5^3 
Knowledge of Rachiopteris cylindrica , Will. 
place of separation the leaf-trace does not appear to possess any adaxial 
metaxylem (PI. XXVI, Figs. 6 and 9 ; Text-figs. 8 and 9). A few elements 
may be developed higher in the petiolar trace, but at a slightly higher level 
still they tend to disappear again ; their formation may be regarded as 
indicating an earlier condition similar to that seen in Botryopteris antiqua , 
in which some adaxial metaxylem is present at the level where the trace 
separates from the stem stele, and which is therefore still more suggestive of 
modified stem-branching. 
This view of the origin of the leaf is further supported by the similar 
behaviour of the protoxylem in branching and in leaf formation. In 
a types, the protoxylem group of both branch- and leaf-traces divides more 
or less definitely; in /3 types, no division normally takes place in either 
instance. 
VI. Summary. 
1. Distribution and Horizon , p. 532. 
Rachiopteris cylindrica appears to be restricted to the Halifax- 
Huddersfield area, where it occurs in the nodules of the Halifax Hard 
Bed of Lower Coal Measure Age. 
2. Description , p. 532. 
The stems and their corresponding petioles may be referred to two 
types, described as a and (3 respectively. 
i. a stems are characterized by a well-developed xylem strand exhibit¬ 
ing a marked tendency towards mesarch structure, with differentiation 
of the central elements ; the inner and middle cortical areas have fairly 
thick-walled cells, while the outer cortex is composed of a few layers of 
thin-walled cells, suggestive of an assimilatory tissue, a petioles also have 
well-developed xylem strands, frequently with distinct diarch structure ; 
their cortex is like that of a stems. 
ii. (3 stems possess only a small monarch, centrarch xylem strand. 
The cortex is wide and composed of thin-walled cells; the middle area 
is more or less lacunar, and the outer layers of the stem seem to be of the 
same nature as those of a stems. The corresponding petioles have also 
a wide cortex, and a reduced xylem strand which is always monarch. 
3. The Significance of the Occurrence of a and [3 types , p. 553. 
It is probable that the differences of structure between the a and 
ft types throw some light on the autecology of Rachiopteris cylindrica , 
which, it is suggested, was amphibious, a and (3 plants being respectively its 
land and water ecads. 
4. Relationships , p. 554. 
R. cylindrica seems to be closely allied to Botryopteris antiqua , 
