46i THE STRUCTURE OF ECONOMIC PLANTS 



the growing point and the primordia of the first foliage leaves. 

 (Fig. i36.) When the petiolar bases diverge, each is crescentic 

 in transection. In the lamina of the cotyledon, the mesophyll is 

 differentiated into palisade and spongy parenchyma. The palisade 

 cells are not as elongated as in the foliage leaf, but they do form a 

 distinct layer beneath the adaxial surface that is more compact 

 and has fewer intercellular spaces than occur in the five or six 

 layers of spongy tissue. 



Anatomy of the Stem. — The fleshy crown stem of the first 

 year constitutes the so-called "heart" of celery. It has a large 



Fig. 136. A, transection of upper hypocotyl showing divergence of six cotyledonary 

 bundles. Two procambial strands of foliar bundles appear in the intercotyledonary plane; 

 B, transection of seedling axis showing cotyledonary collar surrounding growing point of 

 epicotyl and base of first foliage leaf. The xylem of median cotyledonary bundle is tangen- 

 tially oriented rather than endarch at this level. 



central pith, and oil ducts occur in the peripheral portion. Associ- 

 ated with them are numerous medullary bundles which lie centrad 

 to the vascular ring. The occurrence of medullary bundles in the 

 Umbelliferae is not constant, but has been reported by Solereder 

 (ii) for sixteen genera, including Apium. They do not occur 

 in Pastinaca or Daucus. 



At the periphery of the pith is an almost continuous cylinder of 

 vascular tissue comprised of bundles of the leaves which form 

 the rosette of the first year. The leaf traces constitute a ramifying 

 network of vascular strands that continue through the cortex into 

 the bases of the petioles. Because internodal elongation is very 



