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matters. Some, as Boussaingatlt, go so far as to regard the dark mucilaginous matters said to be 

 exuded by roots in water as the eifect of a diseased action, denying the excretion, but this cannot be 

 maintained, for amphibious plants as mints, cress, Myosotis palustris and other species, which are 

 not placed in an abnormal situation when growing in water, yield this substance. The experiments 

 of Gyde appear also explicit on this point. If we are to receive the theory of Dutrochet, that the 

 penetration of tlie soil fluid through the roots is a phenomenon of Endosmosis, there is a necessity of 

 admitting the passage outwards of a portion of the elaborated sap, which Gyde states to be identical 

 in composition with the excrementitious matters collected by himself That none should be obtained 

 from sand, or soil, under certain circumstances, is not surprising, for the exposure of the exuded 

 matter, over a large surface and in contact with oxygen absorbed from the air, would rapidly change 

 it into a new body capable of escaping the ordinary tests — in the same way that alcohol by mere 

 exposure over an extended surface is rapidly converted into acetic acid by oxydation. Although it 

 is premature to deny that a portion of elaborated sap does escape from the root of some plants, it is 

 very evident that this does not create a deposit injurious to the future growth of the species and is 

 not the principle on which rotations are to be devised- 



The natural succession of plants is connected with the presence of organic matter in the soil. 

 The richest weeds which first occupy the surface having the greatest necessity for it, and thus 

 through successive groups to the grasses and forest trees which grow well without any portion in 

 the soil. Other elements of fertility being present, the Chenopodiaceous and allied families thrive 

 only in such localities as yield azotized matters, since they cannot grow without a supply from'the 

 soil. This surmise is sanctioned by the obvious presence of organic matters in the soils where they 

 grow, and by the fact that some species exhale ammoniacal gases, but it is fully established by the 

 experiments of Boussaingault. This chemist grew clover, peas, wheat and oats in a soil 

 completely destitute of organic matter and supplied them with distilled water only ; the clover and 

 peas were found to double their azotized matter during growth, whilst the oats and wheat gained 

 none whatever. As there was but one source of azote present, the atmosphere, it is apparent that the 

 former have the capacity of supplying themselves therefrom, whilst the grain plants are altogether 

 dependent on the soil. Hence in a soil charged with organic matters, rich in azote, those plants 

 which require a supply by their roots will grow freely, and so far exhaust it in time as to render it 

 unfit for the species, which is succeeded by an intermediate class, and finally by the Graminie, Legumi- 

 nosee, and others capable of subsisting on aerial azote, and so far from exhausting, adding it to the soil. 

 From this function of plants, we see an explanation of the natural rotation, and what is of more 

 moment, a means of adapting our succession of crops to the accumulation as well as removal of 

 azotized matters. 



