NUTRIENT SALTS. 71 
water of this spring is hard, and it deposits lime at a little distance from the source. 
Exactly at the spot where it wells out of a fissure in the rock its bed is entirely 
filled by a dark-brown floceulent mass which consists of millions of cells of the 
beautiful Odontidiwm 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 Sturmiana 
and Sawifraga oppositifolia) with leaves aggregated together in elosely-erowded 
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 be 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 extremely 
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 flow 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 colleet 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 silieie acid may become of account in course of time. There is more 
difficulty in understanding how plants with roots in the earth set about utilizing 
substances contained in the soil in scarcely appreciable quantities. These plants 
