78 



INDIVIDUALITY IN ORGANISMS 



in the apical region and decreases basally; as the plant 

 grows older, death may even begin at the basal end and 

 proceed apically while the apical end is still growing 

 actively. Evidences of a transverse gradient in activity 

 appear in a decrease in growth toward the lateral margins 

 and in many forms in a decrease in thickness of the 

 body in the same direction. In the direction of the 



Figs. 23, 24. — Bilaterally symmetrical prothallia of lix^erwort, 

 Marchanlia (dorsal view), and a fern (ventral view). 



dorso-ventral axis which is determined by the action of 

 light and perhaps other external factors, the differences 

 in metabolic activity are indicated by the outgrowth of 

 root-like structures and the sexual organs, and in some 

 forms of scales or leaf-lilve structures on the ventral 

 surface, and also in some forms by the greater density 

 of cellular structure iri the ventral region. 



