168 HYDROPHYTES SECT, iv 



Creeping (migrating) lithophilous species are uncommon, yet are met 

 with among Florideae, Caulerpa, and Podostemaceae. The last named have 

 creeping roots, but agree with algae in their mode of attachment, since 

 the roots only indirectly play a part in fixation by bearing haptera ; they 

 have no organs specially set apart to absorb nutriment. 



Air-containing intercellular spaces are entirely lacking, or at most are very 

 small and contain scarcely any air. Exceptions to this rule occur in the sub- 

 aerial inflorescences of Podostemaceae, and in the floating-apparatus of cer- 

 tain algae, such as Fucus vesiculosus, Halidrys siliquosus, Chorda filum, and 

 Ascophyllum nodosum, which live in the littoral belt or in shallow water. 

 Through this character, lithophilous vegetation stands in sharp contrast with 

 other types of aquatic vegetation. The reason for it is presumably that all 

 the lithophilous plants in question live in troubled water, where they secure 

 rich supplies of air : Podostemaceae mostly find their homes in cascades. 



The necessary power of resistance to rupture is structurally provided 

 in various ways by mechanical, mainly collenchymatous, tissue. 1 



The excretion of calcium carbonate within cell-walls takes place in a 

 number of algae, and with this must be mentioned the silica occurring in 

 Podostemaceae. These excretions in some cases play a mechanical role, 

 while in others they seem to increase the longevity ; certain incrusted 

 algae are perennial, whereas their non-incrusted allies are annual. 



Heavy production of mucilage takes place especially in species growing 

 in the littoral belt, and perhaps serve to prevent desiccation while the 

 tide is down. It is calculated also to diminish friction between water and 

 plants, and thus to preserve the latter from the violence of breakers. 2 



The Plant-shapes are extremely varied; and by no means all seem 

 capable of interpretation as adaptations to environment : 3 



Crustaceous type. There are among algae and Podostemaceae (Erythro- 

 lichen, Lawia, Hydrobryum) crustaceous forms, which are well-suited for 

 existence in very perturbed water ; but many crustaceous Algae grow in 

 deep, and therefore in but slightly disturbed, water. 



Myriophylloid type. There are species among algae and Podostemaceae 

 that are structurally analogous to gills of animals, as they are cut up into 

 many capillary segments, by which the surface and assimilatory activity are 

 both increased. 



Muscoid type. There are moss-shaped forms among mosses (Fontinalis) 

 and Podostemaceae (Tristicha hypnoides, species of Mniopsis and Podoste- 

 mon). 



Filiform type. There occur species shaped like an unbranched thread, 

 which experience passive undulatory movements in the water ; such are 

 Chorda filum, many fresh-water algae, and the podostemaceous Dicraea 

 elongata. 



Phylloid type. Leaf-like forms are presented by Porphyra, Laminaria, 

 Ulva, and Monostroma, as well as by the podostemaceous Marathrum, 

 Oenone, and Mourera. 



Special stress must be laid on the parallelism between the shapes of the 

 marine algae and Podostemaceae, as demonstrating that these shapes are 

 adaptive. In the sea at the southern point of South America there occur 

 peculiar algae (Macrocystis and Durvillea) which have ' floating fronds '. 4 



1 Wille, 1885. 2 Wille, 1885 ; see p. 99. 



See Oltmanns, 1905. 4 Compare Hooker, 1847 a. 



