458 Boodle.—Anatomy of the Hymenophyllaceae . 
lower (/). Between the two xylem-bands is a zone of con¬ 
junctive parenchyma, in which is embedded the protoxylem- 
group (px). The latter is sometimes free, and sometimes 
abuts on the lower xylem-band. A few tracheides (r) form 
a prominence at the left-hand end of the lower band ; they 
have come in recently from a root. The two rows of roots 
are inserted at points about radially opposite to the respective 
ends of the lower band. The protoxylem-elements are spiral 
and annular; the metaxylem consists of scalariform tracheides. 
The xylem-bands lie in horizontal planes. 
The term ‘ metaxylem,’ when used in the present paper, 
is applied to all the xylem in a stele or bundle other than 
protoxylem. The word metaxylem is used by Poirault 1 for 
tracheides, which are formed on the flanks of the diarch plate 
of the roots of several species of Ferns, and have a different 
direction of development from the rest of the xylem-plate. 
The xylem of these roots is therefore composed, according 
to Poirault, of protoxylem, deutoxylem (of Van Tieghem) 
and metaxylem. As, however, there does not appear to be 
any fundamental difference between the elements of the two 
latter, and as ‘ metaxylem ’ has come into use for primary 
xylem excluding the protoxylem, the latter terminology is 
adopted here. 
Figs. 2 and 3 illustrate developmental stages of the rhizome 
of H.scabrum. Fig. 2 shows the protophloem (pph) differen¬ 
tiated in contact with the pericycle ; the small cells succeed¬ 
ing them inwards are the young elements of the remainder 
of the phloem. The protoxylem ( px ) is seen as a small group 
of tracheides near the middle of the stele, while the group of 
four tracheides ( r ) will afterwards form one end of the lower 
xylem-band. They owe their early differentiation to the 
presence of a young root; they begin to differentiate later than 
the true protoxylem. The pericycle is included for a short 
distance on one side of the drawing, it is two or three cells 
in thickness, and is limited on the outer side by the endo- 
1 Poirault, Recherches sur les Cryptogames Vasculaires, Ann. des Sci. Nat., 
Bot., 7 e ser., t. xviii,. p. 141. 
