22 



that nature would employ the spiral vessels in the wood and 

 distinct cells in the bark to effect the respiration in plants. 



In my Vegetable Physiology, I have, on the contrary, endea- 

 voured to show that the intercellular system serves for the re- 

 spiration in plants ; and it is only those elementary organs, sur- 

 rounded by intercellular passages or large air cavities situated 

 in the immediate vicinity, in which an actual assimilation of 

 the absorbed nutriment takes place ; for as the respiration in 

 animals produces an amelioration of the blood, i. e. of the 

 elaborated nutritive and formative sap, so in vegetables the re- 

 spiration causes an amelioration of the nutritive sap, which is 

 elaborated in every cell of the parenchyma : and, if we com- 

 pare the structure of animals with that of plants, we shall at 

 once perceive that a respiration, which is to agree in its re- 

 sults with that of animals, can take place in plants in no other 

 way. If we follow up the connexion which exists between 

 the intercellular passages, air cavities, stomata, and the spiral 

 apertures of the cuticular glands, the view which I take in 

 common with M. Unger, that the intercellular passages carry 

 on the process of respiration in plants, will become more than 

 probable. When the cuticular glands are covered with resins, 

 as is frequently the case in the Conifera, this must be regarded 

 as a purely accidental aggregation of the excretion in the ca- 

 vity of the epidermis. 



The leaves are, however, the organs which possess in the 

 greatest number the above-mentioned arrangements for respi- 

 ration. M. Dutrochet notices the difference of opinion respect- 

 ing the destination of leaves with reference to their function ; 

 some botanists have compared them as absorbing organs to 

 the aerial roots, others on the contrary have declared them to 

 be the lungs of plants. To this latter opinion M. Dutrochet 

 adheres in common with many other botanists. The leaves of 

 plants exert on the nutritive sap contained in them a similar 

 action to that of the act of respiration in the lungs of animals 

 on the blood ; and the sap descending from the leaves, the for- 

 mative sap is that which in some degree may be compared to 

 the blood of animals, as of it the new masses are formed. 



Observations have long since shown that plants during the 

 greatest part of the day-time absorb oxygen : to explain this 

 phenomenon M. Dutrochet draws attention to the important 



