682 SECONDARY GROWTH IN THICKNESS 



Apart from the difference in the size of the component elements, 

 which is primarily responsible for the contrast between spring and 

 autumn wood in the individual annual rinsis, homologous elements 

 increase gradually both in length and in width in successive rings. This 

 statement applies more particularly to the mechanically effective 

 elements, including fibrous tracheides as well as typical wood-fibres, the 

 size of which goes on increasing steadily for a number of years, until a 

 maximum is reached, but thenceforward remains comparatively constant. 

 This general rule is applied to a very variable extent at different 

 heights in the trunk, and in the various branches, twigs and roots of 

 one and the same tree ; such variations in detail cannot be considered 

 in the present work. Most extensive and elaborate measurements with 

 regard to this matter have been carried out by Sanio, 36 ' particularly in 

 the case of Pinus sylvcstris ; the following data are quoted from that 

 observer's results: In a 72-year-old transverse disc of a Pinus trunk, 

 taken about 12 inches from the base, the average length of the fibrous 

 tracheides was *95 mm. in the first annual ring, 2*74 mm. in the 

 seventeenth, 3'87 mm. in the thirty-seventh, 4 mm. in the thirty- 

 ninth, 4*21 mm. in the forty-fifth and also in the seventy-second. 

 The length of the tracheides had evidently attained a maximum value 

 in the forty-fifth ring, and thereafter remained constant. In this case 

 the mean maximum length is more than four times as great as the 

 average length in the first season. The average width of the fibrous 

 tracheides, which is "017 mm. in the first ring and *032 mm. in the 

 seventy-second, likewise undergoes an appreciable amount of increase. 



In Dicotyledonous wood, it is the fibres which display the greatest 

 progressive increase in length ; the vessel-segments and tracheides 

 undergo less change in this respect, while the xylem-parenchyma cells 

 do not deviate appreciably from their initial length. Sanio estimates 

 the average length of the wood-fibres in a 130-year-old trunk of 

 Quercus pedunculata at *42 mm. in the first, and at 1*22 mm. in the 

 three outermost annual rings. In the case of the tracheides, the initial 

 average length was "39 mm., the mean maximum "72 mm. While, 

 therefore, the wood-fibres had progressively increased nearly threefold 

 in longitudinal diameter, the tracheides had not even doubled their 

 length. 



It is most probably this progressive elongation of the elements of 

 the wood in the successive annual rings, that is responsible for the 

 fact, that the strictly longitudinal orientation which prevails in the 

 earlier rings, is gradually replaced by a tangentially oblique position of 

 the cells. 3GS The origin of this obliquity which has been observed by 

 A. Braun in 111 out of 167 woody Gymnosperms and Dicotyledons 

 is, however, not yet quite clear. According to Braun, the inclination 



