COMPOUND OKGANS. 21 



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 important channel for 

 the transmission of sap from one part of the plant to another. 

 The proper juices of plants often collect in these cavities, and 

 by their pressure the latter become enlarged, and afford recep- 

 tacles 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 pecu- 

 liar 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 Ranunculus and Duckweed. 



CHAPTER IT. 



COMPOUND ORGANS. 



34. 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, and the center of a special action, 

 but not independent of the other organs which 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 : the leaf is a compound organ. In de- 

 scribing the various vegetable organs, we will take for an object 

 of demonstration and comparison, one of the most complicated 

 and most perfectly developed vegetables. If we take a tree, for 

 instance, we find it composed of various well-defined parts ; and 

 to describe a tree, taking it part by part, we shall describe all 

 the compound organs which go to form the whole vegetable 

 kingdom. We find it in the first place covered, in its earliest 

 stage at least, by a thin membrane extending over the whole 



With what are these spaces filled ? What collect in them ? — 34. What is 

 an organ ? How illustrated by a leaf? If we examine a tree, of what parts 

 d we find it composed ? 



