DEVELOPMENT OF CONIFEROUS WOOD. 



These cells contain protoplasm. After they have grown some- 

 what in a radial direction, partition walls form across them in 

 the longitudinal tangential direction, so that each cell gives rise 

 to two radially placed towards one another, and, this process 

 being then repeated in one or both of the resultant cells, a radial 





FIG. 16. Diagram illustrating merismatic tissue. I, a merismatic cell 

 ABCD; II, a cross-wall ab has appeared; III, AabB has grown and again 

 equals ABCD in size, whilst aCD6 has also grown ; IV, Aa&B has been divided 

 by a cross-wall cd ; V, AcdB has again grown : it equals ABCD in size and is 

 ready again to divide. Meanwhile cabd and aCD6 have increased in size 

 considerably. From The. Elements of Botany, by Mr. Francis Darwin, by his 

 permission and that of the Syndicate of the Cambridge University Press. 



row is formed (Fig. 16). After several such divisions the inner- 

 most and earliest-formed of these cells ceases to divide, and uses 

 up its protoplasmic contents in lignifying and thickening its 

 walls, except at certain spots which 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 continuing the pith-ray (Fig. 17). 

 In spring, when there is little heat, light, or activity of root and 

 leaf to supply material, and when the bark, split by winter, may 

 exert but little pressure, tracheids are produced with relatively 

 thin walls and wider radial extension, constituting the spring 

 wood; but in summer heat, light, and physiological activity, 



