INTRODUCTION. 5 



the fanciful fabrics of the Aristotelians. They were 

 the founders of that science of vegetable physiology, 

 which, enlarged and carried into practice by the 

 late Mr. Knight and others, has advanced horti- 

 culture to a degree of improvement, undreamed of 

 by their immediate predecessor, Heresbach, when 

 he informed the world that, if the powder of rams' 

 horns is sown, and well watered, " it will come to be 

 good aspai'agus." 



The researches of Hales upon the circulatory 

 power of the sap-vessels, of Bonnet upon the func- 

 tions of the leaves, and of Du Hamel, Priestley, 

 Ingenhousz, Sennebier, Saussure, and others, upon 

 the action of hght, and the nature of the gases de- 

 veloped during the respiration of plants, imparted 

 still more useful knowledge to the gardener, and 

 rendered his art still less empirical. 



The same philosophers directed their attention 

 also to the food of plants imbibed by their roots, 

 and to the examination of their various secretions ; 

 but here they were joined by another band of na- 

 ture's students, and no one conversant with the phi- 

 losophy of plant-culture but will remember the debt 

 he owes to Vauquelin, Lavoisier, Johns, Da^y, Liebig, 

 Lindley, Johnston, and Low. 



It has been my endeavour to concentrate and arrange 

 the results of the researches of the above-named disci- 

 ples of nature in the following pages, — adding such rays, 



