Trans. N. V. Ac. Sci. 42 Jan. 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. iinct07-ia, 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 : (i) 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 xuuo 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."t 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 COi.% 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. 



(i.) The first class of water-plants, to which I have above alluded, 

 is well exemplified by Sititn cictitafolium, Gmelin. In this plant the 



* The Evolutionist at Large, Humboldt Library. Vol. IL. No. 26, p. 10. 

 t .See ■' Water Analysis," by J. Alfped Wanklin, M.R.C.S., p. 90. 

 + Ganot, Elementary Treatise on Physics, p. 136. 



