1904] SARGANT: EVOLUTION OF MONOCOTYLEDONS 343 
lar system of the submerged stem be reduced to a slender cen- 
tral stele with hardly any lignified elements. : 
There are aquatic species with parallel-veined leaves (as Val- 
lisneria, Zostera), but this character is not common among water 
plants. It is almost confined to the monocotyledons among 
them. Circular or oval floating leaves, or the much dissected 
submerged leaves, are far more frequent. 
The primary root of aquatic plants is often replaced early 
by tufted adventitious roots, a character found in many land 
plants with creeping rhizomes. 
On the other hand, aquatic dicotyledons show no tendency to 
the formation of a single seed leaf in place of two, nor do they 
always possess albuminous seeds. 
Great stress has been laid on the primitive floral structure 
displayed by many aquatic monocotyledons. Among the Heli- 
obiae more than two whorls of stamens, and more than one whorl 
of carpels are not uncommon (Stratiotes, Hydrocharis, and 
others). Both stamens and carpels are occasionally indefinite in 
number and arranged spirally on the thalamus (Alisma, Limno- 
charis, and others). It is very probable that such types repro- 
duce the floral symmetry of ancestors which were intermediate 
in character between the Ranales and the Liliiflorae. But even 
if some primitive characters are retained by aquatic species, it 
does not follow that the primitive monocotyledon lived in the 
water, and that its descendants acquired their peculiarities by 
adaptation to that habit. It is more probable that the aquatic 
forms represent ancestors of our modern monocotyledons which 
Were crowded out by the competition on land, and took refuge 
in the water, or on its edge, where competition was less severe.*7 
The vegetative structure of such species would be modified by 
the change of environment and would no longer represent that 
of the ancestor, but the reproductive organs might well remain 
unchanged. Such ancestors, if geophilous, would readily adapt 
themselves to amphibious conditions; their underground stems 
Creeping in the mud which fringed the pond or stream until by 
*7Cf. Darwin on the survival of ganoid fishes and simple vertebrates (Origin of 
Species, Sixth Ed. 1: 130, 154, 155, 163; 2:99, 173- 1888. 
