254 



THE STUDY OF TREES. 



the twig these have a distinct slit in the middle, in the 

 younger parts the slits are not so noticeable. Are these 

 accidental scars ? What reason can we give for think- 

 ing that they are not ? They are too constant in shape, 

 and their appearance suggests some relation to the interior 

 of the twig. That is, we are quite sure they are not im- 

 pressions made on the twig from the outside. 



Fig. 129. Twig of Sycamore. B to E, stages in opening of bud. 



Were we able to cut microscopical sections we would 

 obtain further proof of this. We do not find these struc- 

 tures on leaves for example. But on leaves there is no 

 bark such as we find on twigs and stems. The little scars, 

 our microscopical section shows us, are places where the 

 cork which forms the most important part of the bark shows 

 little breaks. The skin (epidermis) is here broken also, so 

 that the soft tissue within is in direct contact with the 

 outside air. If a twig is dipped in boiling water air will 

 be seen to issue from these scars, which are known as 



