THE LEAF-TRACE IN SOME PINNATE LEAVES. 
21 
This method may be due simply* to lack of space on the abaxial side of the 
leaf-trace, or, more probably, to the basipetal sequence of development in the pinnae. 
Apparently a fair proportion of the Cycads (including species of Encephalartos ) 
develop their pinnae in basipetal succession (Bower, ’84). Where the pinnae which 
develop late are large it is quite likely that the vascular system which sufficed 
for those near the tip of the leaf may be unable to provide strands enough for them. 
For these pinnae a new reinforcing system is required. Such a system could find 
room only on the adaxial face of the leaf-trace, since the abaxial side is quite fully 
occupied by the folds of the abaxial curve. It would seem as if in Encephalartos 
Altensteinii the abaxial curve is used directly to supply reinforcement for a few 
pinnae well below the tip of the leaf while the new reinforcing system on the adaxial 
face is being elaborated to meet the needs of the larger pinnae which occur success- 
ively lower on the rachis. 
The adaxial reinforcing system does not appear in every Cycad leaf in which there 
is a basipetal sequence of pinnae, but its appearance in the leaf of Encephalartos 
Altensteinii and in other species of the genus Encephalartos, where large pinnae are 
found, certainly suggests a connection between the size and sequence of the pinnae on 
the. one hand and the construction of the leaf-trace on the other. 
There is among the Ferns an interesting analogy to this in the Marattiaceae. 
The leaf- traces of Angiopteris , Marattia, and Ar chang iopteris stand aloof from the 
type which prevails among the Filicales -both in general construction and in the 
method of branching below the pinnae (G- Wynne- Vaughan, ’05 ; Davie, T4), though 
they show some resemblance to those of some of the Ophioglossales. In the 
formation of the pinna-traces the strands at the corners of the leaf-trace nearest to 
the pinna are employed together with some of the strands situated directly on the 
adaxial side of the rachis. This is found, for example, in Arcliangiopteris (Gwynne- 
Vaughan, ’05, p. 265). Both in this genus, however, and in Angiopteris the details 
connected with the formation and use of the adaxial reinforcing system differ entirely 
from those in Encephalartos. 
But the suggestion of an analogy is strengthened when it is recalled that in 
Angiopteris the apical growth of the leaf is arrested at an early stage (Bower, ’84, 
pp. 581, 605). 
The aloofness of the type of leaf-trace in Encephalartos from that of the rest of 
the Cycads (except Bowenia, which no doubt owes its isolation to its practically 
unique form of leaf) is thus parallel to the aloofness of the Marattiaceae from the 
rest of the Filicales. 
One can merely note the analogy between the Marattiaceae and the Cycads in 
connection with some of the features of the leaf-trace ; but from both we may con- 
clude that there is some relation between the construction of the leaf- trace and the 
mode of growth of the leaf. 
