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. 
