THE SKIN iy 
and what is more important, they serve as mouths to 
breathe in the air which, as we shall see, is necessary for 
the proper working of the plant factory. 
So far as the leaves are concerned, these skin-cells can 
erow sufficiently for one year, but the skin-cells on the 
branches and twigs are in a different position. They 
cannot go on stretching indefinitely, and they have lost 
the power of dividing themselves and so filling up the 
gap. The result is that the epidermis, or outer skin, 
on the twig splits, and the living part below is exposed. 
To meet this difficulty the cells just below the epidermis 
begin to split with great vigour and divide as fast as they 
can. 
But this time they do not make epidermis. They 
push out layer after layer of cells with the ordinary kind 
of wall changed into cork, and fill up the gaps. This 
is the reason why, on any bush or tree, the twigs after 
a little while turn brown; the cork has taken the place 
of the epidermis. It lets no water or nutriment pass 
through it from inside, and so everything outside of 
it dies and begins to form bark. As you know, bark 
is quite dead matter, and some trees, such as the plane, 
shed it every year. In some trees, on the other hand, 
the cork is produced layer after layer, until you get 
masses of layers, such as are borne by the Cork «Oak. 
Here also all the outside layers are dead, and it is only the 
inner actual cork-growing layer that can be harmed. All 
_ this cork acts as a great-coat for the tree, but the plant 
would be uncomfortable if it were not ventilated. It 
therefore forms small openings in the layer, and in these 
openings the cork-cells lie very loosely, so that the air 
can pass in easily. A most interesting point is that in 
winter these openings are closed at the base by a specially 
c 
