ANNUAL RINGS 



083 



of the fibres to the longitudinal axis of the stem, does not usually 

 exceed 4 or 5 ; in individual cases, however, it may be much greater, 

 e.g. 10 20 in Aesculus Hippocastanum,, 30 in Syringa vulgaris, 40 in 

 JSorbus Aucuparia, and 45 (the maximum value record) in Punica 

 Granatum. 



With regard, finally, to the advantage which the plant derives 

 from the above-described increase in length of the xylem elements, and 

 from the oblique " grain," which is probably a secondary consequence 

 of that increase, attention mav be once more directed to the fact that 

 the increase in length is most pronounced in the case of the 

 mechanically effective elements. That the resistance of the stereome 

 increases with the length of its component cells, is a self-evident 

 proposition. It is highly probable that the oblique course of the fibres 

 also adds to the strength of the trunk ; for, when the stem is bent, the 

 tensile strength of the oblique fibres on the convex side is only tested 

 by a portion of the total stress, the magnitude of this component 

 depending upon the angle of inclination of the fibres ; the other 

 component acts at right angles to the course of the fibres, and thus 

 really constitutes a shearing stress, so far as they are concerned. Thus, 

 a woody cylinder with oblique fibres reproduces on a larger scale the 

 structure of the individual mechanical cells, with walls composed of 

 oblique molecular or micellar series. 



Hitherto we have confined our attention to the width of the 

 annual rings and the dimensions of their component cells in particular 

 transverse sections, that is, at a given level in the trunk. Some 

 consideration must now be given to the differences in the development 

 of the annual rings at different heights in the trunk. 3130 Von Mohl long 

 ago concluded, on the basis of measurements made upon various 

 Conifers, that the average width of the annual rings increases constantly 

 from below upwards ; the rings are also relatively wide right at the 

 base of the trunk. F. Schwarz has carried out numerous measurements 

 upon Finns sylvestris with regard to this point; he records the 

 following average widths of the annual rings at different levels in a 

 Pinus trunk, 30 m. in height, for the period 1885-1894. 



According to Schwarz, the narrowest annual rin^s in Pinus 

 sylvestris are found, on an average, at a. height of 1*3 to 3'4 in. ; from 

 this point onwards the width of the rings increases slowly, but steadily, 



