EVOLUTIONARY PRINCIPLES OF THE COMPOSITAE 435 



magnified. In the cortex, lying outside the series of bundles, may 

 be seen two or three circular spaces surrounded by secretory epithe- 

 lium. These are the oil canals of the tubuliflorous section of the 

 Compositae. Smaller canals of the same type can less readily 

 be distinguished in the medullary region. Such oil canals are 

 found quite generally in the Tubuli- 

 florae and are rarely absent in the root, 

 even when they become abortive in 

 other regions. Fig. 296 illustrates the 

 occurrence of similar canals in the root 

 of Aster. In this genus the canals are 



FIG. 297. Transverse section of a small root of the FIG. 298. Laticiferous system of 



dandelion. Scorzonera (after Sachs). 



not infrequently absent in the stem and leaf, but appear never to 

 be lacking in the root. 



Fig. 297 illustrates the organization of a small root of the 

 common dandelion (Taraxacum}. The phloem of the fibrovascular 

 system is much better developed than the xylem, and in it occur 

 certain dark concentric bands. These indicate the position of 

 the laticiferous elements which elaborate the milky juice, so 

 prominent a feature of the Liguliflorae, to which the dandelion 



