Trans. N. Y. Ac. Sci. 40 ; Fan. 7, 
their own lower ones, closely approximate to, and often touching them ; 
yet here no marginal irregularities are seen, except a mere roughening 
of margins and midrib. Many of the orchids and liliacee grow in 
closely shaded places with leaves of other plants packed densely 
around them, yet their leaves are entire. Only the palms, the high- 
est of these endogens, bear compound leaves, but their pinne are 
entire. If Mr. ALLEN’S hypothesis were true, we should certainly 
expect to find marginal indentations on the leaves of most endogenous 
plants having this habit of growth. The monocotyledons are plants 
of a very old type, having appeared on the earth, geologically, long 
before the angiosperms,* and hence there has been very much more 
time for them to have undergone changes in leaf-forms, but these re- 
main very much as they began. In their case there seems to be a 
perpetuation of an ancient character dependent on a primitive plan 
of parallel venation, which the imaginary “struggle” for carbonic 
acid has failed to alter. 
The leaves of angiosperms hardly appear to have become more ser- 
rated and lobed since their introduction into the earth’s flora at the 
beginning of the Cretaceous epoch. The earliest of them, as repre- 
sented by specimens from the Dakota sandstones and New Jersey 
plastic clays, are probably not less serrated nor lobed than the leaves 
produced by plants of the same genera now living. That there has 
been a change is of course possible, although it may justly be ob- 
jected, that very little is known to us of that early angiospermous 
flora compared with our knowledge of the present one. But we will 
admit that a change in this direction may have taken place from nat- 
ural causes, 7.c., that leaves are now marginally modified to a greater 
extent than they originally were, although the paleontological evi- 
dence of this change is wanting. 
The object of the chorophyll-bearing parenchyma of the leaf is 
primarily the absorption and decomposition of carbonic acid, and it 
effects this in proportion to the amount of stomata-bearing surface ; 
an entire-margined leaf accomplishes this more economically than a 
lobed one with the same system of venation, for the surface is greater, 
and the advantage of lobes, divisions, etc., under this view is difficult 
to apprehend. 
Again, the well-known laws of the phenomena of diffusion of gases + 
render it quite certain that the amount of carbonic acid in all parts 
* The oldest hitherto discovered plant of probable monocotyledonous affinities is Pothocites 
Grantont, Paterson, perhaps allied to the Avace@, remains of which have been found in the 
Carboniferous bituminous shale of Granton, Scotland, and described in Trans. Edinb. Bot. 
Soc., vol. 1. Undoubted endogens occur abundantly in the Trias and Jura. No true angio- 
sperms are known lower than the basal Cretaceous strata. 
+ See Ganot, Elementary Treatise on Physics, p, 135. 
