470 THE STRUCTURE OF ECONOMIC PLANTS 



surface view; but, in general, they are quadrangular and their 

 greatest dimension is parallel to the long axis of the petiole. 

 Stomata occur on both surfaces, being more numerous on the 

 abaxial one where they are arranged in approximate rows in the 

 furrows. The stoma is of the type in which the initial epidermal 



mech 



y-xy 2 



-xy 1 



Fig. 140. Transection of sector of petiole showing vascular bundle and location of mechan- 

 ical tissue underlying abaxial epidermis: ca, cambium; col, collenchyma; cu, cuticle; ep, 

 epidermis; mech, mechanical tissue; gl, oil gland; par, parenchyma; ph, phloem; xy i, 

 primary xylem; xy t, secondary xylem. 



cell is not the mother cell. In its development, the initial cell is 

 divided successively so that two or three accessory cells are formed 

 surrounding the mother cell. A final division of the mother cell 

 results in the two guard cells. (Fig. 6.) 



Ontogeny of the Leaf. — The ontogeny of the leaf is similar 

 to that described by Sachs (19) for Pastinaca and other Umbel- 

 liferae. It first appears as a cone-shaped primordium which 



