90 PRACTICAL BOTANY 



VI. Cut similar transverse sections from a part of a root 

 which has recently turned brown, and has produced lateral 

 roots : comparing with the above sections, note the following 

 changes : 



1. The piliferous layer and cortex have turned brown, and 

 will often be separated from the central cylinder by rupture of 

 the radial walls of the endodermis. 



2. The cells of the pericycle will be found undergoing 

 periclinal divisions, which ultimately result in the formation 

 of a layer of cork. 



3. The groups of primary xylem as before, but united at the 

 centre so as to form a star-shaped mass, with no central pith: 

 the groups of primary phloem alternating with those of primary 

 xylem, as before. 



4. Divisions may be seen in the cells intervening between 

 the xylem and phloem, which indicate the position of the 

 cambium. 



5. If lateral roots be present, they originate from the 

 pericycle. 



VII. Cut transverse sections of still older portions of the 

 root, about ^ of an inch or more in diameter : treat with chlor- 

 zinc-iodine, which brings the starchy medullary rays into 

 prominence. Now examine carefully the central mass of 

 tissue, and recognize the primary xylem in its original position : 

 the further arrangement of the secondary tissues is as in the 

 Scarlet-Runner, but note these points of difference in the root 

 as a whole : 



1. There is no pith. 



2. The primary medullary rays opposite the groups of primary 

 xylem are relatively small and inconspicuous. 



3. The xylem is marked off into annual rings, as in the 

 secondary xylem of the stem. 



It is further to be observed that the cortex has completely 

 disappeared, and the peripheral layer of cork is the product of 

 the pericycle. Numerous sclerenchyma-fibres are to be found 

 scattered through the secondary phloem. 



The diagrammatic Fig. 10 will illustrate the mutual arrange- 

 ment of the primary and secondary tissues of the root. 



