48 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



(in Kalmia, strange to say, the reverse is the case), as 

 well seen in oak ; but in some cases the differences are 

 so slight that we say the vessels are equal all over, e.g. 

 box, birch, willow, alder, &c. The diameter of the 

 vessels varies much in different cases, and very large 

 ones may coexist with very small ones. Neglecting 

 the wood of some climbers, where the vessels are 

 easily seen with the unaided eye, . and may be 

 more than half a millimetre in diameter, we find 

 examples of large vessels in the oak, ash, chest- 

 nut, walnut, &c., where they are visible without a 

 lens, and of extremely small ones in box, birch, 

 willow, maple, horse-chestnut, &c. All sizes between 

 these extremes are to be met with, and Nordlinger 

 has tried to arrange them in six groups, but I cannot 

 recommend this, as it seems impossible to maintain 

 them. The laburnum and the plane afford examples 

 of medium-sized vessels. 



Characters have been obtained from the mode of 

 grouping of the vessels, or " pores," on the transverse 

 section. Thus we have seen how their equal distribu- 

 tion, or concentration in the spring-wood, as the case 

 may be, affects the classification as regards annual 

 rings ; but besides this we find peculiarities of other 

 kinds which are characteristic. In the hornbeam, for 

 instance, there are long, sinuous lines of pores radiat- 



