90 HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 



stem and then cut sections from it and note which 

 tissues if any contain sugar. 



(a) With the low power, starting from the outside of 

 the stem, note (1) the epidermis, consisting of one layer 

 of cells and here and there passing out into (2) large 

 hairs, with thickened bases, the tissue of which is con- 

 tinuous with the epidermis and the underlying stem-tissue 

 while the upper part of the hair is a row of cells besides 

 these there are thinner hairs with smaller bases ; (3) 

 below the epidermis a zone of collenchyma thick- walled 

 but not lignified tissue ; (4) a narrow zone of thin- walled 

 parenchyma, abutting internally on (5) a zone of scler- 

 euchyma with thick lignified walls the tissues lying 

 between this zone and the epidermis form collectively the 

 cortex ; (6) the internal parenchyma in which are em- 

 bedded (7) the vascular bundles ; (8) the central pith 

 cavity. 



(6) Examine one of the larger bundles in detail, and note : 



(1) The very conspicuous wide vessels of the xylem 

 stained yellow with aniline sulphate embedded in thin- 

 walled tissue (xylem parenchyma) ; the outer vessels are 

 very wide, while the inner ones are narrower and are in 

 fairly regular radial rows. 



(2) On the outer side of the wood, the cambium, consist- 

 ing of thin-walled cells elongated tangentially and narrow 

 radially, showing very regular arrangement in radial rows ; 

 the tangential walls are especially thin, thus indicating recent 

 and repeated divisions in this direction. 



(3) The outer phloem, into which the cambium merges 

 on its outer side, with very conspicuous sieve-tubes em- 

 bedded in small-celled tissue. 



(4) The inner phloem, resembling the outer in structure 

 and forming in cross section a crescent-shaped patch on the 

 inner side of the xylem the Marrow is rather exceptional 

 in having bicollateral bundles, with inner phloem as well as 

 the normal outer phloem found in collateral bundles. 



(c) Examine the various tissues with the high power. 

 Note that in the collenchyma, below the epidermis, the 

 walls of the cells are strongly thickened at the angles 

 between adjacent cells, though thin at the middle, so that 

 the cell-cavity appears rounded or oval in section ; at 

 places the collenchyma is interrupted by the underlying 



