94 POPULAR EEEOES. 



being separated in places by tbin plates of cellular 

 tissue called medullary rays or silver grain, which 

 can. be seen glistening upon the surface of bloc-ks of 

 wood which have been split radially from the bark 

 toward the pith. Monocotyledons or Endogens on 

 the other hand, including such plants as corn and the 

 other grains, have their wood in the form of small 

 fibres which pass through the softer tissues of the 

 stem. These fibres have their soft growing portion 

 at their centre, and after this has become old and 

 permanent no further growth can take place. This 

 is the main reason why such stems do not as a 

 rule, grow so large as those of Dicotyledons, in 

 which there is always on the outer surface of the 

 wood beneath the bark a layer of tissue capable of 

 further growth. 



