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



well-marked boundary line between plants which absorb organic compounds and 

 those which absorb inorganic compounds from their respective substrata; and that 

 there undoubtedly exist plants capable of taking up both kinds of material at the 

 same time. This conviction is strengthened still further by the circumstance, 

 which has been repeatedly confirmed by experiment, that plants susceptible of 

 being successfully reared in artificial solutions of mineral salts — to the exclusion 

 of organic compounds — do not entirely reject organic compounds when the latter 

 are tendered to them, but unquestionably assimilate some of them (urea, uric acid, 

 glycocoll, &c.) and work them up into constituents of their own frames. 



But, in spite of the impossibility of drawing a sharp line of demarcation 

 between the two groups, it is convenient to treat of the absorption of organic 

 compounds separately, because this division of the subject affords the best 

 opportunity of inspecting in detail, and of surveying generally, the conditions of 

 food-absorption, the comprehension of which is otherwise difficult. In order to 

 determine in each individual case whether a given plant lives either exclusively 

 or principally upon organic food, derived from decaying animal or vegetable 

 remains, reliance must be placed on experiments with cultures; and, in the absence 

 of better vantage-ground, the results of the rougher experiments made by 

 gardeners should not be neglected, always providing that they are accepted 

 subject to possible correction by subsequent exact experiment. 



SAPfiOPHYTES IN WATEE, ON THE BAEK OF TEEES, AND ON EOCKS. 



Of the special cases of absorption of organic compounds from decaying bodies, 

 we have first of all to consider those occurring amongst water-plants. In the sea, 

 wherever there is an abundance of animal and vegetable life there is also plenty 

 of refuse, for there death and decay hold a rich harvest. The quantity of organic 

 matter dissolved in the water is naturally greater in these places than where 

 vegetation and animal life are less conspicuous. There is a much more varied 

 flora and fauna to be met with in the sea near its coasts, especially in shallow 

 inlets, than at a greater distance from the shore ; and the number of dead organisms 

 is also greater near the coast. A mass of organic remains is thrown up by the 

 tide, and by waves in stormy weather. This mass rots during the ebb. Part of it 

 is dragged out to sea again by the next high tide, and then flung up once more; 

 so that the beach is always strewn with dead remains, and the sea near the shore 

 contains more products of decomposition than in the open. 



In the immediate neighbourhood of seaports, moreover, or wherever people 

 live, the volume of refuse is considerably increased, and the water in harbours and 

 statrnant inlets behind breakwaters, and at the mouths of canals and sewers, contains 

 such a large quantity of organic refuse in a state of decomposition that its presence 

 is revealed by the odour emitted. Now it is just at these places that an abundant 

 vegetation of hydrophytes is developed. Not only the bottom of shallows, but 

 stones, stakes, quays, buoys, and even the keels and planks of boats long anchored 



