CHAP, iv.] The Food of Trees 81 



Pine 1 . Magnesia (MgO) is utilized in the formation of 

 albuminoid substances, and is supposed to have an important 

 influence with regard to the production of seed. Iron (FeO 

 and Fe 2 O 3 ) is requisite, though only in small quantities, for 

 the formation of the chlorophyll in the foliage, by means 

 of which the work of assimilation is carried on with the aid 

 of sunlight. Sulphur, taken up in the form of sulphates, i. e. 

 salts containing sulphuric acid 2 , is, like magnesia, employed 

 in the formation of albuminoid substances, for which also 

 Phosphorus, in the active form of phosphoric acid (P 2 O fl ), is 

 most essentially requisite. The presence of Chlorine (Cl) and 

 of Soda (Na) is most pronounced in plants growing near the 

 sea-coast and on brackish soil. The specific action of these 

 minerals is not very clearly understood ; but they are supposed 

 to assist in the movement of carbo-hydrates within the plant. 

 For the practical purposes of Forestry the most important soil- 

 nutrients are water, then lime, potash, and phosphoric acid ; 

 and the chief nitrogenous compounds are nitrates and am- 

 moniacal salts or certain nitrogenous organic constituents in 

 humus. 



On soils which offer to woodland crops richer supplies of 

 food than can be thoroughly assimilated by them, deposits 

 of mineral substances take place within the plants in excess 

 of their physiological requirements. Wherever this occurs in 

 any excessive degree, it leads to a condition at once pre- 

 disposing to disease and, at the same time, capable of offering 

 least resistance to any attacks that may be made by fungoid 

 parasites or by insect enemies. 



1 Annales de la Station Agronomique de I'Est, 1878, pp. 3 et seq. 



2 A common impurity to a slight extent in the atmosphere is sulphurous 

 acid, which is carried by rainfall into the soil, and oxidizes into sulphuric 

 acid (SO 2 +-H 2 O becomes H 2 SO 4 X When present in the air in anything 

 like excess, as in large towns and at all populous and manufacturing centres, 

 where sulphurous acid is evolved on the combustion of mineral coal, it 

 affects the thriving of trees. The sulphurous acid is imbibed by the foliage, 

 and, on rain falling, becomes transformed into sulphuric acid ; this rapidly 

 acts as an irritant poison with regard to the internal, soft, cellular tissue of 

 foliage. 



G 



