Anatomy of Soienostelic Ferns . 727 
are primarily and essentially endarch. In many Ferns, both 
soienostelic and dictyostelic, the individual protoxylems of the 
leaf-trace are continued downwards into the stem, so that the 
xylem of the stem is also differentiated from a number of 
definite endarch or mesarch centres, just as that of the petiole, 
e. g. Dicksonia ruhiginosa , D. davallioides (Fig. 27, prx .), 
D. adiantoides (Fig. 12), Hypolepis repens , Davallia Novae - 
Zelandiae , Pteris incisa , Dicksonia culcita , Aspleniuni scandens , 
&c. On the other hand, there are a large number of Ferns 
in which the protoxylems of the petiolar bundles gradually 
die out towards the base of the leaf-trace, disappearing entirely 
before, or immediately after, its insertion upon the stele of 
the stem. No definitely localized protoxylem strands are 
to be found in the stems of these Ferns, nor are there any 
spiral or annular tracheides present. The first- formed elements 
of the xylem either form a fairly continuous layer all round 
the external periphery, so that the differentiation is more or 
less centripetal ; or else they appear without order here and 
there throughout the whole xylem mass, so that the differentia- 
tion is quite irregular. In the former case, the small peripheral 
tracheides may be regarded as forming an exarch protoxylem, 
although they are all scalariform and differ from those of the 
metaxylem only in their smaller size and earlier development. 
In the latter case, there is no difference whatever between the 
first-formed elements of the xylem and those formed later on, 
so that a protoxylem as distinct from the metaxylem can 
hardly be said to exist. A continuous exarch protoxylem 
was found in several soienostelic Ferns, Loxsoma Cunning - 
hamii Dicksonia apiifolia (Fig. 28), Davallia platyphylla , 
D. speluncae y D. kirta, D. marginalis , &c. In some dictyo- 
stelic Ferns also the peripheral elements are smaller, and 
upon the whole develop earlier than the rest, but the sub- 
sequent differentiation is usually rather irregular, as in Gymno - 
gramme japonic a, G. vestita y Adiantum trapezij 'or me , Dicksonia 
Barometz , &c. In the L indsaya-type of stele the large ventral 
mass of xylem seems always differentiated more or less 
1 Gwynne-Vaughan, 1 . c., p. 79, Fig. 3. 
3 D 
