124 EVOLUTION OF PLANTS 



bers, the tissues of the sporophyte show a much greater 

 degree of complexity than is found in any of the plants 

 below the ferns. This is especially seen in the develop- 

 ment of the so-called " vascular bundles," which are met 

 with for the first time in their fully developed con- 

 dition in the sporophyte of the ferns. These tissues 

 are, however, hinted at in the sporophytes of some of 

 the mosses. Thus the central strand of tissue in the 

 seta of the moss-sporogonium, and the columella in 

 Anthoceros, both in origin and appearance, suggest the 

 young vascular bundles in the organs of the young 

 fern-embryo, and may probably be fairly considered 

 as the homologues of these. 



It is in the ferns, however, that we first encounter 

 the peculiar tracheary tissue characteristic of the woody 

 portions of the bundles in the vascular plants. This 

 tracheary tissue is made up of empty cells with woody 

 walls, and is a very important element in the conduc- 

 tion of water in the vascular plants. These empty cells 

 are known as tracheids, but occasionally in the ferns 

 there are encountered true vessels, or rows of tracheids 

 whose partition walls have been absorbed. In the ordi- 

 nary ferns the woody tissue or " xylem " is surrounded 

 by a mass of " phloem " or " bast," containing as its 

 most characteristic element the sieve-tubes, similar in 

 appearance to the tracheary tissue of the xylem, but 

 without lignified walls and containing living proto- 

 plasm. The vascular bundles form a complicated system 

 of strands in the stem of the sporophyte, and with these 

 are connected the bundles traversing the roots and 

 leaves. 



A well-marked epidermal tissue is always present, 



