464 PHANEROGAMS. 



of this a 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 tracheYdes with broad bordered 

 pits and hence with a more or less scalariform wall, are very much like the long prosen- 

 chymatous vessels of Vascular Cryptogams; and this resemblance extends even to 

 the tracheides of Coniferae, so far as they are distinctly prosenchymatous, although the 

 smaller number and round form of the bordered pits shows a more marked difference 

 (pp. 25-27). The bordered pits of Coniferae are usually developed only on the wall 

 M'hich faces the medullary rays, in one or two rows, but in Araucaria in larger numbers 

 and densely crowded. In the structure of the secondary wood, as in that of their flowers 

 and in their habit, Gnetaceae approach Dicotyledons ; in Ephedra broad vessels occur 

 in it together with the usual tracheides in the inner part of the ring of wood, but their 

 component cells are separated by oblique septa, and are therefore still prosenchymatous, 

 and are penetrated by several roundish holes ; their lateral walls show bordered pits like 

 the tracheides, and furnish a striking evidence that the true vessels in the secondary 

 wood of Dicotyledons are connected by intermediate forms with the vessels of Vascular 

 Cryptogams formed from prosenchymatous cells. It is stated that in the wood of 

 Wehvitschia tracheides with doubly bordered pits are entirely wanting, and that it 

 contains in their place thick-walled ' porous vessels.' 



The rays of the secondary wood of Coniferae are very narrow, often only one cell 

 in breadth ; the cells are strongly lignified, and their lateral faces in contact with the 

 adjoining tracheides are provided with closed dots. In Cycadeae the rays are broader, 

 and their tissue bears a closer resemblance to the parenchyma of the pith and cortex ; 

 their number and width cause the whole substance of the wood to appear spongy, 

 and its parenchymatous cells to be strongly curved in different directions when cut 

 across. The phloem-portion of the fibro-vascular bundles of Gymnosperms resembles 

 that of Dicotyledons ; it is mostly composed of true strongly-thickened bast-fibres, 

 cambiform cells, latticed-cells, and parenchymatous cells ; while in Conifera3 they are 

 formed in alternate layers. Usually the soft bast predominates. 



The Fimdamental tissue of the stem of Gymnosperms is separated by the ring of wood 

 into pith and primary cortex. Both are very strongly developed in Cycadeae, especially 

 the pith, and consist of true parenchyma, while the woody portion is considerably 

 smaller. In Wehvitschia the parenchymatous tissues appear also to prevail ; but the 

 greater part of their substance can only originate from the meristem-layer of the stem 

 already mentioned. A large number of so-called spicular cells occur dispersed in all the 

 organs of this remarkable plant, they are fusiform or branched and greatly thickened ; 

 and a number of beautifully developed crystals are found imbedded close to one another 

 in their cell-wall. Similar structures also occur in Coniferae (p. 66). 



The parenchymatous fundamental tissue of Coniferae decreases greatly with the 

 increase in age of the stem (and of the root). With the exception of the pith, which is 

 here small, the stem consists exclusively of the products of the cambium-ring, since the 

 primary cortex, and afterwards also the outer layers of the secondary cortex which 

 always have a subsequent growth, are used up in the formation of cork. In the stem of 

 Cycadeae, the increase of which in thickness is inconsiderable, the formation of cork 

 is also very small ; in Wehvitschia it appears to be entirely wanting (?). 



Sap-conducting Intercellular Passages are widely distributed in Gymnosperms; their 

 structure is that which has been explained generally at pp.73 and 115. In Cycadeae 

 they are found in all the organs in large numbers, and contain gum, which exudes 

 from incisions in thick viscid drops ; in Coniferae they contain oil of turpentine and 

 resin. In this latter order they occur in the pith of the stem, in the whole substance 



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



