86 PRACTICAL BOTANY 



hairs are present on the older parts, but not on the youngest 

 parts close to the apex. 



Microscopic Observations. 



Harden the roots in alcohol for two or three days or more. 

 In order to cut the sections it will be necessary to hold 

 the roots between pieces of pith, or, better, to embed in 

 paraffin. 



I. Cut transverse sections of the primary root at a point 

 nearer the apex than the youngest lateral roots, i.e. about two 

 inches from the end : clear the sections with weak potash, or 

 "eau de javelle," and mount in glycerine. 



Observe the following tissues : 



1. At the centre of the circular section is a mass of parenchy- 

 matous pith. At the periphery of this are 



2. Radiating groups of elements of the primary xylem, 

 which are the most strongly marked tissues of the young root. 

 The number of these is most often four, but it is subject to 

 variation, and may be as high as six, while in the Pea it may 

 frequently be only three : the same is the case with the groups 

 of phloem, which alternate with these. The groups of xylem 

 have dark lignified walls (test with chlor-zinc-iodine or aniline 

 sulphate), and resemble the primary xylem of the stem. Note 

 fresh elements in course of formation at their central limit : the 

 development is thus centripetal. Alternating with these may 

 be seen 



3. Four groups of primary phloem, which are not as yet 

 very well marked. These several groups of elements are 

 separated laterally from one another by bands of parenchyma. 

 At the periphery of the central cylinder thus built up is 



4. The pericycie, consisting of thin-walled cells arranged in 

 an undulating band, which is a single layer of cells in thickness 

 peripherally to the phloem, but opposite the xylem it consists 

 of two to three layers of cells. 



5. Immediately outside this is the bundle-sheath, or endo- 

 dermis, consisting of a single layer of cells, having the 

 characteristic dark dot on their radial walls. Then follows 



