YOUNG AND OLD LEAVES- 85 



cotyledons. It takes some time for the parts 

 which hold the green tissue to develop fully, and 

 therefore it requires very effective protective con- 

 trivances to allow the young leaves, exposed to 

 the changes of the weather, to grow normally and 

 form unhurt their green, transpiring tissue. These 

 contrivances are sometimes peculiar to young, 

 undeveloped leaves ; sometimes they may also be 

 observed in full-grown leaves. 



The diminution of the extent of the upper sur- 

 face, which is directly exposed to the air and the 

 wind, the vertical position of the leaves, and the 

 covering of the green tissue under a protecting 

 mantle are the most important means of defence. 



The small amount of surface exposed to the air 

 and sun is necessitated by the position of the leaf 

 in the bud. In the bud the room is very limited, 

 and the leaves are packed tightly into this room, 

 so that their surfaces are rolled, folded, or crumpled. 

 This is also an advantage when they emerge to 

 the light of day : it prevents the green tissue from 

 becoming dry, is continued until other protective 

 appliances are formed, and remains in some cases 

 throughout the life of the plant. 



Many leaves are rolled in the bud, especially 

 in bulbous plants. The midrib, or often quite a 



