Trans. N. Y. Ac. Sci. 44 Jan. 7, 



pbtaca pecthiacea. Lam., are good examples of this class. If we 

 accept as true the proposition that leaves were originally entire, we 

 must assume some system of change from less divided-leaved pro- 

 genital species, not by a search for carbonic acid, as Mr. ALLEN has 

 imagined, but caused primarily by resistance offered by the water. 



The origin of the multiform leaves, borne by plants whose natural 

 habitat is dry land, is much more obscure. The theories advanced 

 by Mr. Allen certainly will not answer. But taking the facts before 

 mentioned in regard to the variable amounts of nutriment received 

 from the roots, together with the existence of a well-defined system, 

 a venation, and the fact that all vegetable growth is affected under 

 the atmospheric pressure of fifteen pounds to the square inch — cer- 

 tainly an appreciable amount of resistance, and also against the at- 

 traction of gravitation — I think we have data which will explain some, 

 if not all of the marginal modifications of leaves. 



Mr. Allen's papers on this subject have been widely, and very 

 generally unfavorably criticised.* I do not here propose to offer other 

 objections to his theories, or his style of imaginative evolutionary de- 

 duction, although there are many statements in the papers here allud- 

 ed to, which should be seriously considered before accepted as exact. 



DISCUSSION. 



Dr. SCHONEY observed that agriculturists give no clue to the cause 

 of this variation in leaf structure. The Japanese maples* have been 

 found capable of a wide modification. However, experimental culti- 

 vation has not thrown light upon the causes of these changes. 



Miss E. G. Knight stated that in the fern leaves the ribs corre- 

 spond to the interspaces between the veins. In Salisburia all the ter- 

 minal leaves and all the older leaves are much more laciniated. 



Prof. Day suggested that plants growing in loose sand, and thus 

 liable to be covered by sand and earth, were found apt to be divided. 



Mr. C. F. Cox said that he had given some attention to a similar sub- 

 ject, and that, if any criticism were to be made upon the paper just 

 read, he thought Dr. Britton had confined himself too exclusively 

 to the merely mechanical or physical view of the matter, and had not 

 allowed sufficient weight to physiological considerations, particularly 

 in reference to the differences between the shapes of aerial and of 

 aquatic leaves. It seemed evident that the parenchymal tissue was 

 spread out upon the fibro-vascular skeleton for the purpose of expos- 



* See W. T. Thistleton-Dyer in Nature, XXVII., 54 ; notes by F. O. Bower, ibid., 552 ; 

 note by Sir John Lubbock, ibid., 603, where he indicates his opinion that the cause of lobing 

 is a mechanical one ; note by E. M. Holmes, ibid., XXVIII., 29 : also, L. P. Gratacap, in 

 Bull Torr. Bot. Club, June, 1883. 



