1884. 43 Teans. IV. V. Ac.S6t: 
naturally pinnate leaves with serrate pinne 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 multifidus, Pursh, some forms of Ranunculus 
aquatilis, L., and Proserpinaca palustris, L., are good examples of my 
second class; and here the same arguments hold good, for, as I have 
repeatedly observed in Ranunculus multifidus, 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 Wympheacee and the genus Limuanthemum. 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 Wuphar parvulum, Smith, and Lim- 
nanthemum lacunosum, Griseb.t 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., Ceratophyllum demersum, L., and Proser- 
*In Calbomba we have small, entire margined, floating leaves, and large, dissected, sub- 
mersed ones, In one or two species of Vymphea the leaf is somewhat dentate. 
+ Gray, Manual, p. 57. 
t¢ Bull. Torr. Bot. Club, x., 34. 
