SAPROPHYTES IN WATER, ON THE BARK OF TREES, AND ON ROCKS. 105 
in harbour, are overgrown by Ulve, wracks, filamentous algw, and Floridew. Not 
a few, as, for instance, the so-called sea-lettuce (Ulva lactuca), several species of 
Gelidium, Bangia, and Ceramium, and the great Cystosira barbata, thrive best 
and in greatest abundance in polluted water of the kind; and there can be no 
doubt that this is to be accounted for by the presence of a greater quantity of 
organic compounds in that water. 
It is not only in contaminated sea-water, but also in other collections of water 
which contain products of putrefaction in solution, that we find a characteristic 
vegetation. We have already alluded to the presence of Euglene in the liquor 
of manure-pits. They occur also at the foot of shady walls, in dirty back 
streets in towns, in the puddles, and on ground which is saturated with urine and 
impurities of every kind. ‘These places are the home of a number of other minute 
plants, which stain the polluted ground after rain with the gayest colours. There, 
side by side with black patches of Oscillaria antliaria and verdigris-coloured films 
of Oscillaria tenuis, are blood-red patches of Palmella cruenta, and brick-red 
patches of Chroococcus cinnamomeus. Equally characteristic is the vegetation 
which covers the earth at the mouths of drains, and is bathed by the trickling 
sewage. Large areas here are overgrown by the green Hormidiwm murale, which 
weaves itself over the mire, and by the dark, actively-oscillating Oscillaria limosa; 
and, above all, the curious Beggiatoa versatilis makes itself conspicuous, sending 
out from a whitish gelatinous ground mass long oscillating filaments, which emerge 
after sundown, and next day split up into innumerable little bacteria-rods. The 
red-snow alga, too (represented in Plate I.), lives at the expense of the pollen- 
grains, bodies of insects, and other decaying matter blown on to snow-fields; 
whilst the nearly allied blood-red alga (Hamutococcus plwvialis or Spheerella 
pluvialis) lives in the water in hollow stones where all sorts of animal and 
vegetable remains collect. Leaves blown into deep pools, and lying rotting 
at the bottom, are everywhere overgrown by green @dogonium, by Plewrococeus 
angulosus, and by the amethyst-coloured Protococeus roseo-persieimus. The 
bottoms of ditches on peat-bogs, which are full of brownish water containing an 
abundance of compounds of humic acid in solution, are covered with this amethyst 
Protococcus, whilst a profusion of small filamentous alge, Oscillarie and so forth 
(Bulbochate parvula, Schizochlamys gelatinosa, Spherozosma vertebrata, Micro- 
cystis ichthyloba, &e.), as well as a group of dusky mosses (Hypnum giganteum, H. 
sarmentosum, H. cordifolium), all have their home exclusively in still water richly 
supplied with organie compounds. When we include also the eurious mould-like 
Saprolegni® produced on dead bodies floating in water—Saprolegnia ferax and 
Achlya prolifera on flies and fishes—some idea is obtained of the great variety 
of saprophytes living in fresh water, as well as of those inhabiting the sea. 
A much more agreeable and attractive picture than that of these aquatic sapro- 
phytes is afforded by plants whose sole habitat is the bark of trees. The dead 
bark does not constitute the nutrient base of all the plants which grow from 
trunks and branches, or climb up them in the form of clinging and twining lianas. 
