14 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



directly as do many of the results obtained by different paths of 

 inquiry which branched out from the more special questions 

 just touched upon. The researches of De Bary, Brefeld, Cohn, 

 Pasteur, and their pupils had at various periods since 1860 led to 

 much investigation of the contents of soils. Among other 

 matters it was soon found that common garden and field soils 

 abound in the spores of fungi, yeasts, and bacteria, and a natural 

 inquiry was, what are these organisms doing in the soil ? 



Vittadini in 1852, and Brefeld and Klebs subsequently, in- 

 vented and developed, and Koch and others have improved, a 

 method of microscopic gardening which has led to most astound- 

 ing results in these directions, and I cannot resist giving you a 

 slight summary of some of these valuable and suggestive results. 



The old Greek naturalists thought that the food of plants 

 was elaborated beforehand in the earth, as in a stomach, and it 

 took centuries of work to establish the fact that what the 

 ordinary plant takes up by its roots in the absorbed water is only 

 mineral matters of the ash, constituting but a minute fraction of 

 the food-materials of the plant (absolutely essential, however), 

 to be worked up in the leaves with the far larger quantities of 

 gases there taken in and assimilated in the chlorophyll apparatus 

 by means of energy obtained from the sun. 



It was part of the price to be paid for rescuing the physiology 

 of plant-nutrition from the grip of the old ideas of Aristotle, in 

 the disastrous more modern form they had assumed in 1835-40 

 when the humus theory held sway, that soil came to be regarded 

 as merely a mineral medium of value to the plant in proportion 

 to its contents in certain chemical salts. 



Soil, as we now know, is really an extremely complex 

 medium. It is true the substances it affords to plants — I am 

 speaking of ordinary green plants— are entirely the small quan- 

 tities of mineral salts needed for the ash constituents, and 

 forming only about 1 per cent, to 2 per cent, as a rule, or a little 

 more, of the whole dry weight, the rest coming entirely from the 

 air ; but since these mere traces of mineral salts are to be found 

 in practically every soil it is clear that their presence or absence 

 il by do means the determining factor of the value of a soil. 

 The structure, porosity, capacity for retaining heat and moisture 

 and various -ases, and a score of other physical properties, are 

 now known to be far more important factors in most cases. 



