530 Boodle.— Comparative Anatomy of the 
Starting with a small protostele and a simple type of 
petiolar bundle, the following appears a probable course 
of advance in structure among the Ferns. Increased leaf- 
surface necessitated increase in sectional area of stele and 
petiolar bundle. But this was achieved in two different ways, 
viz. simply by greater diameter in the case of the protostele, 
and by elongation into a band-shaped structure in the case of 
the petiolar bundle \ For mechanical reasons the peripheral 
part of the petiole had to be occupied by sclerenchymatous 
tissue ; so, to avoid too great diameter in the petiole, the 
band-shaped bundle became arched. To admit of the inser- 
tion of a number of large arched bundles, the stele increased its 
diameter beyond the size required by the exigencies of water- 
conduction, and the central part of the xylem of the stele was 
transformed into parenchyma or other tissues. Such central 
tissue might be parenchymatous or sclerenchymatous at its 
origin and remain so in certain phyla (especially where the 
leaf-traces were collateral) ; in other cases it might be paren- 
chymatous at first, and afterwards have its peripheral part 
converted into phloem ; or inner phloem and pith might arise 
simultaneously ; or possibly inner phloem might be produced 
without an ordinary pith (see Tansley and Lulham, ’02). 
To return to the petiole, its arched bundle was able to 
increase its sectional area 1 2 by an incurving of its ends, thus 
producing the horseshoe-type, which is of such frequent occur- 
rence among Ferns as pointed out by Gwynne- Vaughan 
(’01, p. 95), and as seen by reference to the table of diagrams 
given by Parmentier (’99, p. 340). The division of the petiolar 
bundle into two or more portions, as found in many Poly- 
podiaceae, &c. (see Bertrand et Cornaille, ’02, pp. 53, 20 7, &c.), 
may be connected with the downward extension of the leaf- 
gap in the stem, or may have originated for mechanical reasons, 
because a large petiolar bundle would be subjected to consider- 
1 Convenient for the insertion of numerous distichous branch-bundles. Whether 
the primitive petiolar bundle was concentric or collateral must be left an open 
question, but probably both types existed at a fairly early stage. 
2 Without increasing the diameter of the petiole. 
