58 
into the sap for the production of new arrange- 
ments. 
The tracheae contain fluid matter, which is al- 
ways thin, watery, and pellucid, and these organs, 
as well as the false tracheae, probably carry off 
water from the denser juices, which are thus en- 
abled to consolidate for the production of new 
wood. 
In the arrangement of the fibres of the wood, 
there are two distinct appearances. There are 
series of white and shining laminae which shoot 
from the centre towards the circumference, and 
these constitute what is called the silver grain of 
the wood. 
There are likewise numerous series of concentric 
layers which are usually called the spurious grain , 
and their number denotes the age of the tree. * 
The silver grain is elastic and contractile, and it 
has been supposed by Mr. Knight, that the con- 
tractions produced in it by changes of temper- 
ature are the principal causes of the ascent of 
the sap. 
The silver grain is most distinct in forest trees ; 
but even annual shrubs have a system of fibres 
similar to it. The analogy of nature is constant 
and uniform, and similar effects are usually pro- 
duced by similar organs. 
The pith occupies the centre of the wood ; its 
texture is membranous 5 it is composed of cells, 
* Fig. 8. represents the section of an elm branch, which ex- 
hibits the tubular structure and the silver and spurious grain. 
Fig. 9. represents the section of part of the branch of an oak. 
Fig. 10. that of the branch of an ash. 
