APIUM GRAVEOLENS 



481 



of ioo per square millimeter on the under surface and approximately 

 100 per square millimeter on the upper one. 



The outer walls of the epidermal cells are slightly thickened 

 where the epidermis overlies the chlorophyll parenchyma, and very 

 much thickened over the veins and at the leaf margins. The 

 mesophyll consists of a single adaxial layer of palisade cells and 

 four to six layers of spongy parenchyma which are rather compactly 

 arranged. (Fig. 1.^6, A.^ The principal veins form projecting 



Fig. 147. Floral development : A, transection of young tlovver showing arrangement of 

 parts ; B, transection of young ovary with two carpels and ovules ; C and D, early stages in 

 formation of umbellet showing centripetal development of flowers ; E-J, successive stages 

 in ontogeny of irtdividual flower (in /, filaments of stamens are not in plane of section) : bu, 

 vascular bundle; car, carpel; inv, rudimentary involucel of umbellet; o d, oil duct; ov, 

 ovule; /)«^, petal ; j-^^, stamen. 



ridges on both surfaces of the blade owing to the strands of col- 

 lenchyma which lie immediately beneath the epidermis. The 

 abaxial and adaxial strands are separated from the bundle by several 

 rows of parenchymatous cells, and an oil duct lies above and below 

 the vein. (Fig. 146, B.) 



The vascular bundle resembles that of the petiole and is oriented 

 with the xylem directed toward the adaxial surface. In the 

 larger veins, the phloem cap is well developed and there is also 



