LACTUCA SATIVA 647 



positae from the standpoint of its phylogenetic implications, 

 confirmed the work of previous investigators with respect to 

 the course of the strands in the stem of Lactuca sativa and L. virosa. 

 His summation follows : 



"Medullary strands occur throughout the stem. In the higher part 

 of this organ phloem-strands only occur. In the basal region they 

 acquire xylem and become bundles. In this basal region, at a still 

 lower level, they lose the xylem, and eventually either die out in situ 

 or join the vascular ring. The amount of xylem in the medullary 

 strands varies with the thickness of the stem, and is always secondary 

 in origin. The fate of the strands, if traced upwards, is fourfold. 

 Some of them end blindly in the pith. Some pass into the vascular 

 ring. Others pass out to form the medullary system of a branch; but 

 in one and the same plant the branch-system may arise both in this 

 way and also de novo in its pith-tissue. The small peripheral phloem- 

 strands persisting in the higher part of the stem and peduncle become 

 part of the ordinary vascular system of the flowers." 



The Leaf. — The gross morphology and the differences which 

 exist between the leaves of lettuce varieties have been pointed out. 

 In the New York variety, the sessile and somewhat auriculate, 

 clasping leaves of the floral axis have a central vascular region 

 which is thickened and concave on its adaxial surface. The 

 principal veins extend through this fleshy region parallel to one 

 another with occasional cross-connections occurring between the 

 lateral members and the central bundle. From the lateral veins of 

 this parallel series, branches extend outward to the periphery 

 of the blade in a pinnate arrangement. These in turn branch and 

 rebranch, forming a fine net-veined system in which some of the 

 ultimate veinlets end in small islands in the leaf blade while others 

 terminate in the marginal teeth. In the basal leaves of the heading 

 and bunching types, the principal veins are spreading and pal- 

 mately arranged ; but even the basal leaves of the Cos type have a 

 prominent midrib and a pinnate system of venation. 



Except in the region of the midrib and of the major laterals, 

 the blade is thin. The epidermal cells of both upper and lower 

 surface are extremely sinuous as seen in surface view, except for 

 those which lie over the principal veins where the cells are elon- 

 gated with oblique or somewhat pointed ends. These regions 

 also differ from the remainder of the epidermal surface in that 

 there are few stomata. Elsewhere they occur in large num^bers 

 and in about equal frequency on both surfaces. As pointed out 



