440 THE VASCULAR PLANTS 



Pteridophytes. — Plants with a vascular system, but no 

 seeds. 



Spermatophytes. — Plants with seeds. 



Each of these definitions except the last contains a posi- 

 tive and a negative statement. The positive statement 

 distinguishes the group (except thallophytes) from the 

 one below it in order of evolution, and the negative state- 

 ment distinguishes the group (except spermatophytes) 

 from the one above it. 



The chief contributions which each group makes to the 

 evolution of the plant kingdom may be stated as follows : 



Thallophytes. — The beginnings of plant structures and 

 of three kinds of reproduction. 



Bryophytes. — Growth on land and the alternation of 

 generations. 



Pteridophytes. — Appearance of the vascular system, in- 

 volving true roots, leaves, and stems; appearance of 

 strobili and heterospory. 



Spermatophytes. — Appearance of seeds and flowers." 



80. Angiosperms. — As has just been stated, seeds and 

 flowers are the two things which chiefly distinguish seed 

 plants from all other groups. Both gymnosperms and 

 angiosperms have seeds, but only angiosperms have those 

 structures which are popularly known as flowers. Strictly 

 speaking, gymnosperms also have flowers (see page 260), 

 but these gymnosperm " flowers " are very different in 

 appearance from that structure which is characteristic of 

 angiosperms, and to which the term flower was first applied. 

 The flower is the final organ to be explained, — explained 

 as to its relationships with simpler reproductive structures 

 which are like those from which it evolved. 



