32 THE STORY OF PLANT LIFE 



In each genus there are certain main characters, 

 common to all the species, which are an indication 

 of the retention of the structures evoked under 

 natural selection as best suited to the needs of the 

 plant, whilst the divergent characters of the different 

 species are a result of the particular adjustment of 

 the species to internal or external factors. 



It is thus possible to take as typical of the 

 structure of any organ of different plants that of 

 any particular species. Unlike the forms of the 

 organs of different species the structure remains 

 largely the same for a genus or larger group. 



The structure of a root is thus simpler than that 

 of a stem, and the outer layers of tissue are especially 

 modified to serve the purpose of osmosis where no 

 cuticle is formed. It is possible, however, to recog- 

 nise the difference between a section of a root and 

 that of a stem. The structure of a root, as that of 

 a stem and a leaf, has already been briefly described, 

 and it is not within the scope of this work to enter 

 into details as to the minute anatomy of plant 

 structures. 



Roots of Dicotyledons and Monocotyledons, how- 

 ever, differ in the absence of pith in the former. 

 The first elements of xylem in young roots are on 

 the outside, in stems on the inside. Later they are 

 between the xylem and other elements. Wood and 

 bast are not associated as in the stem but alter- 

 nate till the cambial zone is developed. The 

 number of strands may be two, three, or more. 



