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, &e.) 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. 
SAPROPHYTES IN WATER, ON THE BARK OF TREES, AND ON ROCKS. 
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 
stagnant 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 
