CHAPTER XXXI 

 EVOLUTIONARY PRINCIPLES EXHIBITED BY THE COMPOSITAE 



In earlier pages the principles derived from the comparative 

 anatomical study of existing and extinct plants, particularly 

 vascular cryptogams and gymnosperms, have been emphasized. 

 In the present chapter it will be shown that these principles are 

 as applicable to forms the past of which is unknown as to those 

 historically recorded. It will be advantageous in this connection 

 to consider a very high group among the dicotyledons, namely, 

 the Compositae. This family is commonly divided into two sec- 

 tions, the Tubuliflorae and the Liguliflorae. The former are 

 characterized by the fact that the axial florets of the heads are 

 tubular and not ligulate in their organization. Anatomically 

 they are distinguished by the presence of oil canals, which usually 

 occur in the more conservative organs even when they are lacking 

 elsewhere. In the Liguliflorae, on the other hand, the axial flowers 

 of the inflorescence are invariably ligulate. Histologically this 

 group is characterized by a milky juice which is present in the 

 pericycle and the phloem. Indeed, it often happens that the 

 laticiferous ducts are actual sieve tubes or are at least continuous 

 with elements of this nature. It is an interesting fact that in 

 the higher representatives of the Compositae, marked by the 

 presence of a milky or laticiferous secretory system, frequently 

 oil canals such as occur normally in the Tubuliflorae are present 

 under the same conditions as the ancestral features surviving in 

 vascular cryptogams and gymnosperms. This situation is of 

 great general interest as illustrating the wide validity of the general 

 principles elucidated in chapter xvii. Since the Compositae are 

 readily obtainable by reason of their great abundance in the 

 present flora, either in a wild or in a cultivated state, they may 

 serve advantageously to illustrate pedagogically the fundamental 

 conceptions of comparative anatomy. 



In Fig. 295 is shown a transverse section of part of the 

 stem of the Jerusalem artichoke (Helianthus tuberosus) moderately 



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