CHAPTER III. ORGANS OF PLANTS. 



I 9 7 



same leaf. Compare Figs. 456 and 457. The main veins from 

 which branches diverge to form the reticulum may be pinnately, 

 radiately or costately arranged. In netted leaves the veins are 

 usually more prominent on the dorsal surface of the leaf. 



The parenchyma or mesophyll of the leaf is arranged differently 

 in different leaves, two different types of arrangement being dis- 

 tinguished : (1) the bi-facial, where it is compactly arranged 

 against one surface, usually the upper or ventral, and more loosely 

 farther interior and against the other surface ; and (2) the centric. 

 in which the distribution is nearly uniform throughout. 



Fig. 458. — Portion of transverse section of leaf of Cycas revoluta, illustrating structure 

 of bi-facial leaf, a, cuticle of the thick-walled epidermal cells; b, thick-walled cell of the 

 hypoderma; c, palisade cells containing chlorophyll-bodies: d, loose interior parenchyma; 

 e, large inter-cellular space over stoma; ///, stomata. Magnified about no diameters. 



The bi-facial is much the more common. In this there are 

 usually two or more layers of parenchyma, rich in chlorophyll, 

 compactly arranged next the upper epidermis, or next the hypo- 

 derma, if the latter is present. These cells are elongated in a 

 direction perpendicular to the surface of the leaf, and hence the 

 name palisade tissue has been applied to them. The other paren- 

 chyma cells of the leaf are usually very loosely arranged, being 

 separated from each other by large intercellular spaces. They 

 are hence called spongy parenchyma. In leaves of this class, the 



