i66 
Sargant . — The Reconstruction of a Race of 
and thence through a shorter but still very long period to some living 
Dicotyledons. 
Considering the enormously long epochs with which we are dealing, it 
might be argued that the two cotyledons of modern Dicotyledons, and the 
tetrarch skeleton found in many of their seedlings, are not derived from the 
same source as those characters in living Cycads. The ancestral Pterido- 
sperm may not have possessed them, or if it did perhaps they were lost 
during the gradual evolution of Primitive Angiosperms through a series 
of Hemiangiospermous forms. In either case these characters would not 
have been handed down directly from Pteridosperms to modern Dicotyledons 
on the one hand and Cycads on the other. 
Even on the evidence already given, it must be admitted, however, 
that the theory of a common origin for these characters supplies the 
simplest explanation of the facts. The burden of proof lies with those who 
dispute it. But this is not all. There is evidence from other sources to 
show that the seedlings of Primitive Angiosperms possessed two cotyledons 
and tetrarch symmetry. 
The Bennettiteae probably diverged from the main Angiospermous line 
of descent at a later period than the Cycads. But the embryo of the 
Bennettitean seed is always dicotylar. This increases the presumption 
derived from the two cotyledons of Cycads, that the descendants of the 
ancestral Pteridosperm were dicotylar too, and suggests that they retained 
that character at least as late as the period at which the Bennettitean race 
was given off. This brings us nearer to the Primitive Angiosperms on one 
side. On the other lie the Primitive Dicotyledons and the Primitive 
Monocotyledons. So far as internal evidence goes, the seedling of the 
Primitive Dicotyledon may have shown either diarch or tetrarch symmetry. 
What is known about the seedling of the Primitive Monocotyledon ? 
Among Monocotyledons as among Dicotyledons there is a period in 
the history of the seedling during which it consists of cotyledon, hypocotyl, 
and primary root. The plumular bud is still quite embryonic, and is as 
a rule completely enclosed by the base of the cotyledon, or by a sheath- like 
appendage attached to it. But in Monocotyledons the primary root as well 
as the single cotyledon is commonly a short-lived structure. The hypocotyl 
is almost always much reduced in length, even when it is not transformed 
into a tuber. The young seedling then to all appearance consists only of 
cotyledon and primary root — two short-lived members separated by a brief 
region representing the hypocotylar axis. In many genera this region 
hardly exists at all ; the cotyledon seems to pass directly into the primary 
root. The early internodes of the stem are frequently much suppressed too, 
and the early leaves then appear directly connected with the first cauline 
roots. Such leaves do not receive their water-supply through the main axis 
