SECONDARY THICKENING. NORMAL DICOTYLEDONS. 515 



f. Differences of the secondary wood in non-equivalent members of the same plant. 



Sect. 158. The ligneous body of the stem and its branches has, in the plants in 

 question, the same structure, within the limits of deviation defined in the preceding 

 pages. In trees, however, differences in dimensions exist, owing to the fact that not 

 only the thickness of the annual rings, but also the size of the tissue-elements, is 

 less in the branches than in the stem. This at least is the case according to the 

 more accurate investigations before us, which were carried out on Coniferous trees, 

 'and are mentioned at p. 506 ^- 



A far less general agreement prevails between the special structure of the ligneous 

 body in the stem and its branches on the one hand, and in the roots of the same 

 plant on the other. Here, on the contrary, there are two different extreme cases ; 

 plants with the wood of the root completely similar to that of the stem, and others 

 with the opposfte character; between the two extremes there are of course many 

 intermediate cases. 



The first of the two cases occurs in Gymnospermous and Dicotyledonous trees 

 and shrubs. Although the wood of their roots is never quite similar to that of the 

 stem in form, structure, and distribution of the tissue-elements, yet the differences 

 only affect relative dimensions and subordinate variations of structure ^ Differences of 

 the former class consist, firstly, in a considerable reduction, in the case of the root, of 

 the average thickness of the entire annual ring, which, though subject to great varia- 

 tions, may sink to minimal dimensions, in the White Fir, for example, to o-ii'/'""", 

 although, on the other hand, it sometimes reaches 2-3™™; in Dicotyledonous woods 

 it may even be smaller than the average diameter of the vessels present in the annual 

 ring, in which case the ring must have an undulating outline, widened at the vessels. 

 Secondly, there are differences in the width and thickness of wall of equivalent 

 portions of tissue. These affect details in the structure of the walls, and, in the case 

 of Dicotyledonous woods, the distribution of the non-equivalent forms of tissue in the 

 annual rings. These relations once more appear most clearly and simply in the 

 Coniferous woods. According to V. Mohl's measurements, the tracheides of the 

 spring-wood in the root of the White Fir are distinguished from those of the stem by 

 this fact, that their radial diameter is on the average \, and their tangential diameter 

 \ greater, while their length is also greater ; those of the autumn-wood by their 

 greater radial diameter and wider lumen. To this is often added a diminished 

 absolute thickness of wall in the tracheides of the root, &na, perhaps independently 

 of this, a greater softness of the wood ; further, the fact that the development of the 

 thicker-walled autumn wood-cells diminishes with the thickness of the annual rings, 

 and in very thin ones is almost suppressed. Similar relations, to be compared in 

 V. Mohl and Schacht, I.e., reappear in other Abietineae. The differences between 

 the wood of the root and that of the branches, which Schacht compared with it, are 

 as regards the width of the tracheides even greater than in the case of the stem-wood, 

 for reasons stated above. In the wood of the root in Coniferse, the pits on the radial 

 sides of the tracheides are often arranged in two longitudinal rows, whether the side 



' Compare also von Mohl, Botan. Zeitg. 1862, p. 461. — Schacht, ibid. p. 409, &o. 

 ^ Von Mohl, Botan, Zeitg. 1862, pp. 225, 269. 



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