1884. 45 Trans. N. V. Ac. Sci. 



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 quantify 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 Nymphasaceie stand upon a dividing line between 

 the fully aquatic and the strictly terrestrial orders, the conditions 01 

 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 mor^ 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 Gramineas. Others, which grow in sand, 

 have broader leaves — e.g., the CucurbitacecE (watermelons), several 

 species of Abronia in the Far West, etc. Experiments could be made 

 which would throw much light on this subject, e.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 



