NORMAL FUNCTIONS. 9 



Only the inner portion of the bark, adjoining the sap- 

 wood and separated from it by the very thin cambium, 

 is living; the outer portion consists of an impermeable 

 skin or layer of cork or cork scales. The thicker these 

 layers of cork scales the better protection they offer the 

 living tissues from fungi and mistletoes, but not from 

 insects. 



NORMAL FUNCTIONS OF THE TREE. 



All plants with green leaves or needles contain a 

 green pigment which enables them to live on the very 

 simplest food. They take up water and mineral salts 

 through the roots and carbon dioxide and oxygen 

 through the leaves ; with help of the green pigment the 

 plants produce under the influence of sunlight from 

 these chemically simple materials the bulk of the chemi- 

 cally very complex substances of which they are built 

 up. This function can be carried on only in sunlight. 

 Animals and fungi, on the contrary, live on chemically 

 very complex food, which they break up into simpler 

 substances. 



Large quantities of water containing comparatively 

 little mineral salts are taken up by hairs on the very 

 tips of the roots, which have to be continually renewed. 

 Older roots take no part in this pumping of water 

 from the ground. Their role is to anchor the plant in 

 the soil. The roots, if not too old, also breathe to a cer- 

 tain extent; that is, they take oxygen from the air in 

 the soil. 



The water with the mineral salts is pumped through 

 the roots and then through the trunk in the sapwood 



