NUTRIENT SALTS. 71 



water of this spring is hanl, and it deposits lime at a little distance fi-om the source. 

 Exactly at the spot where it wells out of a fissui-e in the rock its bed is entirely 

 mied by a dai'k-brown flocculent mass which consists of millions of cells of the 

 beautiful Odontidlum hiemale, a species of diatom with siliceous coating. These 

 cells are ranged together in long rows, and are present in numbers and luxuriance 

 such as are scarcely ever to be observed in other situations. Yet the spring water 

 flowing round contains so little silicic acid that no trace of this substance could be 

 discovered in the residue from the evaporation of 10 litres. 



An instance similar to this of silicic acid, is afforded by the iodine in the sea. 

 Most of the sea-wracks inhabiting the North Sea contain iodine, many indeed in 

 considerable quantity, and yet we have not hitherto succeeded in detecting iodine in 

 the water of the North Sea. Similar phenomena, sometimes quite baffling explana- 

 tion, are exhibited by land-plants. The clefts in the rocks of quartziferous slate in 

 the Central Alps are, in many places, overgrown by saxifrages (Saxifraga Sturniiana 

 and Saxifraga oppositifolia) with leaves aggregated together in closely-crowded 

 rosettes, which are conspicuous from afar, owing to their pale colouring. On 

 closer inspection one finds that the apices and edges of these rosulate leaves are 

 covered with little incrustations of carbonate of lime, a substance which will be 

 frequently referred to in connection with its importance to plants. But one seeks 

 in vain for any lime compound in the earth which fills the clefts, and the only 

 traces of lime contained in the adjacent rock itself are those occurring in the little 

 scales of mica scattered about, and these are not readily decomposable. Yet the 

 lime incrusting the saxifrage leaves can only be derived from the underlying rock, 

 just as in former instances the silicic acid in the cell-membranes of diatoms 

 must lie secreted from the spring described, the iodine in sea-weeds from the 

 sea, and the common salt in water-lilies from the pond where they grow, although in 

 each case the substance concerned is only to be found, if at all, in scarcely ponder- 

 able traces in the soil or liquid serving as medium. Facts of this kind have a 

 special interest, because they prove that plants have the power of appropriating a 

 substance, if it is important to them, even when it is only present in exti'emely 

 minute quantities. Where a plant is surrounded by liquid, we can well imagine 

 that fresh portions of the medium are constantly coming into contact with its 

 surface: for, even in water apparently still, compensating currents are con- 

 tinually being caused by changes of temperature. Thus, in the course of a day, 

 thousands of litres of sea-water may fiow over a sea-weed with a surface of 

 one square meter, and, even if only a small portion of the substance, traces of 

 which we are supposing to exist in the water, is wrested from each litre, still, 

 the absorbing plant might collect quite a profitable quantity in a number of 

 days. The volume of water flowing over a plant situated in the source of a 

 spring is still greater, and it is readily conceivable that even the most minute 

 trace of silicic acid may become of account in course of time. There is more 

 difiiculty in understanding how plants with roots in the earth set about utilizing 

 substances contained in the soil in scarcely appreciable quantities. These plants 



