SEASONAL CHANGES 171 



its yellow colour. Cork does not allow water to penetrate it, 

 consequently the tissues outside it dry up and peel off. This is 

 the first bark of the tree. In some trees this is repeated year 

 after year ; bark is formed and peels off. But more generally 

 a new set of cells nearer the wood begins also to form cork, and 

 as everything outside the cork dries up the bark becomes of con- 

 siderable thickness. Bark is impermeable to water and to gases 

 except at certain spots. On twigs, little excrescences of a differ- 

 ent colour from the bark are often seen ; these are lenticels or 

 cork- warts. They are openings in the bark, and correspond to 

 the stomata of leaves. By means of them oxygen can reach the 

 inner tissues of the stem, and carbon dioxide can be got rid of. 

 In the birch and cherry, the lenticels have the appearance of 

 long transverse lines on the bark. In winter the lenticels are 

 closed with cork cells, to exclude air ; in spring the cork cells 

 are very loosely packed, so that interchange of gases between the 

 inner tissues and the air is possible. 



Trees with thin bark often have thick foliage, and cast deep 

 shade; this is true of the 

 beech and hornbeam. Both 

 these trees have smooth grey 

 or brownish-grey bark ; that 

 of the hornbeam is marked 

 in white lines, which give 

 it a tessellated appearance. 



Trees that have a thick, Fia 50--Lenticel closed in winter. 



, r -, -, ITI . Epidermis: ck, cork. 



rough, furrowed bark, like 



the oak, often have less dense foliage, and are light-loving. 



Some trees have bark that peels off regularly in thin scales. 

 The Scots pine, plane, and birch are instances of this. In the 

 Scots pine the scales of the trunk near the ground do not peel 

 off ; the bark in that part of the tree is thicker and shows longi- 

 tudinal fissures, whilst higher up the scales peel off, leaving a 

 brownish -red surface characteristic of the tree. The plane 

 casts off its bark every year in large thin plates, exposing a pale 

 yellow surface. In the birch the greater part of the bark is 

 white ; it peels off in very thin plates horizontally, leaving dark 

 irregular rings, which accentuate the silvery whiteness of the trunk. 



