1884. 39 Trans. N. V. Ac. Set. 



read with much interest by students of natural history; and his theory 

 there advanced, that serrated, lobed, and divided leaves have attained 

 these forms by struggling with others for their required supply of car- 

 bonic acid and sunlight during very long periods of time, by a process 

 of gradual change or evolution from an originally entire-margined 

 leaf-form, has appeared to us, to say the least, ingenious. 



There are, however, considerations which will not allow us to accept 

 this attractive theory without question ; to some of these I propose in 

 this paper to allude, using familiar examples illustrative of what is 

 presented, as well as some observations and ideas of my own, indica- 

 tive of what seem to me to be more probable causes by which the 

 modification of leaf-forms is effected than those advanced by Mr. 

 Allen. 



The various marginal and ultra-marginal modifications of leaves — 

 the serrations, lobations, divisions, etc. — and their plans of venation 

 seem mutually dependent on each other. In nearly all plants bear- 

 ing non-entire leaves, we find that the primary vein system, or strong 

 branches from it, the system of channels which conduct the main 

 supply of sap to the various portions of the lamina;, ends in the teeth 

 or lobes,* and, consequently, these parts obtain a greater amount of 

 nutrition than the portions immediately around the bases of the serra- 

 tures, sinuses, etc. The leaf is but an expansion of a portion of the 

 stem, with a greater development of parenchyma ; the prosenchymal 

 elements are expanded into a reticulum, on which the parenchyma is 

 stretched, and these woody parts, bearing the intimate relation to the 

 marginal indentations above alluded to, should be given more promi- 

 nence in a discussion of this subject than those consisting mostly of 

 parenchyma alone, the cells in which the decomposition of carbonic 

 acid is effected by the aid of sunlight. Mr. Allen has given the 

 parenchyma sole importance in his theory, and assigned the vein 

 system no function in the production of teeth, lobes, etc. In entire- 

 margined leaves we find that the primary veins seldom end in the 

 margin, but curve upward, and ultimately become united with one 

 another, forming the closed areas known as areoles. 



Mr. Allen's views fail to explain the fact that the leaves of mono- 

 cotyledons are almost invariably entire -margined. f Many of these 

 plants, and the grasses and sedges in particular, grow closely packed 

 together, the leaves of individuals overshadowing those of others and 



* There are exceptions to this general statement, as in the genus Cicuta, where the veins end 

 in the serratures, and in some species of Tri/oliutn, where they terminate abruptly in the prac- 

 tically entire margin. 



t There are exceptions, as in Yucca^ Dttsylirion, and Aloe, and other related genera, with 

 filamentous or spiny projections from the leaf-margins, and Stratiotes aloides, L., the Euro- 

 pean '• water soldier." 



