ix THE HISTOLOGY OF THE SHOOT AND ROOT tti 



The Structure of the Leaf. Each leaf consists of the 

 three tissue systems, but by far the largest portion is ground- 

 tissue. The whole of the leaf is covered by the epidermis, and 

 between the upper and lower epidermis comes the Mesophyll. 

 Fig. 138. The mesophyll is built up of palisade cells above, 

 and spongy parenchyma below. Between the spongy paren- 

 chyma and the palisade tissue the vascular bundles run; these 

 bring sap from the root to the cells of the leaf, and carry away 

 the elaborated sap. The palisade tissue consists of regular, 

 fairly elongated cells, which contain a very large number of 

 chloroplasts, and only a few intercellular spaces. The spongy 



H ,US ,UE 



LS/ 



FIG. 137. Transverse section of young 

 root of Wallflower. {After Scott.) 

 P, phloem ; X, xylem ; C, cortex ; P 1 , 

 piliferous layer ; R, root hair. 



FIG. 



38. Transverse section of leaf of 

 Rhododendron. (X 250.) US, upper 

 side of leaf ; LS, lower side of leaf ; 

 P, palisade parenchyma ; S, spongy 

 parenchyma ; VB, vascular bundle ; 

 E, epidermis ; H, hypoderm. 



parenchyma forms a loose tissue full of intercellular spaces. 

 The cells of this tissue are not so well supplied with chloroplasts. 

 The intercellular spaces of the leaf communicate with the 

 stomata, so that any gas which may enter the stomata finds its 

 way into the deeper parts of the leaf. The outside of the epi- 

 dermis of the leaf is always of the nature of cuticle. 



EXPT. 105. Place a piece of the leaf of the Wallflower between 

 little slabs of carrot, and with a sharp razor cut slices right across. 

 Separate the transverse sections of the leaf so obtained in water in a 



