founded on the Structure of their Seedlings . 63 
The homology of the single seed-leaf of Ra 7 iunculus Ficaria 
(PI. VII, Fig. 4) has been much discussed. The reasons 
given by M. Sterckx ( 38 , p. 43) for. considering it a fusion 
of two cotyledons seem very strong. 
He points out that the venation of the bifid blade suggests 
its double origin. I have repeated his observations (Fig. 5) 
and agree in this conclusion. The blade is folded in the seed, 
and retains a well-marked median crease which is easily 
mistaken for the midrib in fresh material. But when the 
blade is blanched by immersion in methylated spirit, and has 
then been made transparent by treatment with phenol, the 
course of the veins can be accurately followed. Two main 
veins traverse the two segments of the blade respectively. 
Branches from both of these run upwards near the median 
crease, and when such branches unite with each other they 
sometimes appear to form a true midrib (B 15 Fig. 5). In 
other specimens its absence is clear (A 3 and A 4 , Fig. 5). 
The formation and maturation of the embryo within the 
seed has been followed and described with great care by 
M. Sterckx ( 38 , p. 42, and Figs. 151-9). The embryo in the 
ripe seed is very small. It is spherical, quite undifferentiated, 
and attached to a short suspensor. The cotyledonary member 
is lateral throughout its development. It is distinctly bi- 
lobed by the end of the summer in which the seed is sown. 
Throughout the following summer the development of the 
embryo continues within the seed. It germinates in the 
second spring, nearly two years after the seed was ripened. 
The ‘cotyledon’ comes above ground at once on germina- 
tion, and acts as the first assimilating organ. Its petiole is 
in a straight line with the hypocotyl and primary root. 
The lower limit of the hypocotyl is rather sharply defined 
externally by the sudden decrease in diameter of the axis 
where the primary root begins, but the upper limit cannot 
be determined until the position of the plumule is ascertained. 
It first appears as a slight swelling at the base of the cotyledon. 
Sometimes the first cauline root, always formed immediately 
below the plumule, shows first as a little tooth pointing 
