INTRODUCTION 



centre of the tree and forming masses of stone weighing many 

 ounces. 



In the course of the building up of the wood by the Cambium, 

 the pores are produced at intervals amongst the wood-cells and 

 arise not in a haphazard fashion but in rhythmic succession. 

 At times they are produced in such abundance and so closely 

 crowded that this rhythm cannot be traced, but in all cases 

 where the pores are separated by spaces occupied by wood-cells 

 a definite arrangement of the pores exists. It may seem to be 

 labouring the point to speak of " rhythm " where the pores on 

 a transverse section seem " peppered " all over the annual ring, 

 yet the more specimens one examines the more one feels con- 

 vinced that order reigns even in apparent chaos. In a great 

 number of cases a definite order can be seen. The pores may 

 be arranged in undulating festoons, as in the Common Elm 

 (Fig. 107), or in radial, tree-like groups, as in most of the Oak 

 tribe (Fig. 152), the Bullet-woods from South America and their 

 allies (Sapotaceae) (Fig. 83), or in radial rows of single pores (Holly) 

 (Fig. 27), or in flame-shaped groups (Buckthorn), or, as most 

 frequently in our European woods, in a ring of pores in the early 

 spring zone of the annual ring. For the most part, as already 

 stated, the pores are smaller if not less numerous in the 

 Summer and Autumn zones, the latter being frequently 

 very poorly provided with them. If it should happen that a 

 wood has more pores in the Autumn than in the Spring zone, 

 that wood is surely Coniferous (Figs. 137, 138), but these pores 

 will be found to be of quite different nature to those just 

 described, in fact they are glands containing resin and are 

 more properly called " Resin-canals or ducts." Their presence 

 is an indication that the species belongs to one of a limited number 

 of genera allied to the Pine-trees. Many kinds of wood have 

 pores in small closed groups which convey the impression that 

 a " mother-pore " has become subdivided into a number of 

 " daughter-pores," not merely pressed or crowded together 

 but clearly indicating a common origin. Usually they are in 

 short radial rows, and sometimes there are as many as ten gradu- 

 ally tapering off from the innermost and largest to the very 

 much smaller, outer pores ; or the largest may be in the middle 

 of the group, or the group may be a cluster or nest of pores. 

 If the wood be cut in a radial direction, a row of closely arranged 

 pores may be exposed at the same moment, so that the wood 

 will appear very " coarse-grained." On the other hand, if 

 the cut be tangential, the radial row of pores will be cut at 

 right angles, and only one of the series will appear on the 

 surface. 



Similar difference of appearance is caused by oval pores 



xxiii 



