THE ANGIOSPERMAE : LEAVES 1027 



as sieve-tube mother cells which have not divided into distinct sieve-tube 

 and companion cell units. The vein sheath extends like a cap over the end 

 of each vein and is thus continuous. 



= ^TRACHEID 



PHLOEM CELL 



SECRETORY. 

 CANAL 



Fig. 1017. — Bundle endings in the mesophyll. A, Fuchsia •^lohosa. Bundle ending in a 

 leaf tooth. B, Ficus elastica. Bundle ending of two rows of tracheids and one row 

 of phloem parenchyma. C, Aralia sieholdii. Transverse section through a bundle end. 

 {After Haherlandt.) 



The vascular tissues of the veins are characteristically arranged with the 

 xylem uppermost. This is a necessar}^ consequence of the position of these 

 tissues in the stem bundles, for as a leaf trace bundle turns outwards into 

 the petiole, its xylem becomes uppermost and this position is usually main- 

 tained, without torsion, through the petiole into the lamina. 



Many accessory structures may occur in the mesophyll. Crystal sacs, 

 containing Calcium oxalate crystals are common, while cystoliths in specially 

 enlarged cells are characteristic of the Urticaceae. Glandular sacs and canals 

 occur in certain families and contain either mucilage or aromatic terpenes. 

 Where the sacs are large they form colourless points in the tissue, as may be 



