650 THE STRUCTURE OF ECONOMIC PLANTS 



cycle; and they may also occur on the inner face of the primary 

 phloem. Both types may develop in young seedlings before the 

 protoxylem is differentiated. In the secondary tissues of the 

 root, the lactiferous ducts appear in the phloem parenchyma, and 

 branches frequently extend through the pericyclic parenchyma 

 adjacent to the phloem. (Fig. 334, C) 



In the hypocotyl, ducts appear adjacent to the primary phloem 

 as in the root, and may occur also in the hypodermal region of 

 the cortex. In the cotyledons, they lie along the abaxial surface 

 of the phloem and extend into the mesophyll. In the stem, ducts 

 are present in the pericyclic arcs outside the outer phloem, and, 

 to a lesser degree, in the outer phloem. They are also found on 

 the centrad surface of the medullary phloem strands and are 

 reported by de Bary as ramifying into these strands. In the leaf, 

 the latex ducts occupy the same position with respect to the 

 phloem as in the cotyledon, and branches extend into the meso- 

 phyll. In the inflorescence, de Bary has noted that the latex tubes 

 accompany the bundles of the floral traces. 



Solereder reports that lactiferous hairs have been observed on 

 involucral and other bracts in the genus Lactuca, and that there 

 is an excretion of latex when the involucral leaves are touched. 

 The lactiferous hairs together with two or more basal epidermal 

 cells become united with the free ends of branches of the lactiferous 

 system which accompany the vascular bundles. This is accom- 

 plished by the absorption of portions of the intervening walls 

 so that the hairs actually represent terminations of the lactiferous 

 system. 



Oil Ducts. — Oil ducts have been reported by van Tieghem (i8) 

 for many species in the series Tubuliflorae, but they are usually 

 lacking in the Liguliflorae, which includes the tribe Cichorieae 

 and the genus Lactuca. Exceptions have been reported in the 

 roots of Scolymus by van Tieghem, and in Cynara by Phillips (xi), 

 where oil ducts are located between the two layers of a double 

 endodermis. These two forms are apparently exceptional cases 

 in which true oil passages as well as lactiferous ducts occur. Rudi- 

 mentary oil passages in the diarch primary roots of Cichorium 

 intybus and Lampsana communis have been reported by de Bary. 

 Port found a somewhat similar situation in Lactuca sativa where 

 intercellular spaces develop between adjacent layers of cortical 

 cells; but, as in Cichorium, no contents were observed. 



