EVOLUTIONARY PRINCIPLES OF THE COMPOSITAE 439 



a higher degree of magnification. External to the fibrovascular 

 tissues, on the upper side, is a row of somewhat imperfect and 

 vestigial oil canals. The interesting fact in the present connection 

 is that the oil canals appear in relation to the leaf trace, although 

 absent in the ordinary bundles of the stem. Sections lower down 

 toward the hypocotyledonary region show the secretory canals 

 somewhat better developed. In no case do they occur in bundles 

 which belong distinctly to the stem, although they may run a 

 short distance down on the foliar traces as they pass into the 

 cylinder of the axis. In the upper regions of the stem a different 

 situation presents itself. Fig. 302 shows a section through the 

 aerial stem of the burdock in transverse section. Here neither 

 the numerous bundles of the axis nor any of the three foliar strands 

 show the presence of oil canals, although small dark dots in the 

 region of the phloem sufficiently vouch for the presence of laticifer- 

 ous elements. Fig. 303 reproduces a magnified view of one of 

 the leaf traces, demonstrating the entire absence of oil canals 

 in the foliar strands given off from the upper part of the axis. 

 If the branches are followed to their tips, where the flowering 

 burs are produced, anatomical investigation reveals the continued 

 absence of oil canals. 



Similar observations may readily be made on the Scotch thistle, 

 Onopordon, or on the Canada thistle, Cirsium, as well as on a 

 number of other representatives of the Cynareae. The burdock 

 is on the whole much more favorable than any of the common 

 thistles and has accordingly been chosen for illustration. It will 

 serve a useful purpose to return now to Cynara Scolymus, the 

 French artichoke. As is well known, the vegetable so designated 

 is a variety of the common Mediterranean thistle in which 

 the scales of the involucre surrounding the head have become 

 hypertrophied at the expense of the floral organs proper. This 

 overgrown involucral covering is eaten as a prized vegetable in 

 Southern Europe. Fig. 304 shows a section through the lower 

 region of the flowering axis of Cynara Scolymus. The bundles on 

 the lower side of the figure either belong to the axis or have very 

 recently taken their departure from the region of the pith. Those 

 j-oward the upper side are passing into the basal portion of the 



