C 5 5 ] 



The porous tubes likewise contain these fluids ; 

 and their use is probably that of conveying them into 

 the sap for the production of new arrangements. 



The trachea? 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 enabled to con- 

 solidate 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 laminse 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 change of 

 volume produced in it by change of temperature Is 

 one of the principal causes of the ascent of the sap* 

 The fibres of it seem always to expand in the morning 

 and contract at night ; and the ascent of the juices, as 

 was stated in the last Lecture, depends principally on 

 the agency of heat. 



The silver grain is most distinct in forest trees ; 

 but even annual shrubs have a system of fibres simi- 

 lar to it. The analogy of nature is constant and uni- 



* Fig. 8 represents the section of an elm branch, which exhibits the tubular 

 structure and the silver and spurious grain, Fig. 9 represents the section of rare 

 of thr; branch of an oak. Fig. \Q, that of the branch of an ash. 



