272 THE STORY OF PLANT LIFE 



in the case of outgoing bundles, the bundles running 

 down the stem and finally diminishing in thickness. 

 In Monocotyledons the apparent so-called endo- 

 genous structure results from the curvature of the 

 bundles in the middle. The cambium, well repre- 

 sented in Dicotyledons, is absent, being arrested. 

 The increase in thickness is due to the activity of 

 the pericycle. The pith is disorganised and not 

 central. 



As a rule the parts of the flower are usually in 

 threes, but in the Naiadaceae they are in fours, and 

 in the Glumacese in twos and threes. They are never 

 in fives. The perianth is petaloid as in Iris, etc., or 

 wanting. It is superior or inferior. Thus there is a 

 sort of parallelism of development in regard to the 

 position and number of parts between what we find 

 here and in Dicotyledons, which is an argument, 

 amongst others, for their derivation from a common 

 ancestor, later divergence and parallel development 

 exhibiting in different directions common characters, 

 present in the original ancestral series, but of different 

 value owing to the opposite direction in which evolu- 

 tion has led each. 



The fruit is largely an achene or nut, sometimes a 

 capsule with small seeds. There is no tap-root in 

 the germinating stage, except in maize, etc. But it 

 does not persist long. The radicle is branching, but 

 adventitious roots are developed. The stem becomes 

 like an inverted cone, as in Nymphaeacese. 



Several modes of classification have been suggested. 



