!8 2 HYDROPHYTES SECT, iv 



Various species are heterophyllous, possessing not only floating leaves 

 but also submerged leaves. According to Askenasy 1 and others, 

 the floating leaves of Batrachium and Cabomba do not appear until 

 the plant is about to flower, so that they may serve specially to raise 

 the blossom above water. 



2. The submerged leaf differs both morphologically and anatomically 

 (particularly as regards epidermis and chlorenchyma) from the floating leaf : 2 



a. The zosteroid, or ribbon-like leaf, which generally occurs among 

 grass-wracks, is less frequent in this formation, though it occurs in Vallis- 

 neria, Sparganium, species of Potamogeton, and others. Convincing 

 evidence is forthcoming that this form of leaf is adapted to and evoked 

 by deep or running water (both these conditions appear to act in the 

 same direction), when we observe that it appears on certain marsh-plants, 

 including Alisma Plantago, Sagittaria sagittifolia, Echinodorus ranuncu- 

 loides, and species of Sparaganium, if these be compelled to grow in such 

 water. Similar forms of leaf are met with under the same conditions 

 in Scirpus lacustris and Potamogeton natans, which produces ' current- 

 leaves ' half a metre long. 3 



b. The elodioid leaf, which is narrow, linear, undivided, flat, sessile, 

 and short, is frequent, as shown by Elodea, Potamogeton densus, P. 

 obtusif olius, P. pusillus and other species, Hippuris, Zannichellia, Callitriche 

 autumnalis and other species, and Naias. In this category may be placed 

 the leaves of the aquatic mosses. Broader forms of leaf are displayed 

 by other species of Potamogeton. 



c. The isoe'toid leaf is linear, undivided, terete, often tubular, and sessile, 

 and occurs in Pilularia, Isoetes, Lobelia Dortmanna, Littorella lacustris, 

 and others, most of which are rosette-plants. Subularia and the Characeae 

 may most fittingly be appended to these. 



It becomes clear that the two, tolerably similar, linear types of leaves 

 just described result, at least partially, from the action of the water, 

 when we observe the behaviour of Juncus supinus, Hippuris vulgaris, 

 Elatine Alsinastrum, Isoetes lacustris, Pilularia, and other plants that 

 assume terrestrial and aquatic forms ; the submerged leaves are much 

 longer, more flaccid than the subaerial leaves. 



d. The myriophylloid leaf, or leaf dissected into filiform or linear segments 

 (like the gills of a fish) is very widespread, occurring in Myriophyllum, 

 Heliosciadium inundatum, Batrachium, and Cabomba, as well as in certain 

 marsh-plants, including Oenanthe Phellandrium, O. fistulosa, and Sium 

 latifolium, when these grow in deeper water. Allied to this type of 

 leaf is the uncommon fenestrated leaf of Ouvirandra fenestralis. Many 

 observations show that the depth of the incisions, and the fineness of the 

 segments are due to the influence of the medium (depth of water, strength 

 of its flow, and so forth) : when the shoots reach the surface there appeal- 

 floating leaves, as in Batrachium, or leaves with shorter, broader, thicker 

 segments, especially when the shoots project out of the water as in the 

 case of Myriophyllum. The physiological cause of this difference must 

 presumably lie in the elongation due to decreased illumination, and in 



1 Askenasy, 1870. 



In reference to the different forms of Polygonum amphibium, see Massart, 1902. 

 )stantin, 1884, 1885, 1886; Gobel, 1889; Raunkiar, 1895-9; Gliick, 1905, 



1906. 



