6 ^^2 PHA NER OGA MS. 



b. With no parenchyma ; Caragana arhorescens, 



c. With both kinds of cells; most foliage-trees and shrubs, e.g. Snlix, 



Populus sp., Lir'iodendron, Magnolia acuminata, Alnus glutinosa, Bftula 

 alba, Juglans regia, Nerium, Tilia, Hakea sua'veoltns, Ailanthus, 

 Robinia, Gleditschia sp., Ulex europceus, &c. 



5. Wood consisting of vessels, fibres, parenchyma, and intermediate cells: — 



a. With no parenchyma; Viscum album. 



b. With no intermediate cells ; Avicennia. 



c. With both kinds of cells ; Fraxinus excelsior, Ornus, Citrus medicuy 



Platanus, &c. 



6. Wood consisting of vessels, fibres, and parenchyma: — 



Cheiranthus Cheiri, Begonia. Also many Grassulaceae and Caryophyllaceae. 



7. W^ood consisting of vessels, fibres, parenchyma, and true woody fibres^ (?) : — 



Coleus Macraei, Eugenia australis, Hydrangea hortensis. 



8. Wood consisting of vessels, tracheides, woody fibres, septate fibres, parenchyma, 



and intermediate cells : — 



Ceratonia siliqua, Bignonia capreolata ; it is however still doubtful if true 

 woody fibres are present.] 



When the increase in thickness of a stem ceases periodically and is renewed 

 with each new period of vegetation, as in our woody plants, a layer of wood is 

 formed during each period of growth (and usually also a secondary cortical layer), 

 which is sharply marked off from those of the preceding and of the following year, 

 and is called an Annual Ring of the wood. These annual rings are usually distinctly 

 visible to the naked eye, because the mass of wood formed in the early part of each 

 period of vegetation has usually a different appearance from that formed in the autumn, 

 the latter being denser, the former less dense and generally with a greater number of 

 vessels. The wood formed in the spring consists also of wider cells than that produced 

 in the autumn, and the radial diameter of the cells is usually greater. The cells formed 

 in the autumn appear compressed radially and broad in the tangential direction ; their 

 cavities are smaller, and hence, other things being equal, the thickness of their wall is 

 greater. A given quantity of wood produced in the autumn is therefore denser than 

 a like volume formed in the spring 2. While Dicotyledons differ so widely from Mono- 



^ [The distinction which is drawn between 'fibres' and 'true woody fibres' is in their contents : 

 the former nearly always contain starch, in some cases {Spircea salicifolia, young wood of Vitis 

 vinifera and Centradenia grandifolia) chlorophyll, and in others {Syringa vulgaris) tannin; the latter 

 contain air and water, and sometimes a mere residue of protoplasm. These two kinds of fibres 

 usually occupy the same relative position in a fibro^vascular bundle, and it is doubtful if they are 

 ever both present in the same bundle: the 'fibres ' gradually become converted into 'woody fibres.'] 

 ^ The cause of this difference is not yet known ; but I suppose that it depends simply on the 

 difference in pressure to which the cambium and the wood are subjected from the surrounding cortex. 

 This pressure is less in the spring, and constantly increases till the autumn. I have no direct 

 measurements of this, but conclude it from the fact that the longitudinal fissures in the bark 

 become wider in F'ebruary and March, as may be clearly seen in the Oak, Maple, Poplar, Walnut, &c. 

 I cannot here explain the cause of this ; but in any case the bark, the longitudinal fissures of which 

 have become wider in winter, must exert less pressure on the cambium in the spring, and the cells of 

 the wood must therefore be able to extend more easily in a radial direction. The pressure which 

 the bark exerts on the cambium must continually increase by the thickening of the ring of wood 

 internally and the drying up of the bark in summer externally, and must affect the radial growth of 

 the young cells of the autumnal M^ood. Further investigations which I am proposing to make will 

 determine whether my theory is correct.' — This hypothesis, which I brought forward in the first 

 edition, has recently been fully confirmed by the researches of H. de Vries. (See Flora, 1872, no. 16, 

 and sect. 15 of Book III of this work.) 



