736 Hens low .— The Origin of Monocotyledons from Dicotyledons , 
Another common feature of nearly all Monocotyledons and aquatic 
Dicotyledons is the total absence of hair, ‘ glabrous 5 being the characteristic 
feature of nearly all alike. 
14. The Reticulated Venation of some Monocotyledons is only 
IMITATIVE OF THAT OF DICOTYLEDONS. 
An important fact must be noticed. The reticulated leaves of certain 
Monocotyledons are not identical with, but only imitate , by approximating 
the structure of, those of Dicotyledons. The typical monocotyledonous leaf 
being really homologous with a petiole only, and therefore phyllodinous, has 
parallel strands. If such be an aquatic plant, it may be so only in com¬ 
paratively deep water, as Sagittaria. If it bear floating leaves, the usual 
change is to widen the intervals at the upper end by curving the strands so 
as to make a curvinerved or elliptical blade. This now introduces trans¬ 
verse cross-bars, thereby strengthening the whole. If the blade becomes 
aerial considerable changes take place ; the first is for the two outermost 
strands to curve downwards, so that the blade assumes a spear-head or 
hastate shape. By the prolongation of the downward points it becomes 
like an arrow-head or sagittate. These points may be rounded off and so 
a cordate form is acquired, being specially adapted, according to Mr. Hiern, 1 
for resisting the strain of running water when floating. 
The last change or advance is to form tissue between the lower points 
or fill in the cordate gap. The blade then becomes peltate, as in Alocasia 
and Nelumbium , as well as our own Hydrocotyle . 2 
The minuter venation changes with these forms ; the first appearance 
of an irregular network is seen between the small divisions of the first 
described. The veins, primarily at right angles to the longitudinal strands, 
become oblique ; the number of main strands may be reduced even to three, 
so that plenty of space is allowed for more uniform reticulations. The blade 
then closely resembles an ordinary one of a Dicotyledon, but a prominent 
midrib, like that of a Dicotyledon, is absent in Monocotyledons ; for when 
the leaf is large, as in Caladium , See., a cluster of strands become crowded 
together , but as their ends successively diverge obliquely across the blade, 
becoming reticulated, the median cluster is reduced to nothing towards the 
apex of the blade. 
We thus discover that aerial reticulated leaf-blades of Monocotyledons 
are not identical but only imitative of the fibro-vascular system of an 
ordinary dicotyledonous leaf. The change is very well seen by comparing 
the small elliptical blades which first arise from the tuber with the subse¬ 
quent sagittate ones of Arum maculatum. 
Indeed, one is tempted to speculate upon this fact. For no Monocotyle¬ 
don would seem to possess a decidedly aquatic form resembling an aerial 
1 Proc. of the Cam. Phil. Soc., Oct. 1872. 2 loc. cit., pp. 515 ff. 
