640 THE STRUCTURE OF ECONOMIC PLANTS 



The secondary phloem is comprised of groups of long, narrow 

 sieve tubes and companion cells surrounded by larger parenchyma- 

 tous cells. There are numerous latex tubes which consist of very 

 slender cells arranged in longitudinal series and forming a con- 

 tinuous system owing to numerous cross anastomoses. (Fig. 334, 

 C) The latex ducts are relatively infrequent in the pericyclic 

 tissue which surrounds the secondary phloem and forms the outer- 

 most zone of the root, and none occur in the secondary xylem. 

 Latex branches may ramify through the pericyclic tissue inter- 

 connecting the latex tubes of adjacent phloem strands, and they 

 also penetrate the pericycle together with the vascular tissue at 

 points where lateral roots are diverged. 



There is an early loss of the cortex and epidermis as a result of 

 secondary thickening, and the outermost cells of the pericycle 

 function as a phellogen forming the periderm. The primary 

 pericyclic cells are rectangular in longisection, and may become 

 much extended tangentially as cellular growth compensates for 

 the increasing size of the axis. 



Vascular Transition. — Among the studies of vascular transi- 

 tion in the Compositae are those of Chauveaud (6), Lee (17), who 

 investigated fifty species including Lactuca sagittata Waldst., 

 Siler (i4) on Arctium minus Bernh., and Phillips (xi), who 

 studied Cynara Scolymus L. 



In Lactuca sativa L., Port (ii) finds that the vascular transition 

 agrees in general plan with that reported by Lee (17) for L. sagit- 

 tata. The primary vascular system of the seedling axis, including 

 the primary root, hypocotyl and cotyledons, is a continuous one 

 which is distinct in its origin from that of the stem; but late in 

 ontogeny the downwardly extended traces of the epicotyledonary 

 leaves anastomose with the vascular strands of the hypocotyl. 

 The vascular transition involves a rather short portion of the 

 seedling axis. It begins in the upper third of the hypocotyl, and 

 the complete endarch condition is attained about halfway up the 

 midrib of the cotyledon. 



The cotyledon has three main veins, a midrib and two small 

 laterals. (Fig. 336, H.) In the distal portion of the cotyledonary 

 blade, the bundles are endarch collateral, and the lateral ones 

 remain so even to a point below the cotyledonary node. In the 

 proximal portion of the cotyledon, the metaxylem of the midrib 

 is differentiated laterally in relation to the protoxylem, forming a 



