GNETACEM. 
■ descend into the stem forming a circle, where a closed cambium-ring is produced by 
the formation of interfascicular cambium. The ascending portion, which curves out into 
the leaf itself, assumes in Cycadeae more or less the character of a closed bundle, while 
in the leaves of many Coniferse it at least retains the appearance of an open bundle. [In 
most cases the growth in thickness of the stem is brought about by the division of the cells 
of the cambium-layer, but in Cycas and Encephalartos the activity of the cambium-layer 
is of limited duration. The subsequent growth in thickness of the stem in these two 
genera is effected by the production of successive new cambium-rings, which make their 
appearance just outside the bast of the older vascular bundles.] No exclusively cauline 
bundles are produced in the stem of Goniferae or of Ephedra ; but in soms Gycadeae acces- 
sory-bundles arise in the older stem which are independent of the common bundles and of 
those formed by the cambium-layers. Thus in the tissue of the pith of Encephalartos slender 
isolated bundles occur ; while in Cycas a system of thicker bundles is developed in the cortex 
which may form there in old age one or more apparent rings of wood. As far as we can 
judge from Hooker's description, bundles occur in the bark of Wekvitschia which owe their 
origin to a layer of meris'em enveloping the whole stem^ The Goniferae, as has been 
mentioned, possess only common bundles, the descending portions passing through a 
number of internodes, and then joining others lower down either unilaterally or on both 
sides by splitting into two arms and turning to both sides. The leaves of Goniferse, 
when narrow, contain only one fibro-vascular bundle from the stem, which then usually 
splits into two halves running parallel to one another (Fig. 102); when the leaves are 
broader, two [Salisburia, Ephedra) or even three bundles occur ; when the leaf forms 
a flat broad lamina, as in Salisburia and Dammara, the bundles ramify in it, but without 
forming a net-work; in Salisburia they repeatedly branch dichotomously. In Goniferae 
these bundles seldom form prominent veins, but run through the middle of the tissues of 
the leaf. In the two gigantic leaves of Wehvitschia there are a number of bundles, the 
parallel ramifications of which run into the middle layer of tissue. In the large pinnate 
leaves of Gycadeae there are also several bundles which curve nearly horizontally within 
the cortical parenchyma, and split into a number of stout bundles in the leaf-stalk when 
it is thick; these bundles exhibit a beautiful arrangement when seen in transverse sec- 
tion (in Cycas renjoluta, e.g. in the form of an inverted ß). They run parallel in the 
rachis of the pinnate leaf, and give off branches into the pinnae, where they either run 
( parallel in the middle layer of tissue (as in Dion) or dichotomise {e.g. Encephalartos) ; 
while in Cycas they form a mid-rib projecting beneath. The course of the bundles in 
the leaf therefore shows a decided resemblance to that of many Ferns. 
The substance of the wood of the stem is formed from the descending bundles, 
which are at first completely isolated, but soon coalesce into a closed ring by portions of 
cambium which cross the medullary rays. The primary wood or xylem, termed the 
Medullary Sheath, which consists of the xylem-portions of the descending limbs of the 
common bundles, contains, in all Gymnosperms, as in Dicotyledons, long narrow vessels 
with annular or spiral thickening-bands, while further outwards occur scalariform or 
reticulately thickened vessels. The secondary wood produced from the cambium-ring 
after the cessation of growth in length consists, in Gycadeae and Goniferae, of long 
tracheides grown one into another in a prosenchymatous manner {cf. p. 25) with a few 
large bordered pits, which are usually circular, at least when the wood is mature. Every 
possible stage of transition occurs between these tracheides and the spiral vessels 
of the medullary sheath. The secondary wood of Gycadeae and Goniferae is distin- 
guished from that of Dicotyledons by the striking peculiarity that it is composed only 
of this prosenchymatous form of cells 2); and that the wide dotted vessels composed of 
short cells are wanting which penetrate the dense narrow-celled masses of the wood of 
Dicotyledons. In the younger stems of Gycadeae the tracheides with broad bordered 
^ [De Bary compares the structure of the stem of Welwitschia to that of Chenopodiace^ and 
Amarantacese, etc. (see infra) ; that of Gtielnm to that of Phytolacca, Polyga'.ere, Dilleniacece, etc.] 
^ Wood-parenchyma is not formed, or only in small quantity. 
]\ I m 2 
