THE LEAF 73 
ers with a lid. In Utricularia bladders are developed 
on the submerged leaves with valves at the mouth 
which allow of ingress, but not egress, of small water 
animals. 
42. Dorsiventral, Isobilateral and Concentric Leaves. 
—In leaves which are placed horizontally the cells un- 
der the epidermis of the upper surface differ in struc- 
ture and function from those under the epidermis of the 
lower surface. Such a leaf is said to be DorsIvENTRAL 
or Biractan. But some leaves hang perpendicularly, 
and usually these have the same kind of tissue on both 
surfaces. Such leaves are said to be ISoBILATERAL. 
Examples are found in the Blue Gum, Silver tree, Iris, 
and some species of Mesembryanthemum and Acacia. 
Lastly, some leaves have a radial arrangement of 
tissue, e.g. the Pine. These leaves are spoken of -as 
CONCENTRIC. 
438. Microscopic Structure of the Leaf.—The internal 
structure of the petiole resembles chiefly that of the 
stem. It can be distinguished from the latter owing to 
its dorsiventral nature. 
A transverse section of the leaf-blade of the Privet 
under the microscope presents the appearance shown 
in fig. 44. The upper and under surfaces are protected 
by a flattened epidermis with a well-developed cuticle. 
The under surface possesses a large number of stomata. 
The cells underneath the upper epidermis are elongated 
and packed tightly together. They contain numerous 
chloroplasts. This tissue is called the PALISADE TISSUE. 
Towards the lower surface the cells are smaller ; rounded 
or stellate and numerous air spaces are present between 
