24 The Nature and Origin of Stipules. 



glossaceae, Marrattiaceae and Osmundaceae and the " ligule " of 

 Selaginella and Isoetes as special developments and as properly 

 placed in a separate category from the appendages bearing these 

 names among the Angiospermse. The Gymnosperma? present noth- 

 ing to represent either stipule or ligule and we have left for our 

 special consideration the ligule, stipule and their homologues as 

 they occur in the vai'ious groups of the Angiospermse only. 



Having thus defined our field, we should have, for the consid- 

 eration of the problem before us, some conception of what sort of 

 plant the earliest Angiosperm was. In the absence of geological 

 evidence this conception must be purely hypothetical and, basing 

 it on a generalization which would admit of the differentiation 

 from it of all the varied forms of the modern group of Angio- 

 sperms, we can see that it must have been a plant of very simple 

 organization indeed. For our present purpose we need not con- 

 cern ourselves with any other organs of this primitive Angio- 

 sperm than the leaves which, from the point of view of the pro- 

 posed generalization, must be conceived of as hardly more than 

 the bare rudiments of leaves, mere sheathing scales at the nodes 

 of the plant, serving slightly, if at all, the function of assimila- 

 tion which was still subserved, as in its ancestors, by the general 

 surface of the plant, but confined chiefly to that of protection. 

 The primitive leaf was probably parallel-veined or approximately 

 so, giving rise in its earlier differentiation to the parallel-veined 

 leaves of the Monocotyledones. The geological evidence indi- 

 cates that these appeared before the Dicotyledones* which must 

 have sprung from them later at one or more unknown points, and 

 netted-veined leaves are of a more recent evolution. Consequently 

 the tendency of aquatic Dicotyledones to revert toward mono- 

 cotyledonous structure is rather a case of atavistic degeneration 

 than an indication of the origin of Monocotyledones from Dico- 

 tyledones in ancient times through the effects of aquatic habit. f 



Now, as advance in evolution proceeded, the need of greater as- 

 similative capacity arose and, as the foliar organ was the one best 



* Professor L. F.Ward. Sketch of Paleobotany. Fifth Ann. Eep. U. S. 

 Geol. Surv. 448, 1885. Professor A. C. Seward, on the contrary, does not be- 

 lieve that we have satisfactory evidence of pre-Cretaceous Monocotyledones. 

 Notes on the Geologic History of Monocotyledones. Annals of Botany, 10 : 

 220. 1896. 



t See Kev. George Henslow. Jour. Linn. Soc. Lond. 29: 485-528. 1893. 



