THE THALLUS OF THE HIGHER ALGM. 29 



39. Summary. — Comparing the thread-form with the thin 

 broad algse, we find the body of the latter often nearly as 

 simple ; but, when the body is thicker, it is often seen to con- 

 sist of unlike regions. The outer parts are arranged so as to 

 enable the plant to make food for itself by getting the proper 

 material from the water and absorbing the light that falls 

 upon the surface. The inner parts, being too much shaded 

 by the outer to serve for food making, are used for other pur- 

 poses. Special organs for floating the plant are formed in 

 some of the brown seaweeds. 



Other algae, of slender form, are more complex by having 

 the older cells of an at first single row divided by partitions 

 parallel to the length into five or more cells. 



With greater complexity of the body, growth in length 

 usually becomes localized at the tips where all the cells are 

 rapidly dividing. 



