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, Owvirandra, 
Myriophyllum, Najas, Zannichellia, Ruppia, Zostera, Elodea, Hydrilla, and several 
species of Potamogeton (P. pectinatus, P. pusillus, P. lucens, P. densus, P. erispus); 
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 Fucacex, 
especially the species of Sargassum and Cystosira, which form regular submarine 
forests, bear upon their branches numerous other small epiphytes, chiefly Floridez, 
and these again are themselves covered by minute Diatomacex. 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 Orchidez and Bromeliacez, 
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, Floridez, Ulve, 
- &e., 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 Cystosiras are indigenous 
deck themselves in Cystosiras, whilst those which inhabit the same places as Ulve, 
carry Ulvze on their backs. This phenomenon has for us a special interest in that 
it shows that the water-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 indifference to all these Fucacex, Floridex, Ulve, &e. 
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 
