CHEMISTRY IN RELATION TO HORTICULTURE. 



105 



formed. Hence the conclusion — since abundantly confirmed — was come 

 to that these plants were able, through the agency of the organisms 

 contained in the nodules on their roots, to take up and assimilate atmo- 

 spheric nitrogen. In this way has been explained what was for a long 

 time a mystery, viz. that clovers, peas, beans, while themselves essentially 

 nitrogenous in composition, were able to dispense with special nitrogenous 

 manuring and at the same time left the soil on which they had been 

 growing richer in nitrogen than before. It would seem possible, too, that 

 there are certain low forms of organism in the soil which possess this 

 same power, but with this exception the capability would appear to be 

 restricted to the Papilionacere. Arising from these discoveries, attention, 

 has been more recently directed to means of inoculating the seed before 

 sowing with materials that may ensure the production of nodules on the 

 roots and the consequent utilisation of the atmospheric nitrogen. 



Besides oxygen, nitrogen, carbonic acid, and a small quantity of 

 ammonia, the atmosphere will contain varying amounts of other con- 

 stituents, such as nitrates, nitrites, chlorides, and sulphates, the quantity 

 of which will differ according to local circumstances. Thus, in proximity 

 to the sea there will be found much more salt (sodium chloride) than 

 elsewhere, and the presence of factories in the neighbourhood will ciuse 

 increase in the sulphur compounds present. Plants of maritime origin, 

 like the cabbage and the mangel, will be favoured by the presence of an 

 atmosphere carrying salt with it, while the same fact accounts for the 

 profitable employment on them of salt as a manure. 



While the useful character of the constituents of the atmosphere has 

 been spoken of, it must be borne in mind that the atmosphere, by reason 

 of the pollution it may undergo at the hands of man, may become the 

 carrier also of ingredients harmful to, or destructive of, plant-life. Of 

 such nature are the emanations from factories and the smoke of densely 

 populated towns. Much has been said against London smoke, but it is 

 probable that the harm done to vegetation is mainly due to the mechanical 

 blocking up of the breathing surfaces of the leaves by the solid particles 

 carried in the air rather than to the emission of any directly poisonous 

 gases, and, despite all, it is astonishing how well trees and plants do 

 grow even amid the untoward surroundings. The subjection of trees 

 and plants to the emanation of gases coming from the chimneys of 

 alkali works, smelting and blast furnaces, and the like, is, however, of a 

 very different nature, and where such exist there is not merely the dis- 

 comfort from the presence of solid black particles carried over, but there 

 is the positive blighting effect of gases such as sulphurous acid and hydro- 

 chloric acid, and the injury done by arsenical fumes and emanations 

 containing copper, lead, zinc, and other harmful metals. To these 

 vegetation shows itself most susceptible, and the devastation caused to 

 trees, hedges, and plant-life generally in a district where factories abound 

 cannot fail to be recognised. Trees are variably sensitive to such 

 influences, conifers in particular being most readily attacked. 



The extent of such damage will depend much upon the surroundings, 

 the force and direction of the prevailing wind, the extent of saturation 

 of the atmosphere, and, of course, the intensity and continuity of the 

 emanations. A sudden emission of a heavily laden blast will do harm 



