132 LECTURES ON 



LECTURE II. 



THE ATMOSPHEEE AND WATER IN THEIR RELATIONS TO VEGETABLE LIFE. 



In the former lecture we have seen that the plant is a collection of 

 cells, and the residence of an organizing up-building agency— the 

 vital principle. We have seen that the cells, are composed of, or 

 occupied with, carbo-hydrates, albuminoids, fats, and salts. The 

 structure of the plant admits the entrance of gases and liquids, and 

 their diffusion throughout its mass. 



We are now prepared to inquire what are the materials employed 

 by the plant in its development — what is the food of vegetation ? 



A seed sown in a moist sand may grow into a perfect plant, and 

 produce a hundred new seeds, each as large and complete as the first, 

 although the sand, the water, and the air, which only can have 

 nourished the plant, contain no traces of cellulose or starch, of al- 

 bumen or oil. 



These proximate elements of vegetation are then obviously con- 

 structed by the plant out of other forms or combinations of matter 

 belonging to the mineral world, and to be sought in the atmosphere, 

 in water, and in the soil. 



Of the entire mass of the plant, but a small portion is derived from 

 the soil, ninety-five to ninety-nine per cent, of it coming originally 

 from the atmosphere. 



The general composition of the pure and dry atmosphere, according 

 to the most reliable data is, by weight, as follows: (To the names of 

 the ingredients are appended their chemical symbols.) 



Oxygen, 23. 18 



Nitrogen, N 7G.82 



100.00 



Besides the above ingredients, whose proportion is very constant, 

 there occur in it the following substances in more variable quantity: 



Water, (as vapor,) HO, average 1-htmdredth. 



Carbonic acid, C0 2 , average 6 ten-thousandths. 



Ammonia, NH 3 , average 23 billionths. 



Nitric acid, N0 5 ? 



Carburetted hydrogen, CH ? 



Nitrous oxyd NO ? 



Let us now inquire with reference to each of these substances, 

 how is it related to the nourishment of the plant ? A number of ex- 

 ceedingly ingenious experiments have been instituted from time to 

 time for the purpose of throwing light on this subject, and we are 

 thus fortunately able to present it in a quite satisfactory manner. 



As to oxygen, we have no evidence that it directly feeds the plant, 

 or is assimilated, so as to increase the mass of its organic matter. 

 On the contrary, plants when growing exhale oxygen, separating it 

 from the carbon and hydrogen of their proper food. 



The nresence of oxygen in the atmosphere is, however, in many 



