188 



A TEXT-BOOK OF BOTANY 



is that it contains a vascular system ( 24) (Fig. 181). 

 The appearance of this system marks some such epoch in 

 the evolution of plants as is marked among animals by the 

 appearance of the backbone. As animals are often grouped 

 as vertebrates and invertebrates, so\plants are often 

 grouped as vascular plants and non-vascular plants, the 

 la-tter being the Thallophytes and the Bryophytes, the for- 

 mer the ferns and the seed-plants. The presence of this 

 vascular system means a special conducting system, and 

 in connection with it there are developed the first roots 



FIG. 180. Portion of the leaf of maidenhair fern, showing the forking veins. 



and the first complex leaves. Such a plant body, with its 

 vascular system and roots and complex leaves, is so dif- 

 ferent from any plant body among Bryophytes that the 

 greatest gap in the whole series of plants, from lowest to 



