Humphreys : An analogy 573 



The opening bud in early spring forecasts the subsequent ar- 

 rangement of the leaves on the stems. The outer ones are entire, 

 while the inner ones are lobed. The upper zone of leaves cannot,' 

 however, be seen, as the buds are not yet sufficiently opened! 

 but as the bud expands and the new branch grows the entire' 

 leaves remain at intervals along the lower part of the branch, 

 while the lobed ones are carried higher up, and finally the second 

 series of entire leaves unfolds at the top. As, therefore, in the 

 cnnoid arm, the simplest forms of structure are found at the base 

 and tip, with the more complex ones between them. 



It seems probable that this arrangement of the leaves in Sassa- 

 fras has, like the arrangement of the plates in the biserial crinoid 

 arm, some phylogenetic significance. In regard to the crinoids it 

 is believed that the grouping of the plates epitomizes the life-his- 

 tory or development of the organisms to which they belong, and 

 that this history, having its climax in the central part of the arm, 

 may be read either upward from the base or downward from the 

 tip. It is inferred, therefore, that the earliest crinoids of any 

 genetic series had simple uniserial arms with quadrangular plates, 

 and that gradually the plates became biserial in arrangement and 

 more complex in form. There are also other facts which indicate 

 that this is the correct interpretation. 



In like manner the leaf arrangement described seems to indi- 

 cate that it also summarizes the development of the Sassafras 

 branch and its leaves, and that this story may be similarly read, 

 either upward or downward, although the change from entire to 

 lobed leaves is more abrupt and less gradual than is the transition 

 from quadrangular to pentagonal plates in the crinoid arm. If this 

 analogy holds true, it leads to the inference, therefore, that the 

 ancestral type of Sassafras had entire leaves and that these are 



the primitive leaf forms, while the lobed ones are a later develop- 

 ment. 



Since writing the above my attention has been called to R. T. 

 Jackson's observations on Sassafras, in his " Localized Stages in 

 Development in Plants and Animals." * The inference above ex- . 

 pressed, however, in regard to the ancestral type of Sassafras leaf, 



*Mem. Boston Soc. Nat. Hist. 5 i 89-153. //. 16-23. 1899. 



