26 OF WOOD IN GENERAL. 



thicker walls are produced, whilst increased pressure of new 

 bark allows less radial extension. As winter comes on, the 

 active growth and division of the cambium cells cease, and its 

 recommencement to form large thin walled tracheids in the 

 following spring, after being dormant for several months, pro- 

 duces the sharp contrast between compressed summer tracheids 

 and larger spring ones that marks a new annual ring. 



FIG. 17. Transverse section of Scots Fir (Pinus sylvestris). After Stras- 

 burger. From The Elements of Botany, by permission of Mr. Francis Darwin 

 and the Syndicate of the Cambridge University Press. 



phi, phloem ; s.p, sieve-plate ; m.r, pith-ray ; c, cambium ; ?', initial cell of 

 cambium ; x, xylem ; 1, 2, 3, successive stages in the development of bordered 

 pits. 



The simple uniformity of structure in coniferous wood con- 

 tributes largely to its great technical value. 



Space does not permit any detailed discussion of the physio- 

 logical uses of the different parts of such a stem as that of a 

 conifer to the growing tree. The following recapitulation must 

 suffice. The vitality of the pith of trees is generally confined to 

 the very earliest stages of their existence, and the spirally- 

 thickened elements of the protoxylem also only serve as 

 conducting tissue when all the xylem is young. Heart-wood 

 has ceased to have any active functions, serving merely for 

 strength. Whilst cortical tissue serves to protect from external 

 action, damp, etc., and to check transpiration, the sieve-tubes of 

 the phloem appear to be the chief carriers of the food-materials 

 elaborated by the leaves to the growing parts of the stem ; and 

 the formation of new phloem and xylem is the one function of 

 the cambium. In the sap-wood of conifers consisting, as it does, 

 so largely of tracheids, it is these tracheids, communicating as 

 they do by the bordered pits on their radial walls, that convey 



