22 OF WOOD IN GENERAL 



become pits. It has, in fact, become a water-and-air-conducting 

 tracheid. A cambium cell in the same radial row as a pith-ray 

 undergoes transverse division into 8-10 superposed cells which 

 elongate radially and retain protoplasmic contents, thus continu- 

 ing the pith-ray (Eig. 17). In spring, when there is httle heat, 

 light, or activity of root and leaf to supply material, and when the 

 bark, spHt by winter, may exert but Httle pressure, tracheids are 

 produced with relatively thin walls and wider radial extension, 

 constituting the spring wood; but in summer heat, Hght, and 

 physiological activity, 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 ceases, and 

 its recommencement to form large thin-walled tracheids in the 

 following spring, after being dormant for several months, produces 

 the sharp contrast between compressed summer tracheids and 

 larger spring ones that marks a new annual ring. 



Fig 17 —Trausvorae section of Scots Fir (Pimis svhtUns) Afioi Strisbiirgor. 

 (From The Elements of Botanij, by permission of Mr. Francis Darwin and the Syndi- 

 cate of tho Cambridge University Prefe'' ) 



pU, pMoem , s p, sie-ve plate , u r, pith ray , c, cambium , a, initial cell of cam» 

 bium , Xi xylern , I, 2, a, successive stages m the development of bordered pits 



The simple uniformity of structure in coniferous wood contri- 

 butes 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 vitahty of the pith of trees is generally confined to the very- 

 earliest stages of their existence, and the spirally-thickened ele- 

 ments 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 



