GNETACE.E. ^6^ 



There is a system of bundles cominon to the stem and leaves ; the portions whicli 

 descend into the stem forming a circle, ^vhere a closed cambium-ring is produced b\ 

 the formation of interfascicular cambium. This ring causes the permanent growth in 

 thickness of the stem. 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. No exclusively 

 cauline bundles are produced in the stem of Coniferae or of Ephedra ; but in Cycadege 

 and Wehvitschia bundles arise in the older stem which are nothing but ramifications 

 of the common bundles, although in their further development, to a great extent, 

 independent of them. Thus in the tissue of the pith of some Cycadese slender isolated 

 bundles occur ; while in some a system of thicker branches of bundles is developed in 

 the bark 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 Wel- 

 witschia which owe their origin to a layer of meristem enveloping the w^hole stem. The 

 Coniferae, 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 Coniferae, when narrow, contain only one fibro-vascular bundle from the stem, 

 which then usually splits into two halves running parallel to one another; 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 Conifera: these bundles seldom form prominent veins, but nm 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 Cycadeae 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 section (in Cycas revoluta, e.g. in the form of an inverted 12). 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 dichoto- 

 mise {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 arms 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 Cycadeae and Coniferse, 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 (p. 99) and the spiral vessels 

 of the medullary sheath. The secondary wood of Cycadeae and Coniferae is distin- 

 guished from that of Dicotyledons by the striking peculiarity that it is composed only 



(Jahrb. f. wiss. Bot. vol. IV. p. 329).-Geyler, Ueber Geflssbundelveilauf bei Coniferen (ditto, 

 vol. VI. p. 68).— Thomas, Vergl. Anat. des Conifer-Blattes (ditto, vol. IV. p. 43).— Mohl, Ueber die 

 grossen get.ipfelten R5hren von Ephedra (Verm. Schr. p. 269).-!. D. Hooker, On Wehvitschia 

 (Trans. Linn Soc. vol. XXIV.).— Dippel, Histologie der Coniferen (Bot. Zeit. 1862 and 1S63).- 

 Rossmann, Bau des Holzes (Frankfurt-a-M. 1863).— Mohl, Bot. Zeit. 1871. 



