74 NUTHIEXT KAi;i'S. 



substances ceases ami detriincutal action Ijcgins, is not known more pi't-ciscly tlian 

 has been stated. We only know that different pkints behave very differently in 

 this respect. Suppose, tor example, that we scatter wood-ash over a fiekl which is 

 ove^•gro^vn by grasses, mosses, and various herl)s and shrubs. The result is that the 

 mosses die; in the case of the grasses growth is somewhat increased; whilst some of 

 the lu'i-lis and shrubs, notably polygonaceous and cruciferous plants, exhibit a sti'ik- 

 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 quantitj- of lime for food, whilst others require similarly silicic 

 acid. Hereupon was founded a division of plants int(j 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 satisfactoiy, at any rate in 

 the case of siliceous plants. It is much more probable that the so-called silica- 

 loving plants are produced on gi-ound composed of ((uartz, granite, or slate, not by 

 reason of the abundance of silicic acid, but because of the absence of lime in an}' 

 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 jiarticular food-salts and onlj' flourish luxuriantly when 

 these nutritive salts are not meted out too sparinglj-. In the case of oraches, 

 thrifts, wormwood species, and cruciferous plants, alkalies, in comparatively large 

 quantities, are necessarj^ for hardj' development. The proper haliitat for these 

 plants, therefore, is on soils which contain an abundance of easily soluble 

 alkaline compounds, in places where the ground is regularlj' saturated by saline 

 solutions, and whei-e crystals of salt effloresce on the dr3-ing surface. Such places 

 are the sea-shore, the salt steppes, and the neighbourhood of salt-mines. The 

 above plants not onlj' flourish in these localities in great aljundance and perfection, 

 but they supplant all other species on w^hich 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 onl}^ thrive in 

 sea-water. Most of the species used by us as edible vegetaViles, as, for instance, 

 cabbages, turnips, cress, «Sic, are reallj' descended from halophj'tes, and accordinglj- 

 require a soil that contains a comparatively rich supply of alkalies. An oppor- 

 tunity will occur, later on, of I'eturning to the question as to how far agriculture 

 has gained by all these discoveries, and of considei-ing what processes, based upon 



