150 PKACTICAL BOTANY 



four, but it is subject to variation, and may be as high 

 as six : 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 centri- 

 petal. 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 pericycle, consisting of thin-walled cells 

 arranged in an undulating band, which is a single layer 

 of cells in thickness peripherally to the phloem, but op- 

 posite the xylem it consists of two to three layers of cells. 



5. Immediately outside this is the bundle-sheath, 

 or endodermis, consisting of a single layer of cells, 

 having the characteristic dark dot on their radial walls. 

 'Then follows 



6. The parenchymatous cortex, a thick band of 

 tissue, with intercellular spaces, and 



7. The piliferous layer, a single superficial layer, 

 not well marked. Single cells of this layer will be 

 seen to have grown out perpendicularly to the surface 

 as root-hairs, which as a rule are not branched, and 

 are of cylindrical form, with thin cell-walls : particles 

 of soil may be found attached to many of them, 

 while they may often be seen to be modified in form 

 so as to apply themselves closely to the grains of sand, 

 &c., with which they have come in contact. 



