HISTOLOGY OF ANGIOSPERM STEM, KOOT, ETC. 91 



thin-walled parenchyma, and at these places stomata 

 occur in the epidermis ; chloroplasts occur in the cortical 

 parenchyma, less abundantly in the collenchyma ; air- 

 spaces occur between the parenchyma cells, but not in the 

 collenchyma ; the innermost layer (endodermis) of the 

 cortex lying immediately outside the sclerenchyma zone 

 consists of cells with the radial walls wavy, and the 

 cells contain starch grains. 



In the phloem note the protein contents of the sieve- 

 tubes in sections treated with iodine or with Millon's 

 reagent, and look for places where the section has passed 

 just above or just below a transverse wall (sieve-plate) 

 in a sieve-tube; the plate has a dotted appearance, the 

 dots being pores in the plate these are often seen better 

 on treating a section with eau de Javelle, which removes 

 the contents of the sieve-tube. Note also that each sieve- 

 tube is associated with a narrow cell (companion-cell) 

 which has been cut off from the sieve- tube by a longitu- 

 dinal wall, and that the remaining tissue of the phloem 

 consists of parenchyma cells differing from the sieve-tubes 

 in their smaller size and the absence of sieve-plates. Also 

 note carefully the appearance of the cells in the cambium. 



101. Radial L. S. of Marrow Stem. Cut radial 

 longitudinal sections of the stem, passing through one of 

 the larger bundles ; mount some sections unstained in 

 glycerine, and of the others treat some with iodine, chlor- 

 zinc-iodine, aniline sulphate, Millon's reagent leaving 

 the rest in alcohol for further treatment. 



(a) Starting from the outside, note the various tissues 

 (compare carefully with what has been seen in the trans- 

 verse sections) : epidermis ; collenchyma and parenchyma 

 of cortex ; endodermis ; sclerenchyma of very long fibres 

 with thick lignified walls and tapering pointed ends ; the 

 inner ground tissue parenchyma within the sclerenchyma ; 

 the bundles. 



(b) In a single vascular bundle, with high power, note : 



(1) The outer phloem, in which the sieve-tubes are 

 easily recognised by their conspicuous transverse walls 

 (sieve-plates). 



