The Trunk 83 



soil, it will have the same effect as a warm season and 

 will produce a thick ring. The rings you see on a slice 

 of beetroot grow in just the same way except that 

 a new one forms every fortnight instead of once a 

 year. 



What is known as bark is really the dead outer skin 

 of the tree. This outside layer cannot stretch to fit the 

 growing size of the trunk and soon becomes too small. 

 It therefore cracks or in some cases peels off in scales, 

 and new bark is formed underneath to take its place. 

 The thick skin which the old and new bark form together 

 is of use to the tree in two ways : it prevents the tree 

 from losing too much water through the trunk in dry 

 weather ; and it guards from the direct heat of the sun 

 and from frost the delicate growing part of the tree 

 which lies all round the trunk just under the bark. 

 The younger branches and twigs sometimes suffer for 

 want of a thicker layer of bark than they have, for, 

 although during the summer the leaves take its place 

 and serve to shade them from the sun's rays, when these 

 leaves are gone, frosts which do not hurt the large 

 branches under their thick coat of bark often nip the 

 twigs. 



Bark depends very much for its thickness on the 

 requirements of the tree on which it grows. Since one 

 of its uses is to protect the trunk from the sun, when 

 any kind of tree usually grows in the sun it will have a 

 thicker bark than the shade-seeking ones. At first sight 

 you might think that the papery looking bark of the 

 birch, the most sun-loving tree of all, was an exception 

 to this rule (Fig. 56). This bark, however, consists of 

 many thin layers one beneath the other and forms a 

 better shelter than one thick coat would do. Besides 



62 



