Cheilanthes and Pellaea . 
\ 
683 
species examined. It possesses a dorsiventral dictyostele with leaves given 
off approximately in pairs, though by interpolation of extra leaves these 
pairs are often converted into groups of three. Among other species of 
Pellaea , , P. falcata has a typical solenostele,and Gwynne-Vaughan describes 
P. rotundifolia as a dictyostele with two dorsal ranks of leaves. 
4. Parallel with the advance towards dictyostely is an increase in the 
thickness of the band of xylem, and in the extent to which the xylem is 
broken up by parenchyma. Thus Cheilanthes Fendleri is solenostelic, and 
has a narrow compact band of tracheides, while Ch. persica is a dictyostele 
with a broad band of mixed tracheides and xylem parenchyma. Intermediate 
between these comes Ch . lanuginosa. 
5. There is no protoxylem in the stems of any of the five species 
investigated. The earliest tracheides to be organized are large and 
scalariform, and have no constant position in the stele. 
6. The internal and external phloems are equally well developed, each 
consisting of a single line of small elements, corresponding in size and 
position with the protophloem of mesophytic Ferns. The internal and 
external phloems are not continuous with one another through the 
leaf-gaps. 
7. The petioles may all be derived from a type with three endarch 
protoxylems such as has been described by Sinnott. This relatively 
primitive condition is found in Pellaea andromedae folia, and at the very 
base of the petiole in Cheilanthes Fendleri. Ch. Fendleri at higher level 
shows in successive sections its median protoxylem passing through the 
stele, becoming exarch, and then completely detached and isolated. The 
other Cheilanthes species all show a secondary simplicity of petiolar 
structure in the main part of the petiole. This can be in every case derived 
from the Ch. Fendleri type, the connexion being clearest in Ch. lanuginosa , 
where the Ch. Fendleri peculiarities appear at the base of the petiole, but 
die out at higher levels. 
Pellaea andromedae folia retains its three endarch protoxylems through- 
out the whole length of its petiole, but the median one is situated at the end 
of a narrow groove, and only a short distance from the abaxial surface of the 
xylem mass. 
8. The petiolar structure, the stem anatomy, and the greater output of 
spores per sporangium all point to Ch. Fendleri as a near approximation to 
an ancestral type from which Ch. gracillima and Ch. lanuginosa have been 
derived. 
Botany School, Cambridge. 
May , 1914. 
