344 FOURTH GROUP. SEED-PLANTS. 



Vascular Cryptogams, in which the vascular bundles, except in a very few cases, 

 contain only tracheides. The bordered pits in the Coniferae are usually developed 

 only on the cell-walls which are turned towards the medullary rays, and in one or 

 two rows ; in Araucaria they are in several rows and closely crowded together. The 

 Gnetaceae approach the Dicotyledons in the structure of their secondary wood, as 

 they do in that of the flower and in habit ; in Ephedra wide vessels are found along 

 with the ordinary tracheides in the inner part of the ring of secondary wood, but their 

 constituent parts are separated by oblique septa and are therefore still prosenchy- 

 matous, and are pierced with several roundish holes ; their lateral walls like those of 

 the tracheides show bordered pits. 



In Gnetum, as in the Cycadeae and many Dicotyledons, the growth in thickness 

 from the first cambium-ring ceases after a time, and a new zone of meristem is formed 

 in the secondary cortex outside the ring ; in this zone xylem-strands are formed on the 

 inside and phloem-strands on the outside alternating with medullary rays. As this 

 process is repeated more than once, a transverse section of an older stem or branch of 

 Gnetum scandens, for example, shows several concentric rings of growth, each con- 

 sisting of a xylem-ring and a phloem-ring. 



The stem of the Cycadeae shows at first the typical structure of the Gymnosperms and 

 Dicotyledons ; a ring of xylem, bast, and cambium proceeds from the primary leaf-trace- 

 ring and separates the outer cortex from the pith. Both pith and cortex contain gum- 

 passages and mucilage-passages. The xylem of the vascular bundles is formed of 

 tracheides ; the innermost and first formed have spiral threads, as in the Coniferae, 

 the others have scalariform thickenings. The tracheal elements of the secondary 

 wood are tracheides, which either have bordered pits arranged transversely in several 

 rows, as in Cycas and Encephalartos, or a combination of scalariform and reticulate 

 thickenings, as in Zamia. Zamia^ Dioon, and Stangeria retain this structure, the net- 

 work of primary bundles and the increase from the normal cambium-ring, all their life. 

 But in Cycas and Encephalartos, as in Gnetum^ growth in thickness from this source 

 is limited, and a new ring of growth is formed on the outer edge of the phloem-layer ; 

 and as this process is repeated, old stems of Cycas are found with from six to eight 

 successive rings of growth. Cycas also has a system of bundles in the cortical tissue, 

 Encephalartos in the pith 1 . 



The gnetaceous genus Welwitschia which is peculiar in many respects has also 

 anomalies in its anatomical structure, on which De Bary should be consulted. 



The medullary rays' 2 of the secondary wood are very narrow in the Coniferae, often 

 not more than one cell broad, and their cells are strongly lignified and have closed 

 pits on the walls which meet the adjoining tracheides. They are broader in the 

 Cycadeae, and their tissue is more like the parenchyma of the pith and cortex ; their 

 number and breadth make the whole woody body appear loose in texture ; and its 

 prosenchymatous elements are much curved in different directions in a tangential 

 section. The phloem-portion of the vascular bundles of Gymnosperms resembles 

 that of the Dicotyledons ; it consists usually of true much thickened fibres, cambiform 

 cells, sieve-tubes and parenchyma, which in the Coniferae are formed in alternate 

 layers ; the soft-bast generally predominates. 



The fundamental tissue of the stem of Gymnosperms is separated by the woody 

 cylinder into pith and primary cortex. Both these are largely developed in the 

 Cycadeae, the pith especially, and consist of true parenchyma ; the mass of wood is 

 much smaller. In Welwitschia also the parenchymatous tissues predominate, but the 

 larger part of them must be formed by an outer meristematic zone in the stem. A large 

 number of the so-called spicular cells are found scattered about in all the organs of 

 this remarkable plant ; they are fusiform or v branched with greatly thickened walls, 



1 See De Bary, Vergl. Anat. p. 612, English translation. 



2 [Kleeberg, Die Markstrahlen d. Coniferen (Bot. Ztg. 1885).] 



