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 “phloém” 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, 
