50 BRITISH PLANTS 



Leaf-Types in Aquatic Plants. 



I. Submerged Leaves. 



1. The Dissected Type — e.g., water-buttercup {Ranun- 

 culus aquatilis), water-dropwort {(Enanthe Phellandrium), 

 water- violet [Hottonia folustris), bladderwort {Utricvy 

 laria. Fig. 46). In tliese 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, Isoetes lacustris (Fig. 110), and 

 the water-fern, Pilularia (the pUlwort). These leaves are 

 short, smooth, thick, and tapering, and contain enormous 

 air-spaces. 



