SCOTS PINE. 367 



Now cut transverse, radial longitudinal, and tan- 

 gential longitudinal sections of a three-year-old stem, 



for detailed examination of the xylem, phloem, cambium, 

 and rays. Treat some sections with chlor-zinc-iodine, 

 others with aniline .sulphate ; others might be stained with 

 safranin and aniline blue and mounted in balsam. 



519. T. S. of Three-year-old Stem (Fig. 89). In 

 beginning to examine the tranverse sections, note that 

 (1) the cells of the cortex show here and there signs of 

 having grown in length tangentially and undergone 

 divisions by radial walls to accommodate the increased 

 volume of the vascular cylinder ; (2) the phloem has 

 grown in thickness, but its outer portion has undergone 

 distortion; (3) the xylem has grown greatly in thickness, 

 and shows three layers (annual rings) owing to the sudden 

 increase in radial diameter of the four- sided lignified 

 elements (tracheids) formed in spring as compared with 

 those formed in autumn, the latter lying immediately 

 within the wide spring tracheids ; (4) in each ring, starting 

 from within, the tracheids show a gradual transition from 

 the wider and thinner-walled tracheids of the spring 

 wood, becoming more flattened and thicker-walled on 

 passing outwards till the autumn wood is reached. 



Starting with the cambium, note that (1) the cambium cells are 

 in very regular radial rows ; (2) their walls are very thin, especially 

 the tangential walls ; (3) the cells contain abundant protoplasm and 

 a nucleus, especially in the active middle region where the cells are 

 radially narrowest ; (4) the cells of the medullary rays in the 

 cambium-zone are radially longer than the other cambium-cells. 



Now trace the ordinary and the ray cells of the cambium inwards 

 into the wood (xylem), and note 



(1) The developing tracheids immediately within the cambium, 

 with relatively thin walls ; the inner layer (that next the cavity) still 

 giving cellulose reactions, while the rest of the wall is more or 

 less strongly lignified. 



(2) The protoplasmic contents of these youngest tracheids, grad- 

 ually diminishing in quantity and disappearing on tracing a row of 

 tracheids inwards. 



(3) The increasing thickness of the tracheid walls and their com- 

 plete ligriification as they grow older. 



(4) The bordered pits on the radial walls, each pit appearing in 



