50 BRITISH PLANTS 
Leaf-Types in Aquatic Plants. 
I. Submerged Leaves. 
1. The Dissected Type —e.g., water-buttercup (Ranun- 
culus aquatilis), water-dropwort (Hnanthe Phellandrium), 
water-violet (Hottonia palustris), bladderwort (Utricu- 
laria, Fig. 46). In these plants the submerged leaves 
are so extensively divided that the ultimate segments 
are almost filamentous. Three reasons have been offered 
in explanation of this dissection—namely : 
(1) That such a finely-divided leaf offers less resistance 
to moving or disturbed water than an entire leaf, and so 
runs less risk of being damaged by tearing. 
(2) That it offers increased surface for the intake of 
the carbonic acid gas dissolved in water, and required 
by the plant for assimilation ; and 
(3) That it offers increased surface for the absorption 
of oxygen required in respiration. The second explana- 
tion is founded on the fact that, although there is more 
carbonic acid gas in water than in air, it is not so avail- 
able. Diffusion is so rapid in air, that as soon as one 
particle of the gas is removed by the plant, another 
at once takes its place ; in water, however, the rate of 
diffusion is much slower, and the particles removed are 
not so quickly replaced by others as in air. The first 
explanation is founded upon an obvious danger in moving 
water, the third upon a still more serious peril in stagnant 
water. 
2. The Ribbon-Type.—In this the leaf is long, undivided, 
and band-shaped. It tends to set itself in the same 
direction as the current, and as it offers no resistance 
to it, it is not likely to be damaged—e.g., water-plantain 
(Alisma Plantago), Vallisneria, Zostera, Potamogeton 
crispus (Fig. 18). 
3. The Awl-Shaped Type, seen in the water-lobelia 
(Lobelia Dortmanna), the shoreweed (Littorella), in the 
spore-bearing cryptogam, Jsortes lacustris (Fig. 110), and 
the water-fern, Pilularia (the pillwort). These leaves are 
short, smooth, thick, and tapering, and contain enormous 
air-spaces. 
