Trans. N. Y. Ac. Sci. 42 Fan. 7, 
to entireness of margins in the younger and consequently more vig- 
orous, better fed leaves of saplings is explainable in the same manner. 
This is well exemplified in some of our native oaks, particularly in 
Quercus coccinea, L., var. tinctoria, Gray, and often in Quercus nigra, 
L. This varying supply of sap-nutriment then produces variations in 
leaf-forms, and it is not improbable that these may now, and in past 
time have become specifically permanent, and here we may have one 
potent cause of the production of marginal modifications. 
The mechanical cause of the origin of leaf-forms which long ago 
suggested itself to me is simply resistance to cell-formation by the 
medium in which growth takes place, be this air or water. 
Let us first consider the case of submerged leaves, which are notably 
the most cut and divided of all, remembering that the density of water 
is many times that of the atmosphere. We may conveniently divide 
these into three classes: (1) plants whose natural habitat is moist 
ground, but which sometimes (by an unusually wet season, or other 
causes) have their lower leaves covered by water; (2) plants growing 
naturally in the water with their lower leaves submerged, their upper 
ones exposed to the air, either floating or immersed; and (3) plants 
wholly submerged. 
Mr. ALLEN has stated that ‘‘ gases are not very abundant in water, 
as it only holds in solution a limited quantity of oxygen and carbonic 
acid.”* Certainly the quantity is small compared with the mass of 
water ; but although water can dissolve only ;2$5 of its volume of oxy- 
gen, there is enough of this gas to support a more redundant animal 
life than exists on the land. As to the actual amount of carbonic acid 
dissolved in waters, there is little exact information, as this substance 
is not determined in ordinary water-analysis. However, ‘‘in the 
autumn of 1859, W. A. MILLER found that a litre of Thames water at 
Woolwich contained 63.05 c. c. of dissolved gases, of which 48.3 c. c. 
was carbonic acid; and at Kingston, a litre from the same river con- 
tained 52.7 c. c. of dissolved gases, of which 30.3 c. c. was carbonic 
acid.”{ If river water contains this percentage, ponds, and particularly 
partially stagnant waters, in which the greatest quantity of submerged 
plants are found, certainly contain much more, water being capable of 
dissolving its own volume of CO,.{ It would appear then that Mr. 
ALLEN has underestimated the percentage of carbonic acid in waters ; 
the percentage certainly far exceeds that carried in the atmosphere. 
(1.) The first class of water-plants, to which I have above alluded, 
is well exemplified by S¢m cicutefolium, Gmelin. In this plant the 
* The Evolutionist at Large, Humboldt Library, Vol. II., No. 26, p. ro. 
t See *‘ Water Analysis,” by J. ALFRED WANKLIN, M.R.C.S., p. go. 
¢~ Ganot, Elementary Treatise on Physics, p. 136. 
