Proceedings of the Linnean Society of New South Wales/Volume 44/On the identity of Polypodium spinulosum Burm.f.
some extent overcome by the osmotic pressure of substances in the soil solution, and a plant may be compelled to take up a larger amount of substances like soda or magnesia than it requires, and which may be decidedly harmful. Such a case is furnished in my own experience by the growth of sugar-cane on soil infiltrated with brackish water, on certain parts of the Clarence River, New South Wales, and on what is known as Tiri land in Fiji—low-lying land subject to inundation with sea-water. In such cases, quite excessive amounts of chlorides are found in the juice when the cane is crushed.
Another interesting point which requires bearing in mind in case of further investigation of the water from plants, is that brought out by Professor H. H. Dixon[1], as to the variation in concentration in different parts of the plant. Dixon finds that the concentration of solids is greatest in the sap from the roots, and becomes less in the upper parts of the plant. This was determined by the electrical conductivity and freezing-point methods in samples of sap obtained by centrifuging pieces of the roots and stems. The figures obtained by both methods are proportional to the amount of dissolved substance present, and are in close agreement. It was found in the case, for instance, of Acer pseudoplatamus, that the sap in the branches at a height of 30 feet had only two-thirds the amount of dissolved substances as that in the roots. In another plant, Colocasia antiquoriim, a similar distribution was observed, and it is interesting to note that the droplets of liquid transpired from the tips of the leaves consisted not of sap, but of practically pure water.
These results, as regards the greater concentration of the sap in the roots, are certainly contrary to what I would have expected, but Professor Dixon's experiments are most lucidly described, bear evidence of careful execution, and seem to me quite convincing. The Rev. W. B. Clarke,[2] without giving his authority, states his belief that it had been ascertained that the specific gravity of the sap is least in the roots. This would be a most interesting subject for further observation. It will be noticed that, in the Red Mallee water, the proportions of soda and of chlorine are high relative to the other ingredients. In a series of analyses of Mallee soils published many years ago by the Agricultural Chemist of Victoria,[3] the amounts of soda and of chlorine vary within wide limits, the chlorine in one case rising as high as 0.20 per cent. of the soil. This corresponds to about 0.33 per cent. of sodium chloride, which is a high percentage for a soil, and is quite sufficient to account for the large proportion of soda and chlorine in the Mallee root-water.
Not all samples of sap obtained in the manner of the Mallee root-water are so dilute; Smith, for instance,[4] found enormously greater amounts of both mineral and organic solids in the sap from the timber of Grevillea robusta, while the juices of such plants as the sugar-cane and sugar-maple, carry large percentages of sugar, though not of mineral constituents; it must, however, be remembered that in the case of these latter plants the sweet sap is contained in special cells or vessels, and is not the ordinary circulating sap, but apparently functions as a food reserve. In the specimen of these roots exhibited by Mr. Fletcher, the cut ends showed a remarkable assemblage of open vascular tubes, reminding one strongly of a section of cane. From a consideration of the circumstances, I am inclined to think that the Mallee root-water may be a reserve supply for the use of the plant during droughty conditions stored in special vessels.