IX THE HISTOLOGY OF THE SHOOT AND ROOT 109 



(ii) The cortex, which is several layers of cells in thickness, 

 (iii) The pericycle, which is very strong and forms a thick ring, 

 (iv) The scattered vascular bundles, which are embedded in the ground 

 tissue. 



EXPT. 102. Cut a transverse section of a two-year-old stem of the 

 Wallflower. Mount in water, and examine under a low power. 

 Note 



(i) The periderm, which is formed from the pericycle. 



(ii) The bark, which is outside the phellogen or cork cambium. 



The Structure of a Monocotyledonous Stem. If a 

 transverse section of a young stem of the Maize be made and 

 examined by a low power the 

 following parts will be seen, 

 Fig. 136. On the outside an 

 epidermis which consists of 

 a single layer of cells. Im- 

 mediately inside this, a broad 

 band of thick-walled paren- 

 chyma forms the cortex. The 

 cortex is a mechanical tissue 

 and is the principal support 

 of the plant. The remain- 

 ing part of the stem is made 

 up of scattered vascular 

 bundles and ground tissue. 

 TJiere is r^ rprnhinm anrl 



FIG. 136. A portion of a transverse section 

 of stem of Maize, (x 20.) E, epi- 

 dermis ; C, cortex ; VB, vascular 

 bundles. 



no_rrwth j n thickness ran take place a 



result of tria 



division of the cambium ring. 



Structure of a Dicotyledonous Root. If a transverse 

 section of a very young tap-root of the Wallflower is examined 

 the structure appears very different to that of the stem. On 

 the outside the piliferous layer is formed ; this is another name 

 for the young epidermis of the root. Many of the cells of the 

 piliferous layer are converted into root-hairs, hence the name. 

 A root-hair is unicellular and is produced from a single cell of 

 the epidermis. Inside the epidermis, and limited internally 

 as in the stem by the endodermis, is the cortex. The centre of 

 the root is occupied by a vascular cylinder which consists of two 

 masses of xylem and two masses of phloem. The xylem masses 

 alternate? with the phloem masses. In the stem the protoxylem 



