THE ANGIOSPERMAE 



1589 





While it is interesting to compare the monocotyledonous embryo with 

 the dicotyledonous and to try to find common terms between them, we 

 should do so with Goebel's remark in mind, that the origin of mono- 

 cotyledony is by no means the same thing as the origin of Monocotyledons. 



To begin with it has been very generally accepted in recent years that 

 the Monocotyledons are a derivative class, though the older morphologists 

 thought otherwise. They appear rather later than the Dicotyledons in the 

 geological record and some structural characters, like the absence of 

 secondary thickening, the adventitious root system, and the prevalence of 

 the geophilous habit, seem to 

 indicate secondary specializa- 

 tion. Their characteristics 

 serve to isolate them from the 

 Gymnosperms, with which, on 

 the other hand, the Dicotyle- 

 dons have much in common. 

 Acceptable evidence, no doubt, 

 but not conclusive. If we re- 

 gard the Monocotyledons as a 

 single, monophyletic class, we 

 may still think of them either 

 as evolved parallel with the 

 Dicotyledons or else as an off- 

 shoot of the latter. We may 

 also, on the contrary, prefer to 

 regard them, with Lotsy, as 

 comprising two groups, 

 evolved independently; one, 

 including Araceae and Palm- 

 aceae, arising from the Piper- 

 ales, and the other, including 

 Helobiae and Liliaceae, arising 

 from the Ranales. 



If the Monocotyledons have 

 in fact arisen from Dicotyle- 

 don ancestors then, so far as 



the seedling character is con- p,^. 1^49.— Heterocotyly as the origin of the mono- 



rprnpri there are at least two cotvledonary condition. Hill's interpretation. 



cerned, tnere are ar leasi IWO ^ ^^^ ^^ Peperomia pellucida. 3 and 4, P. peruviana. 



possibilities. P-.lther the smgle . a^j 5 p parvifoUa. One cotyledon has been 



cotyledon represents a fusion reduced to a haustorium. 7, Arisaema dracon- 



" p ^.^^^^^ Araceae. Germination for comparison 



of two, as Miss Sargent be- ^^jth the above. 8, 9 and 10, Germination of 



lieved or else the single typical monocotyledons. Cj and C2 are the two 



', . , . .- cotyledons, 



cotyledon is the survivor ot an 



original pair, the other member having either disappeared or been altered 

 out of recognition. The latter view was maintained by A. W. Hill on the 

 basis of a comparison of seedlings in Peperomia (Fig. 1449). His argument 



