SIMPLE THALLOPHYTES WITH SPERMATOPHYTES 295 



to be connected and no definite position which the plant must 

 maintain. 



It is now clear that in passing from the unicellular condition 

 to the Spermatophyte stage, evolution was along the following 

 lines: First, plants became multicellular ; second, the cells con- 

 stituting a multicellular plant became somewhat differentiated as 

 to function and structure; third, as plants became more multi- 

 cellular, there was further differentiation which eventually re- 

 sulted in the establishment of definite structures or organs fitted 

 to efficiently perform special functions. Such structures in their 

 most highly organized form are the leaves organized for the 

 manufacture of plant food, the roots organized for absorbing 

 and for anchoring the plant, the flowers organized for reproduc- 

 tion, and the stem organized to support leaves, flowers, and fruit 

 in the air and sunshine. 



Of course the organs as they occur in Spermatophytes did 

 not arise suddenly, but they, too, underwent a gradual process of 

 evolution, at first arising as simple structures and gradually 

 becoming more complex and better defined. Through the 

 Thallophytes, Bryophytes, Pteridophytes, and Spermatophytes, 

 including both living and extinct forms, the organs characteristic 

 of the highest type of Spermatophytes gradually arose. 



