70 



THE PRINCIPLES OF AGRICULTURE. 



of plants, and often also in different parts of the same 

 plant. In their simplest forms they are generally spher- 

 ical, or globular. In the fiber of wood they are long and 

 tapering, firmly joined together by their sides. 



Section of Wood. 



Lengthwise Slice of Wood from an Ailnnthus glandulosa, or " Tree of Heaven," 

 highly magnified. 



Sometimes the covering of the ends of these long cells is 

 removed, so that they open into each other, and form a con- 

 tinuous tube, through which the sap may flow more freely. 

 Fibers of cotton and flax are simply long, single cells. 



