ALLIUM CEPA 199 



smaller ones may end at the base of the blade without joining other 

 veins. In the outer sheaths, the bundles frequently appear green 

 because of a layer of chlorenchyma which partially surrounds them. 



In the mature leaf, the orifice leading to its basal cavity is oval 

 in outline, and appears to be laterally placed owing to the greater 

 elongation of the sheath on the side bearing the blade. At the 

 base of the sheath, the distribution of the vascular bundles is fairly 

 uniform; but near its top, they become more closely aggregated 

 on the side of the sheath from which the blade extends. On the 

 opposite side, there are one or two smaller bundles which, after 

 branching several times and producing cross-connections, end 

 blindly under or around the margin of the sheath. On each side 

 of these smaller bundles is a larger one which parallels them up to 

 the orifice and then curves around its edge. (Fig. 90, /.) 



Lactiferous Cells. — The cotyledon and all other leaves develop 

 longitudinal rows of lactiferous cells. They first appear early in 

 the ontogeny of the plant, and may be found in a longisection of the 

 cotyledon about 14 days after germination. In the leaf base, they 

 are almost invariably separated from the epidermis by two layers 

 of parenchyma, rarely by only one; but in the green tubular portion 

 of the blade, they may be more deeply located, lying just within 

 the chlorophyllose palisade tissue. (Fig. jl, D.) Hanstein (5) 

 described these structures in various species of Allium and called 

 them "vesicular-vessels." Later, Rendle (14) pointed out that 

 the term "vessel" is not appropriate in this case, since the latex- 

 containing cells are "not cell-fusions, and continuity between the 

 contents of adjacent members can very rarely be seen." He sug- 

 gested the term "laticiferous cells" for these structures, which has 

 been shortened to "lactiferous cells" by Hoffman. 



Each row of lactiferous cells is parallel to the epidermis; and 

 in their early stages of development, the transverse septa are not 

 pitted; but become so as the leaf enlarges and the cells elongate. 

 (Fig. 9i, A, D, £.) In the succulent portion of the leaf, these cells 

 do not elongate much, and they are extremely short at the base, 

 where the longitudinal series may be irregularly connected by cross 

 unions. (Fig. 9X, F.) Both Hanstein (5) and Rendle (14) state 

 that row^s of lactiferous cells may occur side by side; and in such 

 cases, the longitudinal wall separating them is also pitted; but 

 where parenchyma abuts these cells, the dividing walls are 

 unpitted. (Fig. 9i, 5.) 



