THE EPIDERMIS 21 



are numerous minute holes too small to be seen by 

 the naked eye or even with the aid of an ordinary 

 magnifying glass. These holes can be opened or shut 

 as a mouth by its lips and are called stomas. They 

 occur on all the fresh green parts, but especially on 

 the leaves, and mostly on their under sides. Closing 

 when the air is dry, and there is a risk of too great 

 loss of water by evaporation, and opening when it 

 is damp and there is no danger of the leaves wither- 

 ing, they regulate the amount of moisture passed into 

 the air. 



As the branch grows, the epidermis peels off, and 

 is replaced by a much thicker brown skin which forms 

 the outside of the bark, and in this there are no 

 stomas. 



6. When wood is burnt in a fire, there is left, as 

 every one knows, a greyish powder. This may contain 

 a good deal of carbonaceous matter that is not com- 

 pletely burnt, but by roasting it in an open dish, we 

 may obtain at last a nearly white substance which is 

 not altered or diminished by further heating. This 

 fine white powder, called the ash, consists of oxides 

 and other compounds of the elements, potassium, 

 calcium, magnesium, phosphorus, sulphur and iron. 

 Sodium and chlorine are also often present, and in 

 some plants other elements such as aluminium, man- 

 ganese, zinc and iodine. 



By analysis of the ash of a large number of differ- 

 ent kinds of plants, it has been found that the first 

 six of the elements named above are always present : 

 and by growing plants in water containing some or all 

 of these in soluble form, it has been found that they 



