162 ON VEGETABLE RESPIRATION. 



lopes, and the fruit, all belong to the coloured division. Having 

 settled this point, we shall next enquire in what manner these 

 different parts act on the atmosphere. Atmospheric air, it is well 

 known, is a mixture of two gases, oxygen and azote. One volume of 

 atmospheric air contains in the 100 parts, 79 of azote and 21 of 

 oxygen ; it also contains some traces of carbonic acid. It is sur- 

 prising, that so small a proportion of the latter gas is capable of per- 

 fonning so important an office in vegetable respiration, as we shall 

 hereafter show; but the surprise vanishes when we consider the 

 immensity of the mass of air by which we are surrounded. In our 

 analytical experiments we are only able to obtain a very small quan- 

 tity of carbonic acid, because we operate on a comparatively small 

 quantity of air ; but, we are assured by calculation, that the entire 

 atmosphere can contain no less than 1,500 billion kilogrammes of 

 carbon. 



Functions of the coloured parts. — The coloured parts of plants 

 absorb oxygen, and exhale carbonic acid, at all times, by day as well 

 as by night. 



We see around us innumerable proofs of this fact; thus the 

 presence of air is indispensable even to roots, which, if they be so 

 deeply imbedded in the soil that the air cannot reach them, the plant 

 perishes. The same effect takes place if a tree be so inundated that 

 a large mass of water is interposed between the atmosphere and the 

 roots. The growth of a hyacinth may be veiy much hastened by 

 inverting a phial of oxygen in the vessel of water containing the roots. 

 Fruits have precisely the same action on atmospheric air, and exhibit 

 identical phenomena even after they are gathered ; the danger of 

 remaining long in a close room in which a large quantity of fruit is 

 stored, is well known. The oxygen of the air being absorbed by the 

 fruit, its place is supplied by carbonic acid, which is a deadly poison. 

 The same objection applies to flowers : it would be extremely impru- 

 dent to pass the night in a green-house, which proves beyond a doubt, 

 that the disengagement of carbonic acid takes place by night as well 

 as by day. The coloured parts respire in the manner of animals ; 

 they absorb oxygen and exhale carbon acid, which vitiates the sur- 

 rounding atmosphere. 



Functions of the green parts. — Here commences a series of phe- 

 nomena the most important to vegetable life, and in producing these 



