APIUM GRAVEOLENS 465 



libriform or connective type of cell also surrounds the inner face 

 of the xylem of each bundle to some extent. The portion of the 

 ray adjacent to the phloem does not mature as connective tissue, 

 but the outer face of the phloem is capped by cells which tend to 

 become thick-walled. 



The xylem region consists of numerous vessels separated from 

 one another by parenchymatous cells. The first-formed proto- 

 xylem elements are of small caliber with open spiral thickenings, 

 while those that mature later are larger with much closer spirals. 

 A rather striking feature of the spiral wall thickenings is that in 

 many cases the bands are double. The vessel segments of the 

 metaxylem have very close spirals or a reticulum. 



One or more ducts are developed in each primary phloem group, 

 and there are three in most of the larger bundles that subtend the 

 angles of the stem. Two or three occur in bundles of intermediate 

 size and there is a single duct in the small bundles. The smaller 

 secretory canals located in the peripheral portion of the pith are 

 bounded by five or six epithelial cells. The fascicular cambium 

 is relatively inactive, producing a limited amount of secondary 

 xylem and phloem; but at the nodes, there is more activity and 

 the nodal plate may become considerably lignified owing to the 

 development of secondary xylem consisting of pitted vessels, 

 fibers and parenchyma. 



Medullary Bundles. — Medullary bundles are distributed in 

 the peripheral portion of the pith, and in a given stem, may be 

 found in several stages of development. In most cases, the phloem 

 is well developed, while the extent of xylem formation is variable. 

 The bundle may be completely amphivasal, half-amphivasal, or 

 collateral, and types are found in which the bundle appears to be 

 bicollateral with xylem on two sides of the intervening phloem. 

 The more completely developed bundles, regardless of type, are 

 surrounded by sheaths of wood parenchyma. (Fig. 138.) 



In most instances, an oil duct is centrally located in the medullary 

 bundle and the phloem is arranged in concentric layers around it. 

 Outside the phloem, the xylem is variously distributed or may be 

 entirely lacking. Preliminary investigations indicate that the 

 ontogeny of the medullary bundle is related to the secretory canals, 

 and it appears that they are initiated by an activation of cells 

 adjacent to the epithelial cells which line the duct. (Fig. 1^,^, A.^ 

 These cells form a meristematic strand by a series of periclinal 



