CHAPTER XX 



The Ascent of the Sap 



We will now inquire how the plant is fed 

 by the substances, the most important of 

 which we have been studying. The substance 

 of any plant is not compact and uniform, 

 without intervening spaces. On the contrary, 

 if we examine it with the microscope we shall 

 see an infinite number of very small cavities 

 called cells. They are receptacles without 

 any opening, sometimes round or oval, more 

 often of no regular shape, and angular by 

 reason of their mutual pressure. Their walls 

 are composed of a very fine membrane. In 

 the pith of the elder, which is riddled like a 

 sponge, there are cells large enough to be seen 

 without a microscope. Other cavities are 

 long, pointed at both ends and swelling out 

 in the middle, like a spindle. These are 

 called fibres. Others are channels of uni- 

 formly equal thickness, and long enough to 

 extend from the roots to the highest leaves. 



"5 



