TORREYA 



March, 1914. 

 Vol. 14 No. 3 



SOME FOSSIL LEAVES AND THEIR SIGNIFICANCE 



By Edwin W. Humphreys 



Anything that will shed light, however feeble, upon the life 

 processes of the past in their relation to those of the present is 

 always of interest; hence, when certain abnormal fossil leaves 

 are found which show the same aberrations that living ones of, 

 apparently, the same genus possess, that fact seems to be worthy 

 of record. 



In the American Naturalist for 1907,* there are three articles 

 on the development of pinnate leaves as shown by examples of 

 arrested development in mature leaves of living plants. The 

 occurrence of similar forms among fossil leaves and their sig- 

 nificance is the subject of this paper. 



In Lesquereux's Flora of the Dakota Groupf two specimens 

 of fossil Rhus leaves {Rhus Powelliana Lesq.) are figured, in one 

 of which, reproduced on plate A, fig. i, a, the terminal leaflet 

 has reached a stage of development similar to that shown by 

 the terminal leaflet of the living Rhus glabra L. (pi. A, fig. 1, b). 

 The other (pi. A, fig. 2, a) shows a stage like that of the sumac 

 leaf depicted in pi. A, fig. 2, b. There is, however, a more ad- 

 vanced stage of development portrayed in the leaf represented 

 in pi. A, fig. I, a, for some of the lateral leaflets are lobed; one of 

 them, in fact, showing a distinct leaflet. In this case, the primary 

 leaflets seem to exhibit a tendency to become pinnate, thereby 

 foreshadowing the formation of a bipinnate leaf. Fig. i, c is a 

 drawing of a portion of a leaf of Rhus glabra L. showing a similar 

 stage of development. 



There is another species of fossil Rhus, R. Uddeni Lesq. (pi. A, 



*F. T. Lewis, Am. Nat. 41: 431, 701, 817. 1907. 

 t Mon. U. S. Geol. Survey 17: 155. pi. 56, figs. 4-5. 1892. 

 [No. 2, Vol. 14, of ToRREYA, Comprising pp. 21-38, was issued 9 February, 1914] 



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