LEAF IN THE VASCULAR CRYPTOGAMS AND GYMNOSPERMS. 
599 
with that of the Cycadaceae it appears that the two structures are not unlike at an 
early period (figs. 49, 38, 33, 24) ; the difference between them when mature depends 
mainly upon the large development of the pinnae, and the almost complete abortion of 
the wings of the phyllopodium in the Cycadaceae, while in Gnetum the pinnae are 
entirely absent, and are replaced by a larger development of the wings. 
A few words must be said upon a certain irregularity in the arrangement of the 
bundle-system in species of Gnetum, though this lies outside the present subject. As 
already described ( l.c ., p. 285), the leaf-trace of Gnetum Gnemon consists of five 
bundles, one being median. I have found the number of bundles of the leaf-trace to be 
uneven, one bundle being median, in the following specimens supplied from the Her¬ 
barium at Kew: G. latifolium, Celebes; G. paniculatum, Brazil; G. scandens, G. venosum, 
America ; also in an unnamed species (Motley, No. 1063) from Borneo. In these plants 
the single median bundle could be traced as continuous from the stem into the upper 
regions of the leaf, and it coalesces with the lateral bundles rarely, if at all. In one 
species, viz., G. Africanum* the central bundle is found to be absent : the leaf-trace 
consists here of but four bundles disposed in two pairs. Tracing these upwards 
into the leaf, they are found to take distinct courses, and in the leaf which I 
investigated, no fusion was seen to take place between the pairs. This difference of 
the bundle-system is not accompanied by any marked modification of outer confor¬ 
mation ; the leaf is even accuminate, while in some species with a median bundle the 
apex of the leaf is emarginate ( e.g ., G. venosum). This inconstancy of arrangement 
of the vascular bundles is particularly interesting as occurring in the Gnetaceae. It is 
well known that a single median bundle is not as a rule to be found in the leaves of 
the Gymnosperms.t Here in one genus (Gnetum), which approaches the Dicoty¬ 
ledons so markedly in the character of its leaves, the gap is bridged over by the 
presence in some species of a single median bundle, as in so many Dicotyledons, while 
in one species (or more) with a similar external conformation of the leaf, the median 
bundle is absent, and the vascular system thus conforms rather to the type of most 
Gymnosperms. 
Other cases such as this, wdiich also occur, though in a less clearly defined manner,J 
show how insecure are those attempts, so frequently made, to solve morphological 
problems by reference to the position of the vascular bundles. 
Welwitschia mirabilis. 
In the cotyledons, from the period of ripeness of the seed, the growth is intercalary, 
and not specially localised at any point. It results in the formation of a flattened. 
* A similar observation lias been made by Strasburger, ‘ Conif. n, Gnet.,’ p. 115. 
f Warming, ‘ Reclierches et remarques sur les Cycadees,’ pp. 22, 23. 
f Compare the cotyledons of Cycas above described, those of Zamia (Warming, l.c., Taf. 3, fig. 28), and 
of certain Dicotyledons (De Bary, ‘ Vergl. Anat.,’ p. 246, &c.). 
