NUTKITION. 



43 



that, for practical purposes, the other ingredients 

 may be almost disregarded. Solutions in water 

 of the mineral ingredients, potash, lime, iron, 

 are absorbed by the roots. Certain substances 

 which are not soluble in water are, nevertheless, 

 dissolved by acid fluids exuded from the roots 

 themselves, and the solution so formed, diluted 

 with water from the soil, is absorbed by the 

 root. The root in this case not only absorbs 

 the solutions it finds in the soil, but actually 

 dissolves the substances in the soil, and is thus 

 enabled to utilize what otherwise would remain 

 insoluble and inert. 



Still more important to the plant than the 

 mineral matters are the soluble organic matters 

 in the soil, and which are yielded by the decay 

 and disintegration in the soil of vegetable and 

 animal substances, such as farmyard manure, 

 for instance. 



These substances yield nitrogen, on the whole 

 the most important substance the plant con- 

 tains, and one absolutely essential to the life 

 and activity of the protoplasm. Nitrogen exists 

 in the soil in the form of combinations of 

 ammonia and of nitric and nitrous acids in 

 association with potash and other bases. Al- 

 though nitrogen forms by far the largest pro- 

 portion of atmospheric air it appears that plants 

 do not, as a rule, obtain their supplies from this 

 source, but they get it from the soil as above 

 explained. Moreover, it is now ascertained that 

 in some cases certain "Bacteria" or "microbes" 

 in the soil play a large and most important part 

 in the nutrition of the plant, by the power they 

 have of converting the insoluble nitrogenous 

 combinations in the soil into soluble Ones avail- 

 able for plant-foods. It had long been observed 

 that nitrogenous manures, though very beneficial 

 to cereal crops, were not proportionately service- 

 able to leguminous plants (peas, beans, clover, 

 &c), so that the application of such manures to 

 leguminous plants is in a large measure un- 

 necessary and wasteful. It had also long been 

 remarked that the roots of leguminous plants 

 always bore little knobs or excrescences such 

 as are only met with rarely in other plants. 

 Further investigations by microscopists have 

 shown that the knobs are caused by the irritation 

 set up by Bacteria or some similar organisms in 

 the tissues of the root. Chemists have shown 

 that these Bacteria (or some of them, for they 

 are manifold and diverse in their action) have 

 the property of secreting and discharging a 

 ferment, and this ferment has the power of 

 rendering soluble, and therefore useful, what 

 was before insoluble and inert. 



The leguminous plants, therefore, owing to 

 their co-operation or symbiosis with the Bacteria, 

 do not require nitrogen from without, but can 

 utilize, as other plants cannot, the sources of 

 supply furnished by the Bacteria. 



Briefly, it may be repeated that root-action 

 consists in the absorption of water containing 

 in solution various ingredients. The absorption 

 takes place above the root-caps at the lower end 

 of the root-fibrils, and especially through the 

 root-hairs when present. The substances in 

 solution are mineral or inorganic, the most im- 

 portant being potash and phosphate of lime, 

 and organic or nitrogenous supplied from de- 

 composing organic matter in the soil, the ab- 

 sorption of nitrogen being in some cases brought 

 about by the agency of Bacteria. Iron is essen- 

 tial to the formation of the chlorophyll of plants, 

 as it also is to the production of healthy blood 

 in animals. 



The Atmosphere as a Source of Food, 



Plants can no more live without air than 

 animals can. In both cases air is essential to 

 the life of the protoplasm. The air contributes 

 directly to the life of plants, and indirectly as 

 the medium through which the sun's light pene- 

 trates. These facts, which demand no confirma- 

 tion, necessarily lead to the inquiry as to what 

 the air is and what it is made of. We cannot 

 see it, we can judge of it by its effects, and by 

 what happens when Ave are deprived of it; but 

 the chemist can analyse it, and he tells us that, 

 roughly speaking, atmospheric air consists of 

 one-fifth of oxygen gas and four-fifths of nitro- 

 gen gas, together with minute quantities of car- 

 bonic acid gas (carbon dioxide), watery vapour, 

 and ammonia. 



The air so constituted enters at any part of 

 the plant that is permeable. The entrance of 

 air is not confined to the leaf or the green parts, 

 but takes place over the whole available surface, 

 The roots and the seeds breathe as well as the 

 leaves — indeed, one of the functions of proto- 

 plasm is to breathe, to inhale atmospheric air, 

 and to exhale carbon dioxide. In this way the 

 plant is provided with the greater part of the 

 oxygen and of the carbon of which it is com- 

 posed. Breathing goes on night and day in the 

 green and in the colourless parts. 



But this respiratory process is, as has been 

 already stated, not the only interchange of gases 

 that takes place. "Where green matter or chloro- 

 phyll exists, and where it is subjected to the 

 influence of sunlight or of the electric light, 



