Cheilanthes and Pellaea . 
679 
becomes' completely detached and runs through the major portion of the 
petiole as a single separate group (Fig. 10). At a higher level, a little 
below the insertion of the first pinna, it splits into three or four smaller 
protoxylem groups, though the pinna trace is given off entirely from the 
lateral portions of the stele. The other two protoxylems become rather 
broader In the higher regions of the petiole, but otherwise they remain 
unchanged throughout. All three protoxylems were seen in longitudinal 
section to consist of spiral and annular elements, while the main mass of the 
tracheides has scalariform thickening. 
Ch. lanuginosa. The series in this species is very easily derived from 
that In Ch. Fendleri . Three protoxylems appear simultaneously just above 
the base, but at this point the centre of the trace is so narrow that the 
median protoxylem occupies the whole thickness of the stele. The stele 
next shows a definite separation into two masses joined only by the pro- 
toxylem. As in Ch. Fendleri , these two masses rejoin and the median 
protoxlem, now exarch, soon becomes separated from the main stele. 
This stage is reached more quickly than in the former species, and there is 
the important difference that the median protoxylem does not persist. 
It is found for some distance as a line of thickening among the parenchyma 
(diagram 5 in the Ch. lanuginosa series, Fig. 11), but in the higher regions 
of the petiole no trace of it can be recognized. 
Ch. gracillima. In this small plant the two lateral protoxylems, 
occupying an endarch position, are the only ones ever definitely organized. 
There is no trace of a median splitting, and only a slight and transitory 
indication (in the presence of somewhat smaller elements at the position 
shown by the dots in diagram 3) of a median protoxylem. True protoxylem 
elements, however, are not present, but this condition is clearly only slightly 
removed from that represented by diagram 5 in the Ch. Fendleri series. 
The higher regions of the petiole in Ch. gracillima show no further change 
in the number or position of the protoxylem groups. 
Ch. persica. In petiolar structure this species is rather different from 
the American forms, but this is only what one would expect from the 
divergences in the stem anatomy, and from the totally dissimilar geo- 
graphical distribution. The petiolar stele has the shape, in transverse 
section, of an obtuse-angled isosceles triangle, the base of which shows 
a slight adaxial curve. At each angle a protoxylem gradually becomes 
distinct as we examine the series of sections from the base upwards, and, 
once established, this structure continues throughout the length of the 
petiole. This type is probably related to that of Ch. Fendleri, being 
modified from some structure similar to that shown in diagram 5 of the 
Ch. Fendleri series. The loss of the two lateral hooks would easily give the 
type with the three protoxylem angles. 
Pellaea andromedaefolia develops three endarch protoxylem groups, 
