106 HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 



usually produces each year two bands of phloem fibres, so that 

 the number of fibrous bands in the phloem is roughly double that 

 of the annual ring in the wood. In each band of soft phloem the 

 wide sieve-tubes occupy the middle of the bands, while abutting 

 on the fibre-bands are the narrower parenchyma-cells, some of 

 which contain starch and other crystals. 



119. Radial L. S. of Old Lime Stem. In a radial 

 longitudinal section of four-year-old (or older) stem, note 

 the various tissues already seen in the transverse section. 



In the xylem, note (1) the narrow protoxylem vessels, nearest 

 the pith, with spiral thickenings ; (2) the wide pitted vessels, with 

 small bordered pits and also spiral or reticulate thickening ; (3) the 

 tracheids, differing from the vessels only in being narrower and in 

 having tapering intact (not absorbed) end-walls ; (4) the fibres, 

 resembling tracheids in shape, but having only small pits scattered 

 sparsely on their walls these fibres are the most abundant of the 

 secondary xylem constituents in the Lime ; (5) the xylem paren- 

 chyma cells, arranged in vertical rows and usually containing 

 starch ; (6) the rays, seen as bands running across (in reality 

 between) the xylem elements and consisting of cells with pitted 

 walls and proteid or starchy contents the ray cells in contact with 

 vessels have most pits and scantiest contents. 



The cambium, pith, cortex, and periderm have much the 

 same appearance as in transverse section. 



In the phloem, note the very long narrow thick-walled lignified 

 fibres ; the sieve-tubes, the oblique end- walls (compound sieve- 

 plates) of which mostly face the radial plane and are therefore 

 seen in surface view here, each plate showing three or more sieve- 

 areas this "compound" type of sieve-plate is common in the 

 secondary phloem of woody plants. 



120. In a Tangential Longitudinal Section of the 



wood of the Lime, note especially the medullary rays, easily 

 distinguished by their spindle-like form and their narrow 

 and fairly thick- walled cells ; some of the rays are but one 

 cell wide throughout, others two or even more cells wide 

 at the middle, while their height varies greatly. 



121. Maceration of Woody Stems (Schultze 

 Method). Place some fairly thick longitudinal sections 

 of stem in a test-tube, add a few crystals of potassium 

 chlorate, and then enough nitric acid to cover them. Heat 

 gently, at a distance from the microscope, so that the fumes 

 may not injure it. After the fumes have ceased add water 



