32 ORGAN OF PLANTS. 



INTERCELLULAR PASSAGES. 



35. In the placing together the various tissues, which are either 

 globular or cylindrical, spaces are necessarily left between the 

 walls of adjacent cells or tubes, which are called intercellular 

 passages. The appearance on a large scale may be illustra- 

 ted by the spaces, that would be seen in a pile of bladders which 

 would exhibit these passages in the cellular tissue, and the 

 space seen in a bunch of cylindrical rods bound together, 

 would exhibit those seen in the vascular tissue. These spaces 

 are always filled with fluid, and are supposed to afford an im- 

 portant channel for the transmission of sap from one part of 

 the plant to another. The proper juices of plants often col- 

 lect in those cavities, and by its pressure, they become enlar- 

 ged, and afford receptacles which contain large quantities of 

 the peculiar juices of plants ; such is the case with the cavities 

 in the bark of the pine and balsam ; in the latter they are 

 very large, and also in the rind of the lemon, and orange, in 

 which are deposited the peculiar secretions of these plants. 

 Air-cells, are cavities built up by cellular tissue in the leaf or 

 stem for the purpose of enabling the plant to float on water. 

 They occur in the leaves of the aquatic varieties of the Ra- 

 nunculus and Duckweed. 



CHAPTER II. 



ORGANS OF PLANTS. 



36. In the preceding chapter we have described in a brief 

 manner the various tissues which enter into the composition 

 of vegetables. Our next object will be to describe in the 

 same manner the various organs these tissues compose. An 

 organ is a part of a living body, the form and limits of which 

 we can describe with precision, but to determine all its func- 

 tions is not in some cases so readily accomplished. It is the 

 center of a special action, but not independent of the other 

 organs which go to make up the being to which it belongs. It 

 may be composed of other organs more simple than itself. 

 Thus the leaf, which is an organ, and the center of a special 

 action is, at the same time composed of more simple organs, as 

 cells and vessels, which are called elementary organs, and the 

 leaf a compound organ. In describing the various vegetable 

 organs, we will take for an object of demonstration and com- 

 parison, one of the most complicated and most perfectly devel- 



