THE PROBLEM 51 



the foliage in the two classes depends chiefly 

 on the venation, though the distinction is by 

 no means constant. 



The mode of growth of the stem, at least 

 in the case of trees and other large plants, is very 

 different as a rule, for the stem of a Dicotyledon 

 can go on growing in thickness indefinitely, as 

 we see in all our native trees and shrubs, and to 

 a less extent in most other members of the class; 

 in the great majority of Monocotyledons, when 

 once the stem is formed, it is incapable of any 

 further growth in diameter. Thus the stem and 

 branches of a Dicotyledonous tree are thickest 

 at the base, where growth has gone on longest, 

 and the general form of the trunk is more or less 

 conical, tapering from below upwards. In most 

 Monocotyledons the stem is cylindrical, and 

 towards the base may even have the form of an 

 inverted cone. 



These external differences go together with 

 a great difference in the internal structure. In 

 Dicotyledons the vascular strands in the young 

 stem are arranged, as a rule, in a ring, with the 

 wood on the inside and the bast on the outside; 

 between the two lies the cambium, a delicate, 

 actively growing tissue which goes on adding 

 new wood internally and new bast externally 

 as long as the plant lives, of course resting dur- 

 ing the winter in plants, such as trees and shrubs, 



