^ On Agricultural Chemistry. 



372 



though I have not at present traced the changes which take place 

 during the growth of wheat, it appears to me that when sown in 

 a soil containing abundance of azotized matter it empk)ys this 

 substance at first in extending its leaf, and that where an excess 

 of ammonia is supplied the production of leaf is increased to an 

 extent greatly injurious to the next operation of the plant, which 

 is to produce stem. If an excess of ammonia is added late in 

 the spring the plant will no longer increase in leaf, but in stem 

 or straw, which also may be increased to an injurious extent. 

 When the azotized and mineral matters are properly balanced, 

 the plant will produce no more of each organ than is essential to 

 the favourable production of its seed. Up to the period of 

 blooming the compounds of nitrogen derived from ammonia are 

 probably in a fluid or suspended state, circulating through the 

 whole of the plant ; but to what extent starch exists in the plant 

 at this period is doubtful. When the time of blooming is passed, 

 it is probable that the wheat derives but little nourishment from 

 the soil, at all events, if a crop shows symptoms of poverty, it is 

 always before this period. The circulating condition which has 

 prevailed throughout the plant, is now changed, and under a 

 favourable condition of climate (heat, light, and dryness) an 

 elaborative action commences ; the compounds of nitrogen are 

 withdrawn from the leaf and stem, and deposited in the seed, 

 while starch is accumulated in a hard granular form. This de- 

 posit of starch only takes place perfectly under the influence of a 

 high temperature ; the seed is then hard, dry, and plump. In a 

 cold and wet summer the interstices of the grain are not perfectly 

 filled ; watery fluids occupy the place of starch, and, when these 

 have evaporated, the grain is thin and shrunk. The wheat that 

 is grown in a wet summer might therefore contain as high a per 

 centage of nitrogenized matters dependent on the sap as that pro- 

 duced during a hot and dry season. The formation and elabora- 

 tion of starch and other carbonaceous compounds which for the 

 most part supply man with his respiratory or heat-producing 

 elements, are, it seems, greatly favoured by a hot climate, and it is 

 probable that the heat capable of being eliminated by the process 

 of animal respiration, must first have been rendered latent during 

 the growth of the plant. 



Looking at the present state of man's existence on the earth, 

 it may appear improbable that the value of corn should ever be 

 in proportion to its carbonaceous product. A time may arrive, 

 however remotely, when the surface of the earth will be peopled 

 with men very far advanced both in their moral and physical 

 condition, compared with its present occupants. Bread and meat 

 will then constitute the chief sources of food — the one supplying 

 respiration, the other nutrition ; and they will doubtless bear a 



