DEVELOPMENT OF CONIFEROUS WOOD 



21 



bordered pits in a single row down their radial walls ; and that 

 they are closed at their ends by a tapering to one side like the 

 cutting edge of a carpenter's chisel. The pith-rays in longitudinal 

 sections are seen to extend only a short way longitudinally, each 

 appearing on radial sections as a band of 8 to 10 rows of cells 

 elongated at right angles to the elongation of the tracheids hke 

 bricks in a wall 8-10 bricks high, with bordered pits on the ceUs 

 of the upper and lower rows, in Pines and Spruces, and simple 

 pits on the others. On tangential sections the rays appear as 

 vertical series of 8-10 pores tapering above and below. In Pines 

 there are some larger pith-rays containing horizontal resin-passages. 

 The development of this comparatively simple type of wood 



A 





D A 



( 



3 



A E 





a 





5 



a 







C 



I 



L 



) C 







A 



B A 



c 

 a 





d 



h c 



a 















C 



IV 



) 



D 



III 



Fig. 16.~ Diagram illustrating merismatic tissue. I, a merifimatic cell ABCD ; 

 II, a cross-wall ab has appeared; III, Aa&B has grown and again equals ABO0 m 

 size, whilst aODb has also grown ; IV, Aa&B has been divided by a cross-wall cd ; 

 V, Ac<^B has again grown : it equals ABCD in size and is ready again to divide. 

 Meanwhile ca&dl and aODh have increased in size considerahly- (From. TM Blements 

 of Botany, by Mr. Francis Darwin, by bis permission and that of the Syndicate of the 

 Cambridge University Press.) 



from the cambium can be readily traced. The cambium is a 

 cyhndrical sheet of very thin-walled cells, each of which is rect- 

 angularly prismatic, broader in a tangential direction and tapering 

 above and below to a radially-directed chisel-edge. These cells 

 contain protoplasm. After they have grown somewhat in a radial 

 direction, partition walls form across them in the longitudinal 

 tangential direction, so that each cell gives rise to two radiaUy 

 placed towards one another, and, this process being then repeated 

 in one or both of the resultant cells, a radial row is formed (Fig. 16). 

 After several such divisions the innermost and earhest-formed of 

 these cells ceases to divide, and uses up its protoplasmic contents 

 in lignifying and thickening its walls, except at certain spots which 



