112 HISTOLOGY OF ANGIOSPERM STEM, ROOT, ETC. 



(c) Cut a series of transverse sections from various points on 

 the main root, at intervals of about 1 cm. starting from the tip ; 

 arrange the sections in order on the slide, and treat them with ani- 

 line sulphate. Note the different appearances presented by the 

 vascular cylinder in the different regions. 



From this series of sections you will learn 



(1) That for some distance behind the tip the central cylin- 

 der shows as many xylem strands as there are longitudinal 

 rows of rootlets (usually four in Phaseolus, five or six in Broad 

 Bean). 



(2) That in passing backwards from the apex these strands 

 of primary xylem increase in size by the formation of addi- 

 tional vessels on the inner side of those first formed, the later- 

 formed (inner) vessels of each strand being wider than the 

 first-formed vessels. 



(3) That the young rootlet begins as a projecting mass of 

 tissue immediately outside one of these primary xylems. 



(4) That the young rootlet pushes its way through the cortex 

 as it grows, eventually bursting through the surface. 



(5) That the xylem of the rootlet is joined on to that of the 

 xylem strand opposite which it arose. 



(6) That in the older part of the root additional xylem 

 vessels appear in tangential bands alternating with the 

 primary xylem strands, and that the cells on the outer side 

 of each of the bands of secondary xylem show the appearance 

 of a cambium (cells arranged in radial rows, with closely- set 

 tangential walls). 



(7) That, still farther from the root-tip, the secondary 

 vascular tissue increases in amount, though the primary 

 xylem strands can still be seen towards the centre of the 

 root. 



13O. T. S. of Young Beau Boot. Cut thin trans- 

 verse sections of a seedling Broad Bean root, at about 5 or 

 6 cm. from the tip; treat different sections with iodine, 

 chlor-zinc-iodine, and aniline sulphate. Note 



(1) The epidermis, or piliferous layer, some cells of 

 which give out a root hair. 



(2) The parenchymatous cortex, of thin-walled and 

 rounded cells, which may contain starch grains, and which 

 are separated at the corners by intercellular spaces. 



(3) The eudodermis, a single layer of cells showing the 

 characteristic radial walls. 



