SECONDARY THICKENING. NORMAL DICOTYLEDONS, 50T 



the two. In addition to this cause of the demarcation of the annual rings, which con- 

 sists in the distribution of non-equivalent tissues, and is not always present, there are 

 tw^o other causes, consisting in differences of form and differences of structure of 

 equivalent tissue-elements, appearing in the successive, and especially in the extreme 

 zones of an annual increment of growth. 



The first of these phenomena consists universally, in a shortening of the radial 

 diameter, and thus in a tangential flattening of the elements, at the outer limit of the 

 autmnn-wood, and this is due to increasing cortical pressured Changes in the 

 average length may, as has been stated, be connected with this. The second cause, 

 not present in all cases, consists in an increase in the thickness of the wall, and some- 

 times in further changes of its structure. These changes, especially the shortening of 

 the radial diameter, affect both the elements of the medullary rays and those of the 

 ligneous bundles. They come on either suddenly or gradually, and in fact this latter 

 difference depends, partly on species, pardy on differences in the vigour of development 

 of the annual rings in one and the same w'ood. 



These conditions are shown most simply and clearly in the woods of the Conifer cb"^, 

 the bulk of which consists only of tracheides and medullary rays. In the procumbent 

 elements of the medullary rays, the shortening of the radial diameter at the autumnal 

 limit is not very conspicuous, though it does occur. The tracheides however are rela- 

 tively wide at the spring limit of each annual layer, usually quadrangular as seen in cross- 

 section, though sometimes pentagonal or hexagonal, and their radial diameter is equal 

 to the tangential, or even somewhat greater ; at the autumnal limit, on the other hand, 

 they are always much flattened, i. e. the radial diameter is shortened. According to 

 N. Miiller ^, a diminution of their length in the autumn-wood is connected with this, in 

 the case of the Fir. To this is further added an increase, accompanying the flattening, 

 not only of the relative, but of the absolute thickness of the wall ; and the appearance 

 of pits on the tangential surfaces of the w-all, while in the spring elements, with a wide 

 lumen, the latter are limited to the radial surfaces. 



By way of example, these conditions may be illustrated by the mean magnitudes found 

 by Mohl* in the case of a well-grown specimen of Pinus sylvestris, thirty years old ; they 

 are expressed in Paris lines : — 



Spring-wood. Autumn-wood. 



( Radial diameter 00204 0-0056 



Stem . . < Tangential 0-0142 0-0143 



( Thickness of wall .... 0-0019 0-0031 



! Radial diameter 0-0232 0-0094 



Tangential o-oi6i ..... 0-0161 



Thickness of wall .... o-ooi8 00035 



The sudden or gradual appearance of these differences, and the relative thickness 

 of the zones with narrow and wide lumen, here depend in general on the thickness of 

 the annular zones, as will be shown below. 



* H. de Vries, I.e. (p. 475). 



"^ Goppert, Monogr. d. foss. Coniferen, I.e. — Von Mohl, Botan. Zeitg. 1862, p. 225, &c.— Kraus, I.e. 



^ Botan. Untersuchungen, IV. 2, p. 190. 



« L.c. p. 237. 



