1904 | SARGANT: EVOLUTION OF MONOCOTYLEDONS 327 
opinion has lost ground. The evidence which supports it has 
been criticised, and more than one observer has brought forward 
facts in favor of the claim of dicotyledons to be considered the 
elder branch. 
> 
EVIDENCE FOR THE PRIMITIVE MONOCOTYLEDON. 
The case for monocotyledons rests on evidence drawn from 
three distinct lines of research: (1) the direct evidence of fossil 
botany as to the geological succession of forms; (2) comparison 
of the stem anatomy in the two classes ; (3) the study of the 
developing embryo within the embryo sac. 
1. Direct historical evidence from the succession of fossil 
forms would of course be more conclusive than any based on 
comparative morphology. But unfortunately the geological 
record is particularly imperfect at the epoch which separates the 
~ gymnospermous flora of the Mesozoic age from the earliest fos- 
sil floras in which angiosperms can be recognized with certainty. 
2. The absence of a normal thickening ring in the stem of 
monocotyledons was formerly considered a primitive character. 
This was a very natural inference at a time when a normal cam- 
bium was unknown except among gymnosperms and dicotyledons. 
It led to the now discarded classification which united gymno- 
sperms with dicotyledons, of which traces may still be found in 
Systematic text-books. 
- But now that the anatomy of many fossil cryptogams is as 
well known as that of any living forms, we realize that cambial 
thickening was a commonplace in vegetable anatomy long before 
the advent of angiosperms. Some of the ancestors common to 
ledons must be regarded as the loss of a primitive character. 
_ 3. The argument from the history of the embryo within the 
Mbryo sac is that which has perhaps had the most weight with 
ot nists, The facts are shortly these: 
In both classes the cotyledons are commonly the first mem- 
3SEWARD, A. C., Presidential address to Section K of the British Association. 
