LACTUCA SATIVA 649 



immediately abutting the vein may have a few plastids. There 

 may also be small plastids in the xylem parenchyma of the larger 

 veins. 



The main bundles are collateral, but there are strands of adaxial 

 phloem which follow the course of the bundles and extend into 

 the medullary region of the stem. Lactiferous ducts occur around 

 the periphery of the abaxial phloem and there may be similar 

 ducts lying in the same relation to the adaxial phloem. The 

 primary xylem elements are chiefly of the spiral type. The 

 larger bundles develop a cambium and the secondary xylem vessels 

 are similar to those of the stem. The smaller veins do not have 

 strands of adaxial phloem associated with them and the ultimate 

 veinlets consist of single tracheids. 



Lactiferous Ducts. — The presence of lactiferous ducts has 

 been noted in descriptions of the various organs of the plant, but 

 they are sufficiently characteristic to warrant a summary of their 

 occurrence and distribution. The development of articulated lac- 

 tiferous ducts in the Cichorieae has long been known and fre- 

 quently investigated. The early research on this point has been 

 summarized by de Bary (i) and Scott (2.3), and the latter has 

 described the origin and ontogeny of the ducts for a member of 

 this tribe. When the ducts occur in primary tissues, they arise 

 early in the ontogeny of the plant and are probably differentiated 

 during the embryogeny in many instances. Scott observed their 

 formation in the seedling during the initial stages in germination. 

 The ducts at first consist of a longitudinal series of cells which 

 have definite end walls. Later in ontogeny, they become con- 

 tinuous, non-septate passages owing to the perforation and resorp- 

 tion of the end walls, as well as to the occurrence of frequent cross 

 anastomoses between adjacent ducts which form connecting canals. 

 Solereder (2.'^') has pointed out that the 



"tubes of the stem and leaf form a special system of their own, which 

 is distinct from the lactiferous system of the root," but notes that "the 

 primary lactiferous system of the root is however connected with the 

 primary system of the stem, for at a certain level both systems are 

 found to be present in a transverse section and united with one another 

 by anastomoses." 



Summing up the situation for Lactuca, it has been noted that 

 the lactiferous ducts occur in the primary tissues of the root, 

 where they form an arc outside the primary phloem in the peri- 



