106 



The Leaf 



the variegated kinds of ivy and holly are grown for 

 ornament the green bodies are arranged in patches, and 

 then the leaves are flecked green and white. A red 

 colour is often to be seen in young leaves and disappears 

 as they become full grown in their case this red skin 

 serves to protect the delicate young from the sun's rays 

 until it has grown strong enough to bear them. 



The outer skin is harder and tougher than the 

 inside of a leaf. This prevents the sap inside from being 

 dried up, as you will realize if 

 you strip a piece of this skin 

 from a leaf and notice how soon 

 the leaf begins to fade without it. 

 Dotted over the skin, especially 

 on the under side of leaves where 

 they are less likely to get choked 

 by rain or dust, are thousands 

 of little pores through which 

 water vapour passes on its way 

 from the inside of the leaf to 

 the drier air outside. These 

 openings are called stomata, 

 from a Greek word meaning 

 mouths, and are so tiny that they can only be seen 

 through a microscope. If you are lucky enough to have 

 one and look through it at the well scraped outer skin 

 from the under side of an ordinary cabbage leaf you 

 will see something like Fig. 53. The stomata are 

 arranged quite regularly and look like round openings 

 with double sides. These sides close up partly across 

 the opening in dry weather and so prevent the water 

 vapour from passing out too quickly. In spite of them 

 there are days in the summer when the leaves get rid of 



Fig. 53. Three stomata from 

 the lower surface of a leaf. 



