ANATOMY. 



Is Organography we have described the fundamental organs whicli provide for the 

 •growth and reproduction of plants ; namely, the root, the stem, the leaves, the floral 

 whorls, and the seed ; but these are themselves composed of parts which cannot be 

 studied without the aid of the microscope. These parts, the structure of which 

 varies but little in different plants, and which are elementary vegetable tissues, are 

 named elementary organs ; and the science which treats of them is called Vegetable 

 Histology, or Vegetable Anatomy, 



ELEMENTARY ORGANS. 



If we examine microscopically the thinnest possible slice of a stem, root, leaf, or 

 floral organ, it wiU present many different cavities, some entirely enclosed in walls, 

 others having no proper walls, but being interspaces between the first ; taken 

 together, they present the appearance of a fabric or tissue : whence the name vege- 

 table tissue. 



The closed cavities present three principal modifications : — 1. Cells. — Their 

 diameter is [originally] nearly equal every way. 2. Fibres. — These are longer than 

 broad, and their two ends are spindle-shaped. 3. Vessels, or lengthened sacs, the 

 two ends of which cannot be seen at once under the microscope. 



Cells are very variable in shape, depending on the manner in which they are 

 arranged. If they are not crowded, they 

 retain their primitive form of spheroids 

 or ovoids (fig. 652) ; but if the contiguous 

 faces become pressed together in the 

 662. Elder. course of their growth, they become poly- 

 hedral, and may be dodecahedrons, or 



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r^ 



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653. Elder. 



Cellular tissue of the 



central pith. 



654. Lily. 



Elongated 



cells. 



four-sided prisms, either lengthened into columns, or 



tabular, or cubical. A transverse section of prismatic 



cells presents equal squares, a vertical section of dode- 



cahedral cells presents hexagons (fig. 653) like a 



honeycomb ; whence the name of cellula/r tissue given to these cells collectively. 



Lastly, the cells may be placed end to end, like superimposed cylinders or barrels 



(fig. 654). 



When the cellular tissue {parenchyma) is very compact, there are no interstices 



I 2 



