STRUCTURE.] CYSTS AIE CELLS. 95 



(Myrtacese), where they are called crypts, or glandules impresses, 

 or reservoirs vesiculaires, or glandes vesiculaires, or receptacles 

 of oil. In Pistacia Terebinthus the receptacles are tubular ; 

 in Conifers they are very irregular in figure, and even position, 

 chiefly forming large hollow cylindrical spaces in the bark. 

 Those in the rind of the orange and lemon are little oblong 

 or spherical cysts; their construction, which is easily examined, 

 gives an accurate idea of that of all the rest. 



These receptacles are most common below the surface of 

 plants ; but they often occur in wood. In Labisia pothoina 

 the tissue is filled in all directions with little tubes, or passages 

 containing a brown substance of unknown nature ; and what 

 is curious, they extend even into the pith, although they miss 

 the wood. 



3. Of Air Cells. 



Besides the common intercellular passages, and the recep- 

 tacles just described, there is another and a very remarkable 

 sort of cavity among the tissue of plants. This is the air 

 cell; the lacuna of Link. Like the receptacles of secretion, 

 the air cells have no proper wall of their own, but are built 

 up of tissue ; and this sometimes takes place with a wonderful 

 degree of uniformity and beauty. Each cell is often con- 

 structed so exactly like its neighbour, that it is impossible to 

 regard it as a mere accidental distension of the tissue : on the 

 contrary, air cells are, in those plants to the existence of which 

 they are necessary, evidently formed upon a plan which is 

 uniform in the species, and which has been wisely contrived 

 by Providence in the manner best adapted to the purpose for 

 which they are destined. 



They differ from receptacles of secretion in containing air 

 only, and not the proper juice of the plant ; a peculiarity 

 which is provided for by a curious contrivance of Nature. 

 In receptacles, the orifices of the intercellular passages through 

 which the fluid that is to be deposited drains, are all open ; 

 but, to prevent any discharge of fluid into the air cells, the 

 orifices of all the intercellular passages that would otherwise 

 open into them are closed up, except in the partitions that 

 divide them from each other. 



