62 



FOUNDATIONS OF BIOLOGY 



with lateral branches of different form. Frequently the 

 branches, though still composed merely of filaments of cells 

 placed end to end, may show, for example, larger chloro- 

 plastids and thus be more active in photosynthesis. This is 

 essentially the same general division of labor that occurs 



between the stem and 

 leaves of higher plants, 

 but without the attend- 

 ant structural differen- 

 tiation of the parts into 

 tissues. (Cf. p. 413.) 



The next specializa- 

 tion we find is in regard 

 to the character of the 

 growth. Whereas, in 

 simple unbranched fila- 

 mentous forms, growth 

 takes place as a rule by 

 the division of all of the 

 cells composing it, in 

 the branched types this 

 is usually restricted to 

 one or more cells near 

 the end of each filament. 

 Thus, depending on the 

 character of the growth 

 from the apical cells,various complex forms of massive branch- 

 ing structures arise as, for instance, in many of the Red 

 Seaweeds. In these plants the chloroplastids are chiefly de- 

 veloped in the cells on -the surface; which again indicates the 

 physiological division of labor referred to above and suggests 

 that many, if not all, of the modifications of the simple plant 

 bodies thus far considered are provisions for the purpose of 



FIG. 23. A common Seaweed (Fucus). One 

 of the Brown Algae, showing comparatively sim- 

 ple thallus structure. (From Coulter.) 



