LACTUCA SATIVA 



645 



m ph 



Ixd 



The Stem. — In the early stages of its development, the stem 

 does not undergo internodal elongation, and the leaves, which are 

 diverged in a spiral phyllotaxy (commonly a % arrangement), are 

 crowded on the axis. As a result, either a head or a close rosette 

 of basal leaves surrounds the crown stem, which becomes large 

 and fleshy, owing to the pronounced increase in the parenchym- 

 atous tissue of the pith. 

 Later in ontogeny, the 

 seed stalk elongates and 

 the buds in the axils of the 

 upper leaves also expand 

 so that a branching crown 

 is formed which bears the 

 inflorescences. 



The young stem is round 

 or oval and somewhat 

 ridged below each node 

 owing to the downwardly 

 extending bundles of the 



. (. rpi -1 • Fig. 339. Diagrammatic transection of partly 



leai trace. ine epiaermiS mature floral axis showing arrangement of bundles, 



is glabrous except for medullary phloem and distribution of lactiferous 



nrrasional sratrered hairs ducts, variety New York Special : f^, cambium; CO, 

 occasional SCatterea nairs ^^^^^^. ,„^ gndodermis ; ./;, epidermis ; /x^, lactif- 



and contains numerous StO- erousduct; w ^^, medullary phloem ; /?c/, pericycle ; 



mata in which the guard ^^^-phioem; xj, xylem. 

 cells are usually oriented with their long axes parallel with that 

 of the stem. The epidermal cells are irregular in outline, having 

 their long dimension in the vertical plane, and the walls are 

 curving as seen in surface view. In transection, they are square 

 or rectangular and their outer surfaces are moderately cutinized. 

 Within the epidermal layer are two or more rows of compact 

 collenchymatous cells. The large parenchymatous cells of the 

 cortex are loosely organized and are limited centripetally by the 

 endodermis. This layer is differentiated from the other cortical 

 tissues by the continuity of its cells, whose radial walls abut 

 without intercellular spaces. (Fig. 338.) 



The vascular tissue of the immature stem constitutes a dissected 

 siphonostele, but the collateral bundles are soon connected, 

 through the development of interfascicular cambium, and a con- 

 tinuous vascular cylinder is formed. The pericycle becomes 

 multiseriate, and numerous lactiferous ducts develop in its inner 



