142 



X. THE CONIFEROUS STEM. 



FIG. 52. Part of a cross-sectiou of a pretty 

 old branch of Pin us sylvestris. The strip passes 

 through the cambium (i, initial layer), and ends 

 on the one side in the young wood, on the other 

 side in the young bast. 1, 2, 3, stages in the 

 development of bordered pits ; m, medullary 

 ray ; e, sieve-plate ; k, flattened cells with brown 

 contents, later on containing crystals ( x 540). 



tain only water, the pits are 

 bilaterally bordered, just as 

 between trachei'de and tra- 

 cheide. 



In the immediate neigh- 

 bourhood of the cambium w r e 

 see the young wood composed 

 of incompletely developed 

 tracheides (Fig 52). The 

 walls of the cells diminish 

 rapidly in thickness towards 

 the cambial zone. Inside 

 the cambial zone the radial 

 walls thicken again. What 

 we have called Cambium 

 consists of an Initial layer 

 of cells (Fig. 52, i) which, 

 through continued tangen- 

 tial divisions, gives off tissue 

 mother-cells towards wood 

 and bast, from which the 

 elements of the wood and 

 the bast have their origin. 

 Upon the wood side the 

 successive stages in the de- 

 velopment of the bordered 

 pits can be followed (Fig. 52, 

 1 , 2 and 5). The radial rows 

 in which the tracheides are 

 formed can be traced out 

 through the cambium into 

 similar radial rows of bast 

 elements, for these show 

 every whit as marked a 

 radial arrangement. On the 

 bast side of the cambium the 

 cell - walls rapidly thicken, 

 and take on a dull- white 

 appearance. On the radial 

 walls of the wide-cavitied 

 elements of the bast (which 



