76 SOUTH AFRICAN BOTANY 
100 per square millimetre. ‘hey are so small that 
neither dust nor water can pass through them into 
the plant, but so numerous that a Sunflower leaf 
contains about 13,000,000 of them. In dorsiventral 
leaves they are usually found on the under surface. In 
isobilateral leaves they are distributed equally on both 
surfaces. . 
The closing or opening of the pore is effected by 
changes in the turgidity of the guard cells. When 
these are turgid the pore is opened to its widest extent. 
When they are flaccid the pore is nearly closed. 
Their chief function is to allow water vapour and 
other gases to pass in and out of the plant. 
45. Functions of Leaves.—The functions of leaves will 
be more fully considered in Chapter IX. But it may 
be mentioned here that they act as— 
(a) Breathing organs ; 
(6) As organs of transpiration ; 
(c) As actual laboratories, in which is built up out of 
carbon dioxide of the air and the nutrient salts of the 
soil the organic building material of which the plant 
body is made. 
PRACTICAL EXERCISES ON CHAPTER IV. 
1. Sketch a Rose leaf and mark the stipules, petiole, leaf-blade. 
Describe the leaf. 
2. Sketch the leaf of the Arum Lily, sketching the veins care- 
fully. Describe the leaf. 
3. Examine a shoot of Salvia, and describe the arrangement of 
leaves. 
4, Pick leaves of the following plants and describe them with 
regard to — 
