514 SECONDARY CHANGES. 



means conspicuous under microscopical investigation, while the middle of the thin-walled 

 zone appears to the naked eye as a sharp line of demarcation. Possibly the same 

 conditions may exist in the stems of A. brasiliensis, which have been the subject of 

 controversy. A branch of the same specimen of A. excelsa, about 2<"" thick, with 

 narrow rings, shows the same characteristics in some parts, while in others there is a 

 decided flattening of the tracheides at the autumnal limit. 



The question whether the formation of two successive rings in one period of vegeta- 

 tion occurs as an individual deviation from the rule, even in our woody plants which 

 typically form a single ring, is essentially foreign to the present anatomical survey. It 

 must be remarked, however, that this phenomenon is stated to occur as a rare exception, 

 and in fact as a consequence of the interruption of the summer growth by external 

 causes (frost, drought, attacks of insects, hailstorms \ &c.). As regards the anatomical 

 characteristics of these anomalous double annual rings, no other statements have been 

 published, than that the boundary between them is usually indistinct : only in the case 

 of shoots of Sambucus nigra, interrupted by a hailstorm in very vigorous vegetation in 

 1846, does Unger mention 'two distinct woody rings.' 



Sect. 157. The existing investigations show that the structure of most woods 

 remains essentially constant, within the limits defined by the preceding statements. 

 But exceptions even to this rule occur, the most remarkable of which are afforded 

 by the Ash (Fraxinus excelsior ''). In a well-grown tree of this species, the annual 

 ring, which is about 2-3™"! in breadth, shows on the inside a zone of spring-wood, 

 consisting of slightly thickened fibres, between which wide vessels surrounded by 

 bundle-parenchyma are inserted; then follows externally the thick middle layer, 

 consisting of thicker-walled fibres, with scattered smaller vessels, likewise surrounded 

 by bundle-parenchyma ; finally, at the outside, there is the autumnal limiting layer, 

 consisting of several rows of bundle-parenchyma, with small, very thick-walled 

 vessels. In very thin annual rings, the reduction of the middle layer takes place as 

 described above. In the case of very luxuriant young trees, grown on damp soil, with 

 annual rings over 12™™ thick, V. Mohl found the fibres less thick-walled, and the 

 vessels, especially the large ones, narrower than in moderately thick rings. Sanio 

 found a specimen which differed conspicuously from the usual form, in having 

 concentric zones of parenchyma, containing narrow vessels in the middle layer ; in one 

 piece only there was an annual ring similar to the usual type. Another stem, which 

 was stunted, and at an age of fourteen years was only 15mm thick, showed a strikingly 

 feeble development, and in the thinnest rings actual suppression, of the charac- 

 teristic spring layer, in contradistinction to the rule holding good for narrow annual 

 rings, while the vessels were everywhere remarkably narrow. The mean width of 

 the largest of the latter in one ring was o-o>j^^, that of the large vessels in V. Mohl's 

 broad-ringed stems was o-iymm^ ^hile in normally grown trees it is about o-26n«u — 

 Sparmannia africana, as more minutely described by Sanio, I.e. p. 399, shows a 

 conspicuously different structure, even in successive transverse portions of the same 

 stem, or on different sides of the same ring, broad bands of irregular large-celled 

 parenchyma being sometimes present, sometimes absent. 



' Unger, Botan. Zeitg. 1847, p. 265.— Nordlinger, Holzring, p. 10. 

 ' Von Mohl, I.e. p. 269.— Sanio, Botan. Zeitg. 1863, p. 398. 



