APIUM GRAVEOLENS 469 



ing to the point of divergence of the lower leaflets. The central 

 axis of the compound lamina and the stalks of the lateral leaflets 

 are also subterete with a grooved adaxial surface. The abaxial 

 surface is prominently ridged, owing to the underlying strands of 

 collenchyma, while the adaxial one is smooth and without re- 

 inforcing strands of collenchyma although there are usually two 

 or more layers of thick-walled subepidermal parenchyma. The 

 winged margins of the petiole are also strengthened with mechani- 

 cal tissue. 



The mesophyll is parenchymatous with large intercellular 

 spaces. In plants that tend to be pithy, the parenchyma breaks 

 down schizogenously at various points, leaving irregular internal 

 cavities. (Fig. 2.39.) The number and distribution of the chloro- 

 plasts depend upon the variety and cultural methods practiced 

 with respect to blanching. In most types, there is a zone of 

 chlorenchyma immediately within the epidermis, except at the 

 ridges and on the adaxial surface, and it also frequently surrounds 

 the vascular bundles. 



The principal vascular bundles lie in a semicircle immediately 

 inside the coUenchymatous strands with their phloem caps directed 

 toward the abaxial surface. Between the major bundles, there 

 may be smaller ones, their number and orientation depending upon 

 the variety of celery and the level at which the transection is 

 taken. A row of small adaxial bundles may occur, these being 

 more evident at the base of the petiole than at its upper limits. 

 (Fig. 139.) At intermediate levels, the smaller bundles occupy a 

 more nearly central position in the mesophyll; and, above, they 

 come to lie approximately in the abaxial ring. As reported by 

 Esau (7), they may consist of phloem only; in some instances 

 solitary xylem cells have been noted; and, in others, there are 

 bundles in reverse orientation in which the phloem of the adaxial 

 bundles is directed toward the adaxial surface. 



Numerous oil ducts occur in the phloem, prominent ones lying 

 centrad to each coUenchymatous strand and outside the phloem 

 caps of the larger bundles. They are also generally distributed 

 throughout the parenchymatous tissue adaxial to the main vascular 

 ring. (Fig. x4o.) 



The walls of the epidermal cells are thickened, especially the 

 outer one over which there is a well-defined cuticle. There is 

 considerable irregularity in the shape of these cells, as seen in 



