Relation of Annual Rings to Age. 165 



tensify whatever color may be peculiar to the tissue as a whole ; 

 or it may arise from both these causes combined. This means of 

 definition, however, is by far the least important, and may very 

 properly be considered as subordinate to and serving only to 

 augment those distinctions which arise from purely structural 

 peculiarities. 



In certain exogens it is to be observed that, ia the growth of' 

 each year, the ducts and vessels are not uniformly distributed- 

 among the wood cells, but that there is often a very strong tendency 

 for them to becon;e localized, preponderating in thst inner 

 portion of the rings which represents the earliest growth of the 

 season, thus forming a layer of more open and porous tissue, while 

 the wood cells preponderate in the outer portions of the rings 

 formed at the close of the growing season, and thus produce a 

 much denser layer. This alternation of more or less dense 

 structure, therefore, offers at once the means of clear and sharp 

 distinction between contiguous rings, since the inner porous portion 

 of each is directly opposed to the dense tissue last produced in. 

 the preceding ring. Moreover, this variation in density is not 

 due alone to distribution of the ducts, but to variations in the 

 size and aggregation of the wood-cells themselves, as will be seen. 

 in the next paragraph. ExamjDles of this kind are to be found 

 in varying degrees of development, in Quercus, Corylus, Ulmus, . 

 Fraxinus, Castanea, Juglans, Rhus, Acer, Zantlioxylimv, Tilia,* 

 etc, of which we may well consider Fraxinus the type. 



In other instances there is a marked absence of ducts and. 

 vessels, or, if present, they are so uniformly distributed that the 

 entire structure becomes homogeneous and the distinction of 

 rings is thus obliterated ; and when rings do appear under such 

 circumstances, their distinction must rest upon variations in the 

 wood-cells themselves. Thus we find that those wood-cells which 

 are first produced each season, are in cross-section relatively large 

 thin-walled, and more loosely aggregated; while those which are 

 produced towards the close of the season are relatively small, thick- 

 walled, closely aggregated, and often flattened in a radial direction. 

 This variation often proceeds at a uniform rate towards the end 

 of growth, or, as in Alnus, the transition from less dense to more 

 dense structure is not unfrequently very abrupt. In any case> 

 there is thus produced an apposition of more and less dense tissue,, 



