ANNUAL RINGS 125 



The major part of each annual addition to the Secondary wood 

 usually consists of vessels and fibres, but the proportion of these 

 latter varies with the kind of plant and during each season's 

 growth. The wood formed in the spring (spring-wood, Fig. 60 

 A, S.) mostly contains a much larger percentage of vessels 

 than that produced later (autumn-wood, A.) ; its vessels, 

 moreover, are often larger and have thinner walls, and 

 the same may be true of the fibres. This difference can be 

 related to the sudden demand on the water-supply in the spring 

 on the part of the newly-expanding leaves, whilst later in the 

 season provision can be made for the growing mechanical re- 

 quirements l of the plant by an increased proportion of fibres. 

 As a consequence there is a sharp boundary between the dense 

 small-celled autumn-wood of one season and the wide-celled 

 spring-wood of the next (Fig. 60), and this leads to the marking 

 out of the secondary wood into a succession of annual rings 

 (Fig. 58), by means of which the approximate age of a trunk 

 can be estimated. Occasionally, however as, for example, when 

 a new set of leaves is produced to replace a first crop killed by 

 frost or devoured by caterpillars their expansion is accompanied 

 by the formation of a second zone of spring-wood, so that two 

 " annual rings " are formed in a single season. The width of 

 the annual ring is mainly determined by nutrition, though the 

 variations in thickness in one and the same ring are probably 

 the result of mechanical strain. Such asymmetrical annual rings, 

 with a maximum development on the upper or lower side, are 

 commonly found in horizontal branches. An extreme condition 

 is seen in the " buttress-roots" observed in many tropical trees 

 (e.g. species of Ficus). 



The constant addition of new wood is probably mainly 

 necessitated by changes in the central earlier-formed xylem, as 

 a result of which it becomes useless for purposes of conduction, 

 although such changes often increase its value as a mechanical 

 support ; it is then spoken of as heart-wood in contrast to the active 

 sap-wood beyond. The elements of the heart-wood often become 

 impregnated with tannins, resins, etc., which are frequently 

 accompanied by dark-coloured pigments ; in some cases the 



1 A feature that is probably also connected with the growing mechanical 

 strain is the increased length of the fibres in the later-formed annual rings. 



