xii CONTENTS. 



CHAPTER II. 



THE ORGANIC PARTS OP PLANTS. 



Four elements (carbon, nitrogen, oxygen, and hydrogen) 

 form the organic parts of plants. Their origin, tho 

 atmospheric air. Plants cannot assimilate these elements 

 in their pure state. The three great suppliers of organic 

 plant-food, water, ammonia, and carbonic acid. 



Carbon. The erroneous belief of the early philosophers that 

 carbon, and the other elements of organic plant-food, 

 were derived directly from the decaying vegetable 

 matter in the soil. The fallacy of the " Humus Theory." 

 Soils can most raise luxuriant vegetation after every 

 trace of vegetable matter has been expelled from them. 

 The certainty that the organic parts have been derived 

 from the atmospheric air. The presence of humus in the 

 soil tends to increase its fertility only by affording a 

 surplus of organic plant-food. Vegetable respiration. 

 The absorption of carbonic acid and the expulsion of 

 oxygen. The connection between animal and vegetable 

 respiration. The replenishment of carbonic acid with- 

 drawn from the atmospheric air. The necessity of car- 

 bonic acid being also present in the soil, it being a 

 solvent of the alkaline earths and insoluble phosphates. 

 Baussingault's experiments on the absorption of carbonic 

 acid. The decomposition of carbonic acid under the 

 influence of light and atmospheric air, and its assimilation 

 by the plant. 



Hydrogen and oxygen. The source whence they are derived. 

 The several functions of water. Its absolute indis- 

 pensability for vegetable life, being the first stimulator 

 and supporter of it. The property of leaves to inhale 

 moisture from the atmosphere when absent in the soil. 



Nitrogen. Its inert nature in the pure state. Ammonia and 

 nitric acid the two sources of nitrogen. Their presence 

 in the atmospheric air. Its balance, how preserved. 

 The relation between nitrogen and our edible crops 40 



