CHAMBERS'S INFORMATION FOR THE PEOPLE. 



a plant is burned, these elements are driven off in 

 the form of gases, and the other elements that had 

 entered into its composition remain as ash. It 

 has been customary to call the four elements that 

 disappear by combustion, the organic elements, 

 and those that remain in the ash, the inorganic 

 elements of plants. But the distinction is without 

 ground. All the elementary substances that are 

 constantly found in a plant, must be considered as 

 necessary parts of its structure or organism, and, 

 in this position, are all equally organic. 



With the exception of oxygen, none of the 

 elements found in plants are taken up in the 

 elementary state, but only in a state of chemical 

 combination with other elements. Thus carbon 

 by itself is not food for a plant ; it must first be 

 united with oxygen, forming carbonic acid, and in 

 this state it is readily imbibed. Similarly, plants 

 do not take up simple nitrogen, but nitrogen com- 

 bined with hydrogen, in the shape of ammonia. 

 The substances thus taken up as food by the plant, 

 become united by means of its vital action into 

 definite chemical compounds, called organic com- 

 pounds, because they exist or are formed only in 

 organised beings. Chemists, however, have now 

 very generally adopted the views of Liebig, that 

 the compounds which constitute the food of plants 

 are all inorganic. The inorganic substances, car- 

 bonic acid, water, and ammonia, with the alka- 

 line and earthy substances found in the ashes 

 of plants, are now regarded as their only nourish- 

 ment 



The organic compounds of plants may be 

 divided into two great classes nitrogenous and 

 non-nitrogenous ; that is, those which contain 

 nitrogen, and those which are devoid of it 



We know little or nothing of the chemical pro- 

 cesses that take place in the interior of plants 

 during their growth. The manner, indeed, in 

 which the particular compounds are determined is 

 still involved in complete mystery. Liebig, how- 

 ever, with his usual sagacity, has pointed out that 

 phosphorus and sulphur are always associated 

 with nitrogenous matter, and that the alkalies and 

 earthy bases direct the formation of substances 

 devoid of nitrogen. 



The growth of all plants takes place by the for- 

 mation of cells, and the nitrogenous structure of 

 cells precedes the non-nitrogenous. In all prob- 

 ability, it is for this reason that phosphates act so 

 favourably on the growth of some plants when 

 applied as manure at an early stage. By phos- 

 phates assisting in the formation of the cells of the 

 rootlets of plants, rapid growth is induced. The 

 plants are put in possession of the soil more 

 quickly by obtaining a ready supply of phosphates, 

 and thus obtain the other matters which are fitted 

 for their growth. 



The nitrogenous substances found in plants and 

 in animals are usually of the same composition, 

 though they differ in their form. All kinds of 

 wood contain quantities of nitrogenous or albu- 

 minous matter, which is identical with the muscle 

 of animals, but most frequently it is not in a fit 

 condition to be assimilated by the larger class of 

 animals. An oak-tree, with its acorns, is composed 

 of nearly the same elements as a ripened plant of 

 wheat. 



It has been -truly said by Dr Anderson, that as 

 the analysis of plants has become more perfect, so 

 has the difficulty of explaining the necessity of a 



514 



rotation of crops, on mere chemical grounds, 

 become greater. 



The analyses of the ashes of plants by Professor 

 Way, contained in the volumes of the Royal 

 Agricultural Society, put this in a strong light. 

 The subjoined table exhibits the quantity of the 

 alkaline and earthy matters found in an acre's 

 produce each of wheat, turnip, flax, mangel-wurzel, 

 beans, and maize. The wheat-crop is reckoned 

 at 28 bushels of grain and 18 hundredweight of 

 straw ; the turnip at 20 tons of bulbs and 4 tons 

 of tops ; the flax at 20 bushels seed and 2 tons of 

 straw ; the mangel at 20 tons of bulbs and 4 tons 

 tops ; the bean at 35 bushels grain and I ton of 

 straw ; the maize at 48 bushels grain and \\ ton 

 of straw. 



The marked feature in this table is the large 

 amount of alkalies and alkaline earths that the 

 fallow crops, turnips and mangel-wurzel, require 

 in comparison with wheat. Silica enters largely 

 into the composition of wheat-straw, and only 

 sparingly into other crops. The turnip has more 

 phosphates in a crop of twenty tons than any of 

 the others in the table ; still, the beneficial influ- 

 ence which phosphates often have when applied 

 as manure to this crop, must be attributed to other 

 causes than to the mere extra quantity in their 

 composition. 



Soils can be fertile only if they contain a supply 

 of alkalies and earthy bases sufficient for the full 

 development of plants. But soils become exhausted 

 for many kinds of plants, even when they contain 

 abundance of these elements. Some plants are 

 more dependent than others on a supply of carbonic 

 acid and ammonia in the soil. Such differences 

 in the requirements of plants lead us to treat of 



THE PHYSIOLOGY OF AGRICULTURE. 



Carbonic acid, water, and ammonia, which con- 

 stitute the chief food of all plants, exist in the 

 atmosphere and in the soil Plants have the 

 power of obtaining them from both these sources 

 taking them from the atmosphere by means of 

 their leaves, and from the soil by their roots. 

 Carbonic acid and ammonia become fixed in the 

 leaves of plants by virtue of chemical affinities 

 directed by the vital agencies. Two essential 

 conditions of atmospheric absorption are first, 

 that the soil supply the requisite quantity of 

 moisture ; second, abundance of the substances 

 found in the ashes of plants. These conditions 

 being favourable, the amount of carbonic and 

 ammonia that a plant will draw from the atmos- 

 phere will, other things being equal, depend upon 

 the surface of leaves which it exposes to the air 

 during the growing season. An oak-tree, which 

 has an immense surface of leaves exposed to the 

 atmosphere, has greater facilities of abstracting 

 carbonic acid and ammonia than the wheat or 



