ABSORPTION OF FOOD-SALTS BY WATER-PLANTS. 77 



Altogether the number of submerged plants which live suspended in water is 

 very small. As has been said before, by far the greater number are attached some- 

 where. Seed-bearing plants or Phanerogamia., such as Vallisneria, Ouvirandra, 

 Myriophyllum, Najas, Zannichellia, Ruppia, Zostera, Elodea, Hydrilla, and several 

 species of Potamogeton (P. pectinatus, P. pusillus, P. lucens, P. densus, P. crispjus); 

 as also Cryptogams, such as the various species of Isoetes and Pilularia and sub- 

 merged mosses, are fastened in the mud under water by means of attachment-roots 

 or of rhizoids, whilst the almost illimitable host of brown and red sea-weeds are 

 fixed by special cells or groups of cells, which are often root-like in appearance. 

 The sea-weeds choose rocks and stones, by preference, for their support, but they 

 also make use of animals and plants. The shells of mussels and snails are often 

 completely overgrown by brown and red sea-weeds. Larger kinds of Fucaceae, 

 especially the species of Sargassum and Cystosira, which form regular submarine 

 forests, bear upon their branches numerous other small epiphytes, chiefly Florideoe, 

 and these again are themselves covered by minute Diatomaceae. Many of the 

 huge and lofty brown sea-weeds which raise themselves from the bottom of the 

 sea, remind one forcibly of tropical trees covered with Orchideae and Bi'omeliaceae, 

 whilst the latter are themselves overgrown by Mosses and Lichens. These epiphytes 

 are for the most part, however, neither parasitic nor saprophytic. In general 

 hydrophytes attached by means of single cells or groups of cells derive no 

 nutriment, i.e. no food-salts, from the support they rest upon. When loosened from 

 the substratum they continue to live in the water for a long time; they increase in 

 size, and if they come into contact with a solid body are apt to attach themselves to 

 it. In this connection it is well worthy of remark that certain Crustacea have their 

 carapaces entirely covered by hydrophytes of this kind, and that it takes a very 

 short time for the plants to establish themselves upon them. For instance, some 

 species of crabs, such as Maja verrucosa, Pisa tetraodon and P. armata, Inachus 

 scorpioides and Stenorrhyncus longirostris, cut off bits of Wracks, Floridese, Ulvae, 

 &c., with their claws, and place them on the top of their carapaces, securing them 

 on peculiar spiky or hooked hairs. The fragments grow firmly to the crabs' 

 chitinous coats, and far from being harmful to the animals are, on the contrary, 

 an important means of protection. The crabs in question escape pursuit in con- 

 sequence of this disguise, and it is to be observed that each species chooses the 

 very material which makes it most unrecognizable to plant upon the exterior of 

 its body: those species which live chiefly in regions where Cj^stosiras are indigenous 

 deck themselves in Cystosiras, whilst those which inhabit the same places as Ulva?, 

 carry Ulvse on their backs. This phenomenon has for us a special interest in that 

 it shows that the w^ater- plants we are discussing draw no food-salts from their 

 place of attachment, and that accordingly the chemical composition of the support 

 is a matter of utter indiflerence to all these Fucaceae, Florideae, Ulvae, &c. 



There is no doubt that food-salts are absorbed by these hydrophytes from the 

 surrounding water through their whole surface. Accordingly the structure of their 

 peripheral cells is much simpler than is the case in land-plants. In the latter very 



