594 D tit hie. — Anatomy of Gnetum africanum. 
by Welwitsch in the Cazengo district of Central Angola in 1858, and 
was described by him in the Linnean Transactions of 1869. The species 
is a climber with long slender stems which are swollen and jointed 
at the nodes ; the much elongated internodes are sometimes more than 
15 cm. long. The plant exhibits marked heterophylly. In the material 
examined the lateral branches spring from the axils of short, opposite scale 
leaves ; while the expanded foliage leaves produce only abortive buds 
in their axils. On superficial examination the scale leaves resemble the 
persistent bases of the petioles of foliage leaves, and were apparently 
mistaken for such by Welwitsch. 1 No root material was available. 
Stem. 
The oldest material examined shows a stem 5 mm. in diameter, 
its surface covered with a thin layer of cork. The younger internodes 
are marked with numerous, irregularly arranged dots and striations of a 
dark brown colour, while the youngest branches are smooth and green. 
The secondary thickening is perfectly normal ; there is no trace of the 
successively renewed thickening rings which are present in G. scandens 2 
and G. Thoa. It is possible that this abnormality may occur in older stems 
than those investigated. The number of vascular bundles seen in transverse 
section of the axis varies considerably ; from 12 to 20 were observed in the 
internodes examined. 
Xylem. The wedge-shaped xylem-masses are separated by conspicuous 
medullary rays (PI. LVII, Fig. 1). The first-formed elements of the proto- 
xylem have loosely coiled, spiral thickenings occasionally combined with 
annular markings. Those formed later have very close spiral or reticulate 
thickenings and often communicate by means of a row of large pits on the 
oblique walls. Reticulately thickened tracheides are specially abundant in 
the region of the node. Boodle and Worsdell 3 have pointed out that in 
Gnetum the vessels first formed by the cambium have usually several per- 
forations in their end walls, resembling in this respect the well-known 
vessels of Ephedra. In several cases two rows of large, circular, bordered 
pits were seen in the sloping end of a reticulate nodal tracheide of G. 
africanum , and one of the vessels was found to possess two rows of perfora- 
tions on the oblique wall, as described by the above authors in the node 
of G. paniculatum . In general, however, only a single row of perforations 
occurs in these first-formed vessels. Transitional stages between pitted 
tracheides with a row of large, closed, bordered pits on their terminal wall, 
and true vessels were observed, the central pits of the oblique walls being 
perforated, those at the ends closed. Transitional stages between vessels 
with several circular perforations and those with a single long narrow one 
1 Welwitsch (9). 
2 De Bary (3). 
3 Boodle and Worsdell (1). 
