214 STRUCTURE AND VITAL PROCESSES OF PLANTS 



1 gram of potassium nitrate, 0-5 gram of sodium chloride, 0-5 gram 

 of calcium sulphate, 0-5 gram of magnesium sulphate and 0-5 

 gram of calcium phosphate, in 1000 grams of distilled water, 

 and add one or two drops of iron chloride. If a seedling, say of 

 Pea, is nursed in this food-solution, it will he found that it grows 

 healthily and even develops flowers and seeds. But if one or 

 the other of the minerals is omitted, the growth will he stunted 

 and dwarfed. If, for instance, there be no iron in the solution, 

 the leaves and the stem of the plant will become pale yellow, 

 but they turn green when a drop of liquid containing iron is 

 added to the solution. 



The minerals thus absorbed with the water rise to the leaves, 

 and are there assimilated to form elaborate food for the formation 

 of new structures. 



II. Absorption of Carbon by the Leaves. 



(a) Source of Carbon in Carbonic Acid. — The greater part of 

 the plant-body consists of carbon (charcoal is carbon). The 

 plant nourished in the food-solution, as described above, could 

 not derive it from the water. It must, therefore, have had some 

 other source of carbon supply. 



Atmospheric air always contains, more or less, (0-03 — 04'^ o) 

 carbonic acid gas, a compound of carbon and oxygen, and given 

 off from the lungs of animals and men, and from the burning of 

 wood and coal. This is the source of the carbon supply. The 

 leaves absorb the gas, and because there is so little of it, each 

 tree needs to spread out an immense amount of foliage, so that 

 it may drink in all the carbonic acid gas that can possibly 

 be- obtained. 



Some very instructive evidence is furnished by a small 

 aquarium. If animals, especially fish, alone be kept, it will be 

 found necessary to renew the water daily, or the animals will 

 soon die. If, however, some water-plants are introduced, the 

 same water may be kept in for months, and the animals will 

 continue healthy, thus showing that animals soon make the water 

 fatal to themselves, and that plants restore and maintain the 

 balance, evidently taking from the water what the animals give 

 to it, viz.^ carbonic acid gas. 



