512 SECOND A Rr CHANGES. 



ring, of a iroe \vhich has otherwise grown vigorously, shows essentially the same 

 character as the whole of the rings of a feebly thickened tree. This character is 

 different in Coniferous woods, and in the Dicotyledonous woods investigated. 



In the former, according to Mohl, the relative thickness of the spring-wood 

 with wide lumina, and the autumn-wood with narrow lumina, and the more or less 

 sudden transition from one to the other, varies quite universally, according to the 

 thickness of the rings, the variation in the stem being, as a general rule, in the 

 opposite direction to that in the root, subject to specific or perhaps individual 

 modifications ^ In the former, the outer zone of the ring, with narrow lumina and 

 thick walls, forms a larger portion of the whole, and is the less sharply marked off 

 from the interior the thinner the ring is. In the root it is the more developed the 

 thicker the ring ; in the feeble annual rings which are prevalent in roots, and which 

 in the White Fir (Abies pectinata), for example, are on the average only about o-2nini 

 broad, it often consists of only 3-1 layers, and is sharply limited towards the internal 

 zone, which has wide cavities. 



In the Dicotyledonous woods investigated (Fraxinus, Fagus, Quercus peduncu- 

 lata, Morus, Broussonetia, Rhus, Sophora, Gymnocladus, etc.) ^, the middle portion 

 of the ring becomes reduced, as the thickness of the whole diminishes, in such a 

 degree that in extreme cases the ring consists exclusively of spring-wood and the 

 autumnal limiting layer. This condition appears most sharply in Morus, Rhus, and 

 the Leguminosce mentioned, where the woody fibres are confined to the middle region 

 in well-developed rings, but wholly disappear in feeble ones. In this respect the 

 wood of stem and root shows a general agreement, and as the annual rings, in roots 

 which have once attained a thickness of two or three inches, are, as a rule, extremely 

 thin (o-25^""i and less in thickness), it follows at once that there is a considerable 

 difference in the general structure of the wood of stem and root, which is increased 

 by further differences in the structure and distribution of the elements, which will be 

 described below. 



As in Coniferous woods the outer portion of the annual ring contains relatively 

 the largest mass of lignified membranes, and consequently possesses the greatest 

 density, strength, and hardness, while in Dicotyledonous woods, owing to the structure 

 and distribution of their elements, this is the case with the middle parts of the ring, 

 the lesser density and strength of the wood of the root, as compared with that of the 

 stem, is a necessary consequence of the facts above stated. The stem-wood, however, 

 as technically made use of, when taken from well-grown trees, for which the excep- 

 tions to be mentioned below do not hold good, diminishes in the case of the Coniferae 

 in density and strength as the thickness of the annual rings increases, while in 

 Dicotyledonous woods the reverse is the case. 



These rules, however, undergo a considerable modification in the case of stems, 

 owing to local changes in the relative breadth of the autumnal wood at different 

 levels on the stem, as Sanio ■' found in some well-grown main axes of Pinus 

 sylvestris. In these cases the relative breadth of the autumnal wood increases in 



' Compare von Mohl, /. c. p. 238. , 



^ Compare von Mohl, Botan. Zeitg. 1862, /. c. — Sanio, ibid. 1863, p. 397. 



^ Pringsheim's Jahrb. IX. p. 115. 



