SCOTS PINE. 369 



transverse section like a biconvex swelling of the wall. On close 

 inspection some of the pits will show the biconvex lens-like pit- 

 cavity across which there stretches the thin pit -membrane with 

 a thickening (torus) in the middle. 



(5) The rays, consisting mostly of single rows of radially elongated 

 cells with cellulose walls, arid either containing protoplasm and 

 nucleus, or appearing empty ; the ray contains both kinds of cells, 

 as will be better seen in longitudinal sections. 



(6) The resin-ducts of the wood, lined by epithelium and usually 

 associated with a patch of xylem-parenchyma and often also with 

 one of the rays ; the ray may either be interrupted by the paren- 

 chyma which encloses the resin-duct, or the duct may be joined up 

 to the ray by parenchyma. 



Next trace the cells of the cambium outwards into the bast 

 (phloem), and note : 



(1) The phloem constituents are arranged in radial rows, quite as 

 regular as those in the cambiun and xylem. 



(2) There is a gradual transition from cambium to phloem. 



(3) The cell-walls on the phloem side of the cambium become 

 thicker, but are not lignified. 



(4) The medullary rays are continuous through cambium into 

 xylem, and have protein and starchy contents. 



(5) The sieve-tubes are radially narrow, with finely -perforated 

 sieve-areas on their radial walls. 



(6) Here and there are large, and usually starch -containing, 

 phloem-parenchyma cells. 



(7) The sieve-tubes lose their turgidity and become much crumpled 

 and distorted and almost obliterated by the pressure of the cortex 

 on which these older elements of the phloem abut ; though the 

 starch -containing phloem-parenchyma cells and those of the medul- 

 lary rays may remain almost unaltered or even grow larger. 



(8) Sometimes the outer end of a ray, just outside the phloem, 

 joins on to the parenchyma and epithelium around a resin-duct in 

 the cortex. 



520. In Radial Longitudinal Sections of Three- 

 year-old Stem (Fig. 90), which must be cut very care- 

 fully so as to be exactly radial, note : 



(1) The outer tissues, consisting of epidermis, cork, and 

 cortex parenchyma. 



(2) The sieve-tubes, elongated, with scanty contents, 

 and showing here and there the sieve-plates which are 

 confined to the radial walls ; each plate is circular or oval, 

 covered with callus (stained deeply by aniline blue) when 

 young and functional, and shows a number of finely dotted 

 sieve-areas. 



p. B, 24 



