founded on the Structure of their Seedlings. 67 
by the union of two seed-leaves, which were distinct in a 
remote ancestor, and perhaps partially united in a more 
recent one. If this conclusion is justified, the vascular system 
of such ancestors would certainly be bilaterally symmetrical, 
and might probably resemble that of Eranthis. But in the 
seedling of R. Ficaria the vascular system of the cotyledon 
is unilateral in a very marked degree ; quite as one-sided 
as that of Zygadenus , which indeed it closely resembles. We 
need not be startled then by the presence of a midrib in 
the cotyledon of Zygadenus , nor by its lateral position with 
regard to the axis, since both characters are found in the 
seedling of Ranunculus Ficaria , together with independent 
evidence of the double origin of its cotyledon. 
The observations condensed in the first two Parts of this 
paper have led me to the conclusion that the single seed-leaf 
of the Liliaceae and allied orders is a compound member 
formed from the two seed-leaves of a remote ancestor. If 
this be admitted, the probability is that the seed-leaf of 
all Monocotyledons has a similar origin, and my observations 
on the Palms and Scitamineae confirm this view so far as 
they go. 
In the third and last Part of this paper I propose to discuss 
the whole theory of the origin of Monocotyledons which 
naturally arises from the view I have just expressed concern- 
ing the origin of their seed-leaf. 
This discussion will raise three questions, which can be 
treated separately : — 
1. The comparative antiquity of the Monocotyledons and 
Dicotyledons. 
2. Assuming the superior antiquity of Dicotyledons, can 
the single seed-leaf of Monocotyledons have arisen otherwise 
than by the fusion of two cotyledons into one member ? 
3. Assuming the double origin of the seed-leaf in Mono- 
cotyledons, can we form any hypothesis as to the way in 
which the fusion first began, and concerning the correlation 
of this character with the others which distinguish Mono- 
cotyledons ? 
