SAPROPHYTES IN WATER, ON THE BARK OF TREES, AND ON ROCKS. 105 



in harbour are overgrown by Ulvce, wracks, filamentous alg, and Florid**. Not 

 a few, as, for instance, the so-called sea-lettuce ( Ulva lactuca), several species o f 

 Qehd^m, Bangia, and Ceramic, and the great Cystosira barbata, thrive best 

 and m 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 Euglence 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 

 ide by side with black patches of Oscillaria antliaria and verdigris-coloured films 

 Oscillaria tenuis, are blood-red patches of Palmetto, 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 Hormidium murale, which 

 weaves itself over the mire, and by the dark, actively-oscillating Oscillaria limosa; 

 id, above all, the curious Beggiatoa versatilis makes itself conspicuous, sending 

 out from a whitish gelatinous ground mass long oscillating filaments, which emerge 

 ,fter sundown, and next day split up into innumerable little bacteria-rods. The 

 red-snow alga, too (represented in fig. 25A), 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 (Hcematococcus pluvialis or Sphcerella 

 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 (Edogonium, by Pleurococcus 

 angulosus, and by the amethyst-coloured Protococcus roseo-persicinus. 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 algae, Oscillariae and so forth 

 (Bulbochccte parvula, Schizochlamys gelatinosa, Sphcerozosma vertebrata, Micro- 

 cystis ichthyloba, &c.), 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 organic compounds. When we include also the curious mould-like 

 Saprolegnice produced on dead bodies floating in waierSaprolegnia 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. 



