Trans. N. Y. Ac. Set. 46 Jan. 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 SpJieno- 

 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. In a 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 palaeontologist, 

 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 aquaiilis, 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 coniferte 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 



