110 



THE BIOLOGY OF A PLANT. 



FIG. 45. An entire 

 plant of Ptcri*. One 

 of the leaves is 

 young and small, 

 and a comparison of 

 the figure wit-?i Fig. 

 44 will show some of 

 the differences be- 

 tween leaves of dif- 

 ferent ages. 



joins the rhizome. The enlarge- 

 ment is of considerable interest, 

 for it occurs at precisely the 

 point of greatest strain when the 

 leaf is bent by the wind or other- 

 wise, and must serve to strength- 



O 



en the stipe. 



It will appear from the fol- 

 lowing description that the plant 



body exhibits in some measure 



j 



certain general forms of sym- 

 metry and differentiation which 

 in a broad sense may be regarded 

 as analogous to those occurring in 

 the animal. The rhizome grows 

 only at one end, and in its struc- 

 ture suggests the antero-posterior 

 differentiation of the animal. It 

 also shows a slight differentiation 

 between the upper and lower 

 surfaces, which appears both in 

 the external form and in the ar- 

 rangement of the internal lines. 

 It is furthermore distinctly bilat- 



j 



eral, a vertical plane dividing it 

 into closely similar halves. These 

 features are, however, far less 

 prominent in the fern than in 

 the earthworm, and in plants 

 they never attain a high degree 

 of development, while in the 

 higher animals they are' among 

 the most conspicuous and im- 

 portant features of the body. 

 Of more general importance in 

 the fern is the repetition of 

 similar parts (branches, roots, 

 leaves) along the axis, which 

 suggests, perhaps, a certain an- 



