CHAPTER XXn 



ASCENDING SAP 



' TyjOW let us see how the plant is nourished by the 

 ±\ various substances of which we have just stud- 

 ied the most important. Every form of plant-life 

 is made up, not of a compact and uniform mass of 

 matter with no occasional empty spaces, but, on the 

 contrary, with the aid of a microscope it is seen 

 that an infinite number of very minute cavities called 

 cells are interspersed throughout the body of the 

 plant. These cells may be regarded as extremely 

 small closed sacs, sometimes round, sometimes oval, 

 but more often with irregular and angular outlines 

 by reason of the mutual pressure exerted by the cells. 

 The cell-wall is composed of an excessively fine mem- 

 brane. In the pith of the elder, all riddled like a 

 sponge, you have an example of cells large enough 

 to be seen without a microscope. Other cavities 

 are long, pointed at both ends and swollen in the 

 middle like a spindle. They are called fibers. Still 

 others form canals of uniform size throughout, as 

 fine as a hair and long enough to extend from the 

 roots to the topmost leaves. These canals are called 

 ducts. Look closely at the cross-section of a very 

 dry vine-branch, and you will see a multitude of 

 orifices into which it would be possible to thrust a 



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