SCOTS PINE. 373 



In A note (1) the sieve- tubes, in which the tangential 

 walls, seen in surface view, are quite uniform, while the 

 cut radial walls appear wavy owing to the sieve-plates ; 



(2) the phloem-parenchyma cells arranged in longitudi- 

 nal rows ; (3) the brown crystal-containing cells ; (4) the 

 phloem medullary rays, seen as vertically elongated 

 rows of cells, narrower above and below, all with dense 

 contents. 



In B note (1) the ordinary cambium cells, elongated 

 vertically, pointed above and below, thin-walled, with 

 dense protoplasm and elongated nucleus ; (2) the cells of 

 the cambium medullary rays, resembling those of the 

 rays seen in the phloem, and clearly formed by repeated 

 transverse divisions of an ordinary cambium cell. 



In C note (1) the very long tracheids with pointed 

 ends, showing no, or very few, bordered pits in surface 

 view on the tangential walls, but showing many cut longi- 

 tudinally on the radial walls with the same appearance as 

 in the transverse section of the stem ; (2) the medullary 

 rays, each tapering and pointed above and below, and 

 therefore spindle-shaped (like a biconvex lens cut across). 



The height of the ray varies from a single cell up to over 

 a dozen cells, and in the larger rays there may be noted 



(3) minute intercellular air-spaces, (4) two or more cells 

 abreast at the middle of the ray, (5) a resin-duct travers- 

 ing the ray in the radial direction and therefore cut across 

 in the tangential section of the stem, (6) a differentia- 

 tion of the ray cells into " starch cells " and " tracheidal 

 cells." 



Note that in both radial and tangential longitudinal 

 sections one or more resin- ducts may be seen cut longi- 

 tudinally in the cortex and the secondary xylem ; note the 

 cavity of the duct, and the epithelium layer lining it. 



522. T. S. of Root (Young and Old). Cut transverse sec- 

 tions of the young root of a Pine seedling, and note (1) the badly 

 defined piliferous layer, which soon becomes disorganised root- 

 hairs are very seldom present ; (2) the cortex parenchyma ; (3) the 

 endodermis, a layer of cells with folded radial walls ; (4) the 

 pericycle, which consists of several layers of parenchymatous cells, 

 and which merges imperceptibly into the almost indistinguishable 



