ONETACEM. 



531 



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 Coniferae 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 Coniferae or oi Ephedra ; but in some Cycadeae 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 Wehvitschia which owe their 

 origin to a layer of meristem enveloping the whole 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 (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 Coniferae 

 these bundles seldom form prominent veins, but run through the middle of the tissues of 

 the leaf. In the two gigantic leaves of Welivitschia 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 sec- 

 tion (in Cycas re^oluta, e,g. in the form of an inverted Q). 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 foVmed from the descending bundles, 

 which are at lirst 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 Cycadeae and Coniferae, 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 Cycadeae ajid Coniferae is distin- 

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

 of this prosenchymatous form of cells ^); 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 Cycadeae the tracheides with broad bordered 



^ [De Bary compares the structure of the stem of Welwitschia to that of Chenopodiaceae and 

 Amarantacese, etc. (see infra) ; that of Gnetnm to that of Phytolacca, Polyga!e?e, Dilleniaceoe, etc.] 

 ^ Wood-parenchyma is not formed, or only in small quantity. 



M rn 2 



