OTHER VOLATILE SUBSTANCES EXHALED. 91 



chemical influence it exercises over the growth of the plant. As the waler 

 disappears from the leaf, the roots must absorb from the soil at least an 

 equal supply. ^This water brings with it the soluble substances, organ- 

 ic and inorganic, which the soil contains, and thus in proportion to tlie 

 activity with which the leaves lose their watery vapour, will be the 

 quantity of those substances which enter from the soil into the general 

 circulation of the plant. This enables us to understand how substances, 

 very sparingljf soluble in water, should yet be found in the interior of 

 plants, and in very considerable quantity, at almost every stage of their 

 growth. 



2°. Besides watery vapour, however, the leaves of nearly all plants 

 exhale at the sameb'me other volatile compounds in greater or less 

 abundance. In the petals of flowers, we are familiar with such exha- 

 lations — often of an agreeable and odoriferous character. In the case of 

 plants and trees also which emit a sensible odour, we readily recognise 

 the fact of volatile substances being given off' by the leaves. But even 

 when the sense of smell gives us no indication of their emission from a 

 single leaf or a single plant, the introduction of a number of such in- 

 odorous plants into the confined atmosphere of a small room after a time 

 satisfies us that even they part with some volatile matter from their 

 leaves, which makes itself perceptible to our imperfect organs only when 

 in a concentrated state. The probability therefore is, that the leaves of 

 all plants emit, along with the watery vapour which they evolve, cer- 

 tain other volatile substances also, though often in quantities so minute 

 as to escape detection by our unaided senses. By the emission of these 

 substances the plant probably relieves itself of what would prove inju- 

 rious if retained, though of the chemical nature and composition of these 

 exhalations little or nothing has yet been ascertained. 



3°. If the branch of a living plant bo so bent that some of its leaves 

 can be introduced beneath the edge of an inverted tumbler full of water, 

 and if the leaves be then exposed to the rays of tlje sun, bubbles of gas 

 will be seen to form on the leaf, and gradually to rise through the water 

 and collect in the bottom of the tumbler. If this gas be examined it 

 will be found to be pure oxygen. 



If the water contain carbonic acid gas, or if during the experiment a 

 little carbonic acid be introduced, this gas will be found gradually to dis- 

 appear, while the oxygen will continue to accumulate. 



Or if the experiment be made by introducing a living plant into a large 

 Dell-glass full of common atmospheric air, allowing it to grow there for 

 12 hours in the sunshine, and tlien examining or analysing the air con- 

 tained in the glass, the result will be of a precisely similar kind. The 

 per centage of oxygen in the air will have increased.* And if the ex- 

 periment be varied by the introduction of a small quantity of carbonic 

 acid gas into the jar, this gas will be found as before to diminish in quan- 

 tity, while the oxygen increases. The conclusion drawn from these 

 exi)eriments, therefore, is, that the leaves of plants, tvhcn exposed to the 

 rays of the sun, absorb carbonic acid from the air and give off pure oxy- 

 gen gas. 



It has been already stated that the proportion of carbonic acid present 



• It will be remembered that atmospheric air contains about 21 per ceat. of oxygen gjir 



