232 Lady Isabel Browne. 
comparison, that endarchy is the primitive condition, it is more 
difficult to agree with him. He concludes that among Algae and 
Bryophyta there is a tendency to develop conducting tissue at the 
centre of the axis ; that the primitive protostele originated thus ; and 
that if this strand became too bulky for its elements to develop 
simultaneously, the tendency for the more central ones to mature 
would still hold good, thus giving rise to endarchy. But in using 
comparative evidence the structure of the more primitive members 
of the other Pteridophytic phyla should be brought into consi¬ 
deration. Exarchy appears to be primitive in the Sphenophyllales, 
Psilotales and Lycopodiales, while traces of centripetal xylem have 
been found in an ancient member of the Equisetales. So that, 
a priori , we should expect the Ferns to be primitively exarch, since 
all Pteridophyta probably possess a common vascular ancestor. 
Though some botanists would deny this, few, if any, would deny 
the origin of Ferns and Pteridosperms from a common vascular 
ancestor. But it is among the Pteridosperms and allied forms that 
the gradual replacement of the primitive centripetal xylem by 
centrifugal wood is most clearly illustrated (23), (25), (26). Thus on 
comparative grounds there is a presumption that the Botryopterideae 
were primitively exarch. This in no way invalidates the view that 
vascular tissue originated at the centre of the stem, since there are 
indications among the lower plants that this is so. But though the 
Botryopterideae appear to be somewhat primitive Ferns there are 
few indications that they are such very primitive vascular plants. 
Mr. Tansley admits that their fronds are elaborate and highly 
a 
evolved (31); they are often dimorphic, and in one genus, not yet 
fully described, a considerable amount of secondary xylem is 
present (24). Further, if exarchy is primitive it is easier to under¬ 
stand the structure of the stele in the Zygopterese, with its sharp 
differentiation into inner and outer wood, the latter presumably 
representing centrifugal xylem acquired later than and causing the 
reduction of the inner centripetal xylem. The outer wood is clearly 
not secondary, and if the inner wood developed centrifugally such 
a sharp line of demarcation between the two would be hard to 
understand. 
Hymenophyllace.®. 
As regards the Hymenophyllaceas, both Mr. Boodle and Mr. 
Tansley regard the type of stele of Trichomanes reniforme and of 
the larger Hymenophyllums as primitive for the order (3), (32). In 
this type the endarch protoxylem and a few other woody cells are 
