a 
18: A TEXT-BOOK OF BOTANY 
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is that it contains a vascular system (§ 24) (Fig. iS1). 
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 
latter being the Thallophytes and the Bryophytes, the for- 
mer the ferns and the xeed-plants. The presence of this 
vascular system means a special conducting system, and 
in connection with it there ure developed the first roots 
Fic. 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 
