35.] CHAPTER II. THE TISSUES. 197 



may eventually become chambered by the formation of delicate 

 transverse septa (see p. 133). 



The structure of the secondary wood of the root is in some cases 

 (e.g. Conifers) almost identical with that of the corresponding 

 stem ; this is the case, to a somewhat less degree, in woody Dicoty- 

 ledons ; whilst in herbaceous Dicotyledons the structure may be 

 very different in the two members, owing, chiefly, to the develop- 

 ment of more wood-parenchyma, but less fibrous tissue, in the root 

 (see above p. 194). 



The following is a brief enumeration, with examples, of the chief varieties of 

 structure presented by the secondary wood of the stem. 

 The secondary wood may consist 



1. Solely of tracheids : Taxus baccata (Yew), Drimys Winter i. 



2. Of tracheids and wood-parenchyma : Conifers (except Taxus). 



3. Of vessels, tracheids, and wood-parenchyma: Ilex, Staphylea, Eosa, 

 Pyrus, Cratasgus. 



4. Of vessels, tracheids, wood-parenchyma, and thin-walled fibrous cells : 

 Jasminum, Kerria, Potentilla, Casuarina, Aristolochia. 



5. Of vessels, tracheids, thick- walled fibrous cells, and wood-parenchyma : 

 Acer, Sambucus, Euonymus, Vitis, Fuchsia, Hedera. 



6. Of vessels, tracheids, and thin- and thick-walled fibrous cells : Ephedra, 

 Mahonia, Berberis. 



7. Of vessels, tracheids, woody fibre, and wood-parenchyma : Calycanthus, 

 Khamnus, Ribes, Quercus, Carpiuus, Prunus. 



8. Of vessels, tracheids, woody fibre, wood-parenchyma, and thin-walled 

 fibrous cells ; this is the most common type of structure, and is to be found 

 in most dicotyledonous trees and shrubs, e.g. Salix, Populus, Aluus, Betula, 

 Juglans, Tilia, Ailanthus, Magnolia, Eobinia, Gleditschia, Ulex. 



9. Of vessels, woody fibre, and wood-parenchyma : Avicennia. 



10. Of vessels, woody fibre, and thin-walled fibrous cells : Viscum. 



11. Of vessels, woody fibre, wood-parenchyma, thin-walled fibrous cells: 

 Platanus, Fraxinus, Citrus. 



12. Of vessels, thick- walled fibrous cells, and wood-parenchyma : Cheiranthus, 

 Begonia. 



A transverse section of a stem or a root of most coniferous or 

 dicotyledonous trees or shrubs exhibits, even to the naked eye, a 

 series of concentric layers in the secondary wood known as the 

 annual rings. These layers result from the fact that the wood 

 formed in the spring is differently constituted from that which is 

 formed later in the year. The anatomical cause of the distinct- 

 ness of the annual rings is the same in all cases, namely, that the 

 last-formed xylem-elements of an annual ring have a very small 

 radial diameter. In Conifers this distinction is emphasized by the 



