54 RESPIRATION. 



which the respiration of animals is unceasingly diffusing through 

 the air, and consequently to purify the atmosphere. 



39. The green parts alone possess the property of decompos- 

 ing carbo'nic acid in this way, and they cannot effect this decom- 

 position without the direct influence of the light of the sun. 

 Thus, a plant which is put in an obscure place ceases to respire, 

 languishes, bleaches, and dies, after a shorter or longer time. 



40. Consequently, the leaves are the principal seat of respira- 

 tion, and this function is only carried on during the day. 



41. It is easy to demonstrate the influence of light upon the 

 respiration of plants ; a simple experiment is sufficient to do this : 

 if we place leaves in water containing a small quantity of car- 

 bo'nic acid in solution, and expose them to the sun, we see bub- 

 bles of air rise from them ; but if we place them in the shade, 

 this disengagement of gas is arrested. 



42. In leaves exposed to the air, the absorption of carbo'nic 

 acid takes place chiefly through the stomata, and this fluid acts 

 upon the sap in the interior of the cavities which exist in the 

 paren'chyma of the leaf, and abandons its carbon lo pass to the 

 state of free ox'ygen. The intercellular passages (meatus) of 

 the leaves consequently perform, in the respiration of plants, 

 functions analogous to those of the pulmonary cells in terres- 

 trial animals ; and it is remarkable that in aquatic plants, the 

 leaves of which are submerged, there are no similar cavities, and 

 respiration is carried on by the surface of the leaves, just in the 

 same manner as the skin or projecting branchiae perform this 

 function in aquatic animals. 



43. During the night, the leaves, instead of purifying the air, 

 absorb ox'ygen, and consequently contribute towards its vitiation. 

 For this reason, as well as on account of the odour they exhale, 

 it is often dangerous to place plants or even bouquets of flowers 

 in sleeping apartments. 



44. The absorption of ox'ygen by the parts of plants that are 

 not green is feeble, but takes place by day as well as by night, 

 and it is necessary to the life of all plants. It is because roots 

 do not obtain the air which they require that they die, when too 

 deeply buried ; and it is for the same reason that a seed will not 

 germinate when removed from the action of the atmosphere. 



39. Do all parts of a plant decompose carbo'nic acid ? Do plants decom- 

 pose carbo'nic acid under all circumstances ? 



40. Do plants respire at all times ? 



41. How is it shown that light influences the respiration of plants? 



42. In what part of the plant does the carbo'nic acid act on the sap ? 

 What is remarkable in the respiration of aquatic plants ? 



43. Why is it improper to keep plants in apartments in which we sleep ? 



44. Why do roots and seeds die when too deeply buried ? 



