THE LEAF 63 



is thus pefmeated by a ventilating system of air-channels, which may 

 communicate through the pores of the stomata with the air outside. 



Most foliage leaves have such a structure as that described. But 

 the leaf-blade of different plants fluctuates almost as much in the 

 details of its internal construction as it does in its outline : this may 

 even be seen in some degree in those of the same individual plant. 

 Leaves may vary in thickness from the delicate, almost filmy leaf of 

 shade-loving plants, to the leathery texture of those exposed to the 

 sun in dry climates. The leaves may be smooth in surface, as is usual 

 in water-plants, or covered with rough hairs like the Sunflower, or 

 with a dense woolly protection like the Alpine Edelweiss. In- 

 ternally they may have only a single layer of palisade parenchyma, 

 as in most shaded leaves ; or two, which is common in leaves exposed 

 to the full sun ; or more. They may be strengthened by mechanically 

 effective tissue, often placed just below the upper epidermis (hypo- 

 derma), as in the leathery Cherry-Laurel, or distended by water- 

 storage cells, usually occupying the middle of the leaf, as in the 

 succulent Stonecrop, or Aloe. Notwithstanding such differences 

 as these, and many others, the construction is as a rule based upon 

 the same essential scheme as that described. 



The leaf is the chief organ of nutrition in green plants. An essential 

 point of structure to this end is the perforation of the epidermis by 

 the stomatal pores, for this gives the opportunity of gaseous inter- 

 change between the intercellular spaces and the outer air. Stomata 

 occur here and there in the epidermis of the stem and petiole, and 

 even upon the various parts of the flower. But it is on the surfaces 

 of the leaf-blade that they are found in the greatest numbers. Some- 

 times, as in the Sunflower and many herbaceous plants, they are 

 present on both sides of the blade ; but frequently they are fewer on 

 the upper epidermis, or even absent, as in many woody plants. They 

 are often very minute, especially in the Dicotyledons, so that large 

 numbers, usually IOO to 300, may be counted on each square millimetre 

 of surface. The following table gives the result of countings per sq. 

 mm. on the leaves named : 



UPPER. LOWER. 



Lilac .... ioo 150 



Oak - - o 346 



Apple ... o 246 



Water Lily - - - 460 o (submerged). 



Wheat 47 52 



K Examined microscopically in surface view the epidermis of a 

 Dicotyledon appears as a film of tabular cells, often with sinuous 



