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ROYAL HORTICULTURAL SOCIETY. 



weights of carbon, nitrogen, and hydrogen in plants, which 

 proceed on such fundamentally erroneous prejudices, are alto- 

 gether worthless, and lead only to perplexity in things of every- 

 day experience. The carbonic-acid theory has long retarded and 

 confused the physiology of plants, and separated science from 

 practice, as its theories are practically useless. Such errors and 

 such ignorance must be expressly and clearly exposed, in order 

 first to get rid of them and then to make progress possible to 

 something better. 



The fertility of the soil depends upon its capability of furnish- 

 ing nutritive dissolvable substances, with which the surface-water 

 may be impregnated. The humus must be capable of being 

 dissolved. The insoluble condition of ulmin in peat does not 

 allow water to extract from it nutritive matter ; therefore peat- 

 moss, though moist, is sterile without contact with the air through 

 desiccation. The soluble humous constituents of the soil must 

 be difficult of solution, in order to their entering into the water in 

 a very weak proportion, since plants can receive nourishment in 

 very weak solutions only. This is a very important point, that 

 plants can endure no concentrated food. The richer the soil, the 

 greater must be the quantity of moisture to attenuate properly 

 the nutritive liquid. This is the reason why strong manure is so 

 injurious in a dry soil. Inquiries into the injury of plants from 

 concentrated manure, suggested the idea to Ingenhousz that dung 

 does not enter principally into plants in the form of a solution, 

 but must be converted into a gaseous form by decomposition, and 

 that carbonic acid and nitrogen are the peculiar nutritive elements 

 of plants, — a theory which cannot be made to agree with the 

 practice of manuring. 



A consequence of the reception of very attenuated nutriment 

 is the great need of water, and the great quantity which plants 

 consume. The strong evaporation of plants, on which we have 

 the old statistical experiments of Hales, Duhamel, and Bonnet, as 

 well as the more recent ones of Schubler, corresponds with this 

 large quantity of water. A surface of water covered with Pistia 

 Stratiotes, according to Isert, gives off six times as much as that 

 which is free. The negroes in Gruinea, as Forster relates, place 

 this plant in pots of water at the doors of their houses to cool the 

 air. Schubler found that a square foot of meadow-ground, covered 

 with grass, gave off twice as much evaporation as 40-45 cubic 

 inches of water. The evaporation of a moderately large potato- 



