LACTUCA SATIVA 



643 



with a broadened apex which is frequently notched at its extreme 

 tip. Lee (17) has observed that the venation of the cotyledons 

 in the composites is characterized by two facts: (i) the main 

 bundles never end blindly in the mesophyll, but anastomose with 

 one another in the terminal por- 

 tion of the blade; (i) the lateral 

 bundles on either side of the mid- 

 vein always unite with it at the 

 apex of the cotyledon. Lettuce 

 corresponds in this respect with 

 the Arctium type in which there 

 are two lateral veins and a midrib 

 at the base of the cotyledon. 

 The laterals follow the margins 

 of the blade, uniting with the 

 midrib at the apex of the cotyle- 

 don; and, in addition to them, 

 two major branches of the midrib 

 arise near the middle of the coty- 

 ledonary blade, pursue a course 

 parallel to the laterals for some 

 distance, and join them near the 

 tip of the cotyledon. (Fig. 331, 

 P.) All minor branches arising 

 in the basal portion of the blade 

 anastomose with one of the ma- 

 jor veins at some point so that 

 the venation constitutes a closed 



■sz^lx d 



Fig. 337. A, transection of main vein 

 near base of ten-day-old cotyledon showing 

 location of lactiferous ducts ; B, transec- 

 tion through portion of same cotyledon 

 nearer apex of blade, variety New York 

 Regular : ep, epidermis ; hr, hair ; Ix d, 

 lactiferous duct; pal, palisade; ph i, pri- 

 mary phloem; spo, spongy tissue; sto, 

 stoma; xy i, primary xylem. 



system. 



The mesophyll of the fleshy 

 blade consists of a palisade region 

 a single layer in thickness, and a spongy zone of several layers. 

 (Fig. 337, B.) The spongy cells adjacent to the palisade layer 

 are frequently elongated in the same plane as the palisade cells 

 so that they form a transitional type. As the cotyledon expands, 

 the intercellular spaces enlarge and the mesophyll becomes porous. 

 The vascular bundles are collateral, and neither Jean (ii) nor 

 Port (xi) found any evidence of an inner phloem associated with 

 them, such as is present in the stem and leaf. Lactiferous ducts 

 occur on the abaxial face of the phloem and occasionally in the 



