74 NUTRIENT SALTS. 
substances ceases and detrimental action begins, is not known more precisely than 
has been stated. We only know that different plants behave very differently in 
this respect. Suppose, for example, that we scatter wood-ash over a field which is 
overgrown by grasses, mosses, and various herbs and shrubs. The result is that the 
mosses die; in the case of the grasses growth is somewhat increased; whilst some of 
the herbs and shrubs, notably polygonaceous and cruciferous plants, exhibit a strik- 
ingly luxuriant growth. If we scatter gypsum instead, the development of clover 
is enhanced, and, on the other hand, there are certain ferns and grasses that die 
earlier when gypsum is supplied, or, at least, are considerably stunted in their 
growth. 
The fact that certain plants predominate on calcareous and others on siliceous 
ground has been the subject of very thorough investigation; and these researches 
were regarded as justifying the assumption that particular species require a more or 
less considerable quantity of lime for food, whilst others require similarly silicic 
acid. Hereupon was founded a division of plants into those which required and 
were tolerant of lime, and into such as required and tolerated silica. The explana- 
tion given of these facts does not seem, however, to be satisfactory, at any rate in 
the case of siliceous plants. It is much more probable that the so-called silica- 
loving plants are produced on ground composed of quartz, granite, or slate, not by 
reason of the abundance of silicic acid, but because of the absence of lime in any 
large quantity, such as would be liable to injure plants of the kind; for only traces 
of lime are found, and its presence to this extent is absolutely necessary for every 
plant. This is not of course inconsistent with the fact that individual species 
require larger quantities of particular food-salts and only flourish luxuriantly when 
these nutritive salts are not meted out too sparingly. In the case of oraches, 
thrifts, wormwood species, and cruciferous plants, alkalies, in comparatively large 
quantities, are necessary for hardy development. The proper habitat for these 
plants, therefore, is on soils which contain an abundance of easily soluble 
alkaline compounds, in places where the ground is regularly saturated by saline 
solutions, and where crystals of salt eflloresce on the drying surface. Such places 
are the sea-shore, the salt steppes, and the neighbourhood of salt-mines. The 
above plants not only flourish in these localities in great abundance and perfection, 
but they supplant all other species on which the excessive provision of soluble 
alkaline salts is not beneficial. If the seeds of such plants happen to fall upon the 
salt ground they germinate, but only drag out a miserable existence for a short 
time, and in the end are crowded out by the luxuriant oraches and crucifers. 
Plants which only flourish abundantly on soils rich in alkaline salts are called 
halophytes. The same name has also been applied to plants which only thrive in 
sea-water. Most of the species used by us as edible vegetables, as, for instance, 
cabbages, turnips, cress, &e., are really descended from halophytes, and accordingly 
require a soil that contains a comparatively rich supply of alkalies. An oppor- 
tunity will occur, later on, of returning to the question as to how far agriculture 
has gained by all these discoveries, and of considering what processes, based upon 
