SECONDARY THICKENING. NORMAL DICOTI'LEDONS. 495. 



stem-wood of the rest of the Coniferae they form only a single longitudinal row, apart 

 fi'om individual exceptions, which are frequent, for example, in Larix. In the Coniferae 

 pits only occur on the tangential surfaces in the autumn wood. Taxus, Cephalotaxus, 

 and Torreya further show spiral or annular thickenings on the inner surface of the wall 

 of the tracheide\ Comp. Chap. IV. 



The resin-canals, surrounded by parenchyma as epithelium, occur in the ligneous 

 bundles of the same Abietins which possess horizontal canals in the medullary rays 

 (p. 490). They run longitudinally, and, as seen in transverse section, lie scattered in a 

 ring in the external region of every annular layer. Their number varies according to 

 species and individual; Von Mohl- counted, for example, on an equal transverse surface 

 embracing several annual layers, in Pinus nigricans 190, P. silvestris 124, Larix europaea 

 128, Picea excelsa 78. 



2. The ligneous bundle of all Dicotyledonous woods, except those mentioned 

 above, and of the Gnetaceae (Ephedra, Gnetum) always contains vessels, and at least 

 one of the forms of cells distinguished; of the latter bundle-parenchyma and inter- 

 mediate fibres are usually present together. These are further accompanied, accord- 

 ing to the species, by some or all of the other tissues characterised in Sects. 142-144. 

 Of the combinations thus possible, the following have been demonstrated, according 

 to Sanio's statements, in the trees and shrubs investigated. 



1. Vessels, Tracheides, Bundle-parenchyma, Intermediate Jibres. 



{a) Only with bundle-parenchyma: Ilex Aquifolium, Staphylea pinnata, Rosa 

 canina, Crataegus monogyna, Pyrus communis. Spiraea opulifolia, Camellia, &c. 



(b) Only intermediate fibres : Porlieria. 



{c) Both forms of cell : Jasminum revolutum, Kerria, Potentilla fruticosa, Casua- 

 rlna equisetifoha and torulosa, Aristolochia Sipho, and many others. 



2. Vessels, Tracheides, Fibrous cells, Bundle-parencl?yma, Intermediate Jibres . 



(a) Only bundle-parenchyma: fibrous cells non-septate: e.g. Sambucus nigra, 

 racemosa, Acer platanoides, pseudoplatanus, campestre. 



(b) Bundle -parenchyma and intermediate fibres, fibrous cells non-septate : 

 Ephedra monostachya, Berberis vulgaris, Mahonia^. 



(c) Bundle-parenchyma, fibrous cells septate and non-septate: Punica, Euonymus 

 latifolius, europaeus, Celastrus scandens, Vitis vinifera. Fuchsia globosa, Centradenia 

 grandifolia, Hedera Helix, &c. 



(d) All four forms of cell: Miihlenbeckia complexa, Ficus (?). 



3. Vessels, Tracheides, Woody Jibres, Bundle-parenchyma, Intermediate Jibres. This is 

 the prevalent, one may almost say the typical, combination. 



{a) Only bundle-parenchyma : Sparmannia africana, Calycanthus, Rhamnus ca- 

 thartica, Ribes riibrum, Quercus, Castanea, Carpinus spec, Amygdaleae, Melaleuca, 

 Callistemon spec, &c. 



{b) Only intermediate fibres : Caragana arborescens. 



{c) Both forms of cell. Most Dicotyledonous woods no doubt belong to this series, 

 e. g. Salix, Populus spec, Liriodendron, Magnolia acuminata, Alnus glutinosa, Betula alba, 

 Juglans regia, Nerium, Tilia, Hakea suaveolens, Ailantus, Robinia, Gleditschia spec, 

 Ulex europaeus, &c 



' Compare Hartig, Kraus, Goppert, I.e. — Von Mohl, Schacht, Botan. Zeitg. 1862. 



^ Bolan. Zeitg. 1859, p. 340. 



" Compare Sanio, in Pringsheim's Jahrb. IX. p. 55. 



