RESPIRATION OF PLANTS. 137 



the air and the carbon of the plant. This may occur in some plants 

 without leaves, as Fungi, where a direct process of oxidation takes 

 place in the organic matters which have been assimilated. The 

 quantity of this acid given off during night, is by no means equal to 

 that which is absorbed by the plant during the day. 



288. The parts of plants which are not green, seem to absorb oxygen. 

 Thus, roots and subterranean organs act in this way, and the presence 

 of oxygen seems to be necessary for their growth. There are also 

 certain periods in the life of a plant when carbonic acid is given off 

 in large quantity, even during the day, depending on a chemical 

 change taking place in the starch of the plant, by which it is con- 

 verted into sugar. These periods are germination, flowering, and 

 fruiting. The changes produced will be alluded to when these 

 subjects are considered. When plants are decaying, or are hi an 

 unhealthy state, they undergo chemical changes, by which carbonic 

 acid is formed. This was found by Burnett to have effected the 

 results of some of Mr. EUis's early experiments. 



289. Certain plants have a great power of decomposing carbonic 

 acid under the action of light. This is particularly the case with 

 aquatics. It is thus that they keep up the purity of the pools and 

 ponds in which they grow. Pistia Stratiotes has this effect in the 

 Batavian ponds, and Sir H. Davy notices the great vigour of aquatic 

 plants in the lake Solfatara, where carbonic acid was constantly 

 bubbling up on the surface. The oxygenation of the water by aquatics 

 has also been observed by Morren of Geneva. 



290. Experiments have been made as to the effect of the different 

 rays of the spectrum in aiding the decomposition of carbonic acid, by 

 the green parts of plants. Draper states that the light-giving rays, or 

 those nearest the yellow, have the greatest effect; while the heat-giving 

 and the tithonic, or chemical rays, had scarcely any influence. The 

 experiments of Hunt also lead to the conclusion, that the yellow rays 

 have most effect in the fixation of carbon, and in the production of 

 woody matter. 



291. While the breathing of man and animals, and the various 

 processes of combustion, are constantly abstracting oxygen from the 

 atmosphere, and substituting carbonic acid, plants are decomposing 

 this noxious gas, and restoring the oxygen. In tropical countries, 

 where the vegetation is luxuriant and the light intense, the fixation of 

 carbon and evolution of oxygen goes on with great vigour, thus fur- 

 nishing a supply to those regions where, during certain periods of the 

 year, both vegetation and heat are deficient. 



Effects of certain Gases on living Plants. 



292. It has been already stated that plants can live in an atmos- 

 phere containing a considerable proportion of carbonic acid, provided 



