124 BOTANY 



even to the naked eye, and so serve as a means of computing the age 

 of a plant. 



Under certain conditions the number of annual rings may exceed the number of 

 years of growth, as, for instance, when midsfmmek growth occurs, such as commonly 

 happens in the Oak, when, after the destruction of leaves by caterpillars, a second 

 formation of spring wood is occasioned by the new outgrowths thus induced. In the 

 wood of tropical plants the annual rings may be entirely absent. This occurs, for 

 example, in the tropical Conifers of the genus Araucaria, which, in this respect, 

 show a marked contrast to the Conifers of the northern zone. Any interruption of 

 growth, such as would occur during a drought, followed by a period of renewed 

 activity, may occasion the formation of annual rings even in tropical plants. 



Although a cessation in the formation of wood takes place so 

 early, the cambium tissue continues to form bast so long as climatic 

 conditions permit. As a rule, however, fewer elements are added 

 to the bast than to the wood. Up to a certain period, in the age 

 of woody plants, the elements of both wood and bast exhibit a 

 progressive increase in size. 



The living elements may remain in a state of greater or less activity 

 throughout the whole of the wood, extending even to the pith ; such 

 wood is called splint wood : the Beech (Fagus sylvatka) may be quoted 

 as an example ; in the other wood, the heart-wood, the living elements 

 die after a certain time, so that only dead tissues are found within a 

 certain distance of the cambium. Before the death of the living cells, 

 they usually produce certain substances, such as tannin and gums, 

 which penetrate the cell walls of the surrounding elements, and also 

 partially close their cavities. 



The tannins impart to the dead wood a distinct colour, often very 

 characteristic, especially when it has been transformed into wood 

 dyes, or so-called xylochrome. The tannin in the woody walls acts 

 as a preservative against decay, while the gums close the functionless 

 water-courses of the dead wood. The dead portion of the wood of 

 a stem is called the heart-wood or duramen, in contrast to the living 

 sap-wood or alburnum. Usually the splint or sap-wood is at once 

 distinguishable from the heart-wood by its lighter colour. In some 

 stems, however, the heart-wood does not change its colour. In that 

 case, as the protecting materials are generally absent, it is liable to 

 decay, and then, as so often occurs in the willow, the stem becomes 

 hollow. 



The sap-wood is limited, according to the kind of wood, to a larger or smaller 

 number of the younger annual rings, and to it falls the task of water-conduction. 

 The distinction between sap- and heart -wood is sharpest where the latter is 

 dark-coloured, as in the Oak, with its brown heart-wood, and in species of Dios- 

 pyros, whose black heart-wood furnishes ebony. The darker the heart-wood, the 

 harder and more durable it usually is. The following may be mentioned as examples 

 of woods which yield dyes and colouring principles — Haematoxylon ca-nvpechianum, 



