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 effloresce 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, &c., 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 



