464 GNETALES [CH. 



integument, c, is united to the nucellus for about two- thirds of the 

 length of the seed-body. The nucellus consists of a few layers of 

 cells and at the apex forms a nucellar cap, n, the cells of which are 

 lignified ; this cap is supported by a short tent-pole produced from 

 the summit of the endosperm. A ring of vascular bundles enters 

 the base of the seed and forms three series, the two outer pass up 

 to the tip of both the two outer coverings, v, v, and the inner 

 series extends up the inner integument as far as the level where 

 nucellus and integument part company. In the seed shown in 

 fig. 817 the micropyle is closed and the tissue in the closed region 

 of the canal is lignified and dark. Among other features in which 

 this seed agrees with that of Bennettites Morierei is the inner zone 

 of the outer integument, composed of a palisade and a fibrous 

 layer; the fibrous layer becomes five-angled in the upper part of 

 the seed 1 and in transverse section presents a striking similarity to 

 sections through the same region of a Bennettites seed (figs. 524, 527, 

 Vol. in. pp. 397, 402). The oval fleshy seeds are able to germinate 

 after lying some months in sea- water. There is a fairly close 

 resemblance between Gnetum and Ephedra as regards anatomical 

 characters, but some species of Gnetum (sect. Thoa) are character- 

 ised by the formation of successive cambial cylinders as in Cycas. 

 Strasburger 2 pointed out that in the vascular bundles of the leaves 

 the parenchyma of the medullary rays forms continuous plates, a 

 gymnospermous character. 



Welwitschia. 



This remarkable genus, discovered by Welwitsch in 1860 and 

 described by Sir Joseph Hooker 3 , presents striking peculiarities in 

 the habit of- the vegetative body. A Welwitschia plant has been 

 aptly termed an adult seedling 4 ; the large and squat tuberous 

 stem, morphologically the swollen hypocotyl, may be as much 

 as 4-5 m. in girth. The seedling has two cotyledons and an ex- 

 ceptionally long radicle: at an early stage a pair of isobilateral 

 leaves is produced at right-angles to the cotyledons and these 



1 Thoday (Sykes) (11) p. 1116, text-fig. 11. 



2 Strasburger (91) p. 148. For an account of the anatomy of Gnetum, see 

 Duthie (12); La Riviere (16); Thomson, M. R. H. (16). 



3 Hooker, J. D. (63). For figures of Welwitschia, see also Gard. Chron. Jan. 22, 

 p. 49, 1910. 4 Sykes (10 2 ) p. 333. 



I 



