ON THEORY AND PRACTICE. I 



knowledge to be required, by imitating- a practitioner ; but the know- 

 ledge to be obtained from mere practise, however successful, is little 

 to be relied upon ; for although the cause of success may assist, the 

 practitioner may be ignorant of it, and therefore, although the same 

 practice may be followed in another situation, it may fall altogether, 

 because the cause is not there established. If it be an axiom, that 

 every production of nature and of art, is the effect of some cause— it 

 must follow, that before any effect can be produced, the cause must be 

 established ; and before any effect can be effectually prevented, the 

 cause must be removed ; and consequently, before any person can 

 establish or remove a cause, they must know what it is. Then how is 

 a knowledge of the causes of effects to be obtained ? Certainly by no 

 other means than by first forming a Theory, and then putting it to the 

 (est of practical demonstration, to ascertain its truth or falsehood. If 

 a theory be thus proved to be true, the knowledge of it is science. I 

 consider the causes of all effects to be certain elementary principles 

 established in nature, and which are brought into action or rest, and 

 made to exist in a separate or combined state, and to undergo certain 

 changes in form and duration of their existence, by certain immutable 

 laws of nature. My efforts have long been directed to the ascertaining 

 the true causes of the different effects it is desirable to produce by the 

 cultivation of plants ; as well as the true causes of those effects, it is 

 desired to prevent ; and for this purpose, I have not only put my own 

 Theories to the test of repeated practical experiment, but also most 

 of the Theories of the celebrated Physiologists and Chemists, and 

 practical Professors of Horticulture. And in this, all who have done 

 me the honour to visit my garden, admit, that I have established 

 many important principles of practice : my practical elucidations are 

 more particularly exhibited in the training and feeding of fruit-trees ; 

 I say feeding, because it is an obvious fact, not only that plants require 

 food, to sustain tliem, as much as animals do, but that their growth 

 and productions are determined by the quality and the quantity of the 

 food they are supplied with. As to give such an explanation of the 

 nature and properties of different soils, and of different manures, or 

 the elements of the food of plants, would occupy more of your pages 

 than you can afford, you will probably allow me to refer such as wish 

 to make a minute enquiry into those subjects, to a little work I have 

 lately published, " On the Causes of the Barrenness and Fruitfulness of 

 Plants and Trees." I will, however, beg leave now to oiler the 9th law of 

 nature, in my arrangement, for the immediate consideration of your 

 leaders, and shall be ready to give any further information in my power : 

 tlie 9th law "The leaves form the excretory Organs of a Plant orTree; 

 and whether the supply of food be great or small, aplant or tree cannot 

 attain, nor sustain itself in, a perfect state of fructification, until it is 

 furnished with a surface of leaves duly proportioned to the sap sup- 

 plied bj the roots. To enable them (o perform their fiincfh 



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