THE AIR CAVITIES. Ill 



Coniferee, &c. They have not as yet been observed 

 with certainty either in Monocotyledons or Acotyle- 

 dons. 



The different Receptacles are generally placed in the 

 cellular tissue of the bark, and consequently are con- 

 tinually thrown towards tlie surface by the distension 

 produced by the growth of the wood ; whence it results, 

 that they are often wanting in ver}-- old bark : these 

 are the cortical receptacles which Hill has described 

 under the name of Vaisseaux Propres Extirieiirs ; but 

 under the denomination of Vaisseaux Propres Tnterieio's 

 et Intimes he describes indifferently very dissimilar 

 organs, where the proper juice is sometimes lodged, 

 and situated in the wood and pith. The true Proper 

 Juices appear to be entirely secreted in the green, and 

 consequently external, part of plants. 



CHAPTER XII. 



OF THE AIR CAVITIES. 



We have seen that the cellular tissue is sometimes 

 distended so as to form cavities where the Proper Juices 

 are lodged ; and this distension, caused by a known and 

 visible agent, offers no difficulty; but it also often 

 happens, as a necessary consequence of the increase in 

 size and of growth, that the cellular tissue is distended 

 or broken so as to form spaces, or rather cavities, full of 

 air. Grew first observed this phenomenon, and its 

 analogy with the formation of the Receptacles of Proper 



