18 
DR R. C. DAVIE ON 
Dioon spinulosum, Dyer. 
In Dioon spinulosum , Dyer, the leaf-trace has an outline resembling that of 
Cycas revoluta , but the abaxial curve is folded inwards on itself as it joins the 
antero-posterior series of strands (text-fig. 12a). The pinna -traces are given off 
from the margins of the adaxial extensions of the leaf-trace (text-fig. 126, c, d, e). 
As in Cycas revoluta, the abaxial curve alters and simplifies its shape as it passes 
up the rachis, and its strands are drawn in to replace those that pass out to the 
pinnae. The outline of the leaf- trace below the ultimate pinna (text-fig. 12/) is 
less simple than that of Cycas revoluta at the same level (see text-fig. 10c), but 
the supply of the pinna in the two cases is provided on the same system. The 
leaf of Dioon spinulosum is longer than, that of Cycas revoluta ; its pinnae are 
larger and more numerous. Both plants have leaves longer than those of Stangeria 
c d e f 
Text-fig. 12 . 
paradoxa, which also possesses a much smaller number of pinnae. The pinna-traces 
are given off in exactly the same fashion in all three. The differences in the leaf- 
traces are in the abaxial : curves and their elaborations. A comparison of text- 
figs. 9, 10, and 12, made in' view of the different lengths of the leaves of the three 
genera, makes it difficult to resist the conclusion that the abaxial developments in 
the Cycad leaf-trace are related to the length of the leaf. In the Cycads, the 
longer the leaf the more elaborate is the system of vascular strands on the abaxial 
curve of the leaf-trace. 
At this point in the consideration of the features of the leaf-trace and pinna- 
trace among the Cycads, we stand in the position corresponding to that which we 
reached in the discussion of the Fern leaf- trace at the close of the account of the 
genus Polypodium. From the information which we have obtained from the study 
of the genera described above, we may draw conclusions which are generally 
applicable to the leaf- traces of the Cycads as a whole, but which require to be 
supplemented in order that they may apply to two genera, of which we have not 
yet considered any examples. These genera are Encephalartos and Bowenia. 
