Trans. NAY. Aéi\Stt. 46 Fan. 7 
diversity have not yet been investigated with the thoroughness and 
philosophical method through which the truth will be discovered if 
within our reach. Certain broad generalizations could be made in 
regard to the forms of leaves, which were based upon striking and 
suggestive facts, but the explanations offered, were for the most part 
mere speculations. As a general rule the submerged leaves of aquatic 
plants were dissected, while the emerged or floating leaves were broad. 
This is true not only of the cases cited by Mr. BRITTON but of many 
others in the recent flora, and it had prevailed through all geological 
ages. When a boy he discovered that the well-known plant Spheno- 
phyllum of the Coal flora had dimorphous foliage ; the lower leaves 
were capillary, constituting many of the species of the genus Astero- 
phyllites, while the summit leaves were broad wedge-shaped ; and on 
terminal branches carrying such leaves, the genus Sphenophyllum was 
founded. From these facts he inferred that this was an aquatic plant 
growing in the lagoons of the Coal marshes, with only its terminal 
branches and leaves exposed to the air. Ina paper read before the 
American Association in 1853, these facts were given and illustrated 
by figures in the Report of the Proceedings of that meeting. Ten 
years later the same facts were reported by the Belgian paleontologist, 
Coemans, as discovered by him. 
The functions performed by the emerged and submerged leaves 
of aquatic plants are evidently different. They are exposed to dif- 
ferent media, air and water, and the differences in form and structure 
evidently hinge upon the differences in the media, and in the functions 
performed, the blades of the emerged leaves are both exhalent and 
absorbent organs, exhaling moisture and oxygen and inhaling carbonic 
acid, sometimes oxygen, and often absorbing water; the submerged 
leaves have probably a more limited range of function, are simpler 
machines, and they have less to do and that of a simpler kind. The 
pressure of the water may determine the form of the leaf, but it can 
hardly be accepted as counting as a mechanical impediment to the 
formation of parenchyma; for in some submerged plants, such as 
Myriophyllum ceratophyllum and Ranunculus aguatilis, the leaves are 
so numerous that the area of parenchymatous surface must be as large 
as in almost any aerial plant. The thread of prosenchyma in a sub- 
merged leaf filament bears perhaps as small a ratio to the parenchyma 
which encircles it, as the skeleton does to the parenchyma of an 
emerged leaf. 
Among terrestrial plants the most striking and yet unexplained 
difference of form is observable ; for example, the conifer as a rule 
have extremely narrow leaves and derive their German name of needle 
trees from their acicular form. But to this general rule we have 
