1884. 45 Tganss INA Vi AL Sct: 
ing a certain amount of cellular structure to the surrounding air or 
water, with relation to the favorable operation of either one or both of 
the great physiological functions—the assimilative and the metastatic 
processes. Since in nature the employment of material, like the ex- 
penditure of energy, is directly proportioned to some necessary end to 
be attained—there being neither waste nor deficiency, —we are forced 
to the conclusion that the differences in at least the guantity of leaf 
substance have an essential connection with the varying physiological 
requirements of plant life; and when the problem of leaf-forms is 
fully solved, it can hardly be doubted that here, also, physiological 
causes will be found to be the most potent and the most important. 
We know already that, as to their structure, the partially submerged 
plants of the order Nymphzacee stand upon a dividing line between 
the fully aquatic and the strictly terrestrial orders, the conditions or 
their environment necessitating certain adaptations of material means 
to physiological ends,—as, for example, the apparent transference of 
the epidermis with some of its organs (stomata) exclusively to the 
upper side of the leaf, and of some other organs, usually external 
(trichomes), to the intercellular air passages, which are uncommonly 
large. These peculiarities of structure are plain indications that 
physiological functions, which are performed by certain forms of leaves 
growing in the air, may require a change in the forms of the leaves 
when growing partially in the water; and they establish at least a 
strong probability that totally submerged leaves become entirely in- 
capable of performing some of the functions of aerial leaves, and con- 
sequently differ from them in both structure and shape. It is there- 
fore quite likely, as Prof. Day has suggested, that the submerged fili- 
form leaves have reverted to a more fibro-vascular and less cellular 
structure, simply because of a lack of physiological use for a greater 
parenchymal system, by reason of the nature of their habitat. 
Dr. BRITTON stated that he had not combated the idea of the in- 
fluence of an insufficiency of carbonic acid in the water. 
Mr. Cox further remarked that the stomata were almost wanting 
in submerged leaves. 
The PRESIDENT observed that the plants which check the progress 
of drifting sand are chiefly the Graminez. Others, which grow in sand, 
have broader leaves—e.g., the Cucurbitacee (watermelons), several 
species of Adronia in the Far West, etc. Experiments could be made 
which would throw much light on this subject, ¢.g., by means of tanks 
of water containing varying amounts of carbonic acid, or by growing 
plants under pressure, etc. 
The forms of leaves were so infinitely diversified as to constitute an 
almost inexhaustible subject of study, and the cause or causes of this 
