Outlines of Horticultural Chemistry. 203 



being united together by excentric rays of ligneous tissue, 

 which diverge from the pith, and extend to the outer bark. 



This hypothesis (for I shall not call it sound doctrine) is 

 plausible, in accounting for several of the phenomena of 

 .vegetation; the individuality of buds as perfect plants or 

 essences ; the cause of the different appearance in the closing 

 lips of a wound ; the circumstance of variegated plants rising 

 from the roots of unvariegated plants. It gets rid of the 

 greatest difficulty of all; namely, the convertibility of the 

 matured sap into all the parts and properties of plants; and 

 also accounts for the circumstance of buds placed on alien or 

 unsuitable stocks, forming roots for themselves. Neither is it 

 admitted by Dr. Darwin that there is any return of the sap 

 downwards, but that all the time it is in motion its course is 

 from the root to the extremities of the branches ; and that by 

 the increasing cold of autumn it is gradually inspissated, losing 

 its fluidity, and at last is wholly arrested by the cold of winter, 



I am, Sir, yours, 

 Chelsea, May 22. James Main. 



Art. IV. Outlines of Hortiadtural Chemistry : — Manures. 

 By G. W. Johnston, Esq., of Great Totham, Essex. 



(Continued from p. 81.) 



Manures are of benefit to plants, by affbrding some of the 

 gases of the atmosphere to their roots, in a concentrated form. 

 A soil, when first turned up by the spade or plough, has 

 generally a red tint, of various intensity, which, by a few 

 hours' exposure to the air, subsides into a grey or black hue. 

 The first colour appears to arise from the oxide of iron, which 

 all soils contain, being in the state of the red, or protoxide ; 

 by absorbing more oxygen by the exposure, it is converted into 

 the black, or peroxide. Hence, one of the benefits of fre- 

 quently stirring soils : the roots of incumbent plants abstract 

 the extra-dose of oxygen, and reconvert it to the protoxide. 

 Coal-ashes, in common with all carbonaceous matters, have 

 the power of strongly attracting oxygen. Every gardener may 

 have observed how rapidly a bright spade becomes covered 

 with rust, or red oxide, which is left foul with coal-ashes. All 

 animal and vegetable manures absorb oxygen from the air 

 during putrefaction. If it is enquired of what benefit this 

 property is to plants, since the gases are freely presented to 

 them in the atmosphere, it admits the ready answer, — that they 

 enjoy the additional quantity which is thus collected to the 



