



they are more difficult to break across than to 

 rend asunder lengthwise, this is what workmen 

 call following the grain of the wood. These 

 fibres constitute what is termed Woody Tissue, 

 or Pleurenchyma. It is also found in the young 

 bark, and in the nervures of leaves, " and gives 

 strength to the vegetable fabric." * When many 

 fibres are distributed circularly round an axis, 

 whether real or imaginary, the whole together 

 is called a Layer. It is thus that the annual 

 rings of Dicotyledonous trees are formed. 



6. Skin, called also Cuticle, or Epidermis. 

 The whole surface of the plant, wherever it is 



* " 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 it is by a peculiar 



set of dots, seen along their course, that these woody 



tubes may be readily distinguished from all others 



\V hatever be their character, they are of great interest 

 as tending to establish the true nature of coal. 



" That this substance had a vegetable origin has long 

 been generally admitted ; but from the comparative fre- 

 quency and perfection, 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 



