1884. 43 Trans. N, V. Ac. Set. 



naturally pinnate leaves with serrate pinnae become much incised and 

 laciniated by mere submergence alone, without the necessity of invok- 

 ing evolutionary processes through intermediate forms ; and this is 

 wholly due to lesser amounts of parenchymal development, for the 

 vein system is practically alike in the two forms of leaves. Here 

 there is evidently a greater resistance offered to the formation of tissue 

 by the denser medium, water, in which it takes place, and the chlo- 

 rophyll-bearing cells forming the stomata-bearing surfaces are re- 

 stricted to narrow bands bordering the stronger veins. I have twice 

 observed this fact. 



(2.) Ranunculus multi/idus, Pursh, some forms of Ranunctilus 

 aquatilis, L., and Proscrpinaca palustris, L., are good examples of my 

 second class ; and here the same arguments hold good, for, as I have 

 repeatedly observed in Ranunculus jnultifidus, Pursh, and the Pro- 

 serpinaca, it depends entirely on the depth of water and the conse- 

 quent relative amount of submergence of the plants, how many of the 

 leaves are dissected into narrow lobes, and how many remain normal. 

 In the yellow Ranunculus, moreover, we find plants growing on 

 shores, and not at all submerged, bearing only normal leaves ; and 

 this character is so well marked that this form has been given the 

 rank of a variety in the Manuals of Botany [var. terrestre). In both 

 Ranunculi the dissected lobes of the sub-aqueous leaves are somewhat 

 prolonged by the stronger growth at the ends of the main veins as com- 

 pared with that of their lateral branches. In all such leaves the for- 

 mation of cellular, stomata-bearing tissue is reduced to very small 

 amounts by the resistance of the water. 



Another kind of dicotyledonous water plants may here be noticed. 

 I refer to the A'ymphceacecE?ind. the genus Lhnnanthemufn. These bear 

 nearly always entire-margined leaves,* the growth taking place at 

 the contact of air and water, where the resistance is very small. 

 Another interesting feature of these plants is the existence of thin, 

 delicate, submerged leaves on Nuphar parvulum, Smith,! and Lini- 

 nanthejnum lacunosum, Griseb.:]: In these leaves the failure to pro- 

 duce the normal amount of parenchyma is evidenced in a reduction of 

 the number of layers of cells, the resistance here causing a diminu- 

 tion of growth vertically instead of laterally. 



(3.) In plants bearing leaves all normally submerged there is less 

 direct evidence of the manner and cause of their capillary origin. 

 Hottonia inflata, Ell., Ceratophylluni demersum, L. , and Proser- 



* In Cahomba we have small, entire margined, floating leaves, and large, dissected, sub. 

 mersed ones. In one or two species o^ Nympkcea the leaf is somewhat dentate. 

 t Gray, Manual, p. 57. 

 X Hull. Torr. Bot. Club, x., 34. 



