Cross section in a leaf of Ilex aquifolium (Holly)
Holly is an evergreen shrub, with dark green, leathery, glossy leaves. The leaf margins are entire, but the leaves are spiny, the edge of the blade being covered with teeth, a defense against herbivores.
On a same plant a variation of leaf forms can be observed, and this is known as heterophylly: some leaves are prickly while others are non-prickly with smooth margins.
The leaf is bifacial with upper (adaxial) and lower (abaxial) surfaces differing from each other. Under the light microscope the mesophyll is heterogeneous (see below) with palisade parenchyma toward the adaxial face and spongy parenchyma towards the abaxial face.
The sections below are paraffin sections (8 μm thick) stained with safranin – alcian blue. The cuticle and the fibers are stained red and tissues with cellulosic walls (parenchyma, epidermis) are stained blue.
2: epidermis of adaxial face
4: pallisade arenchyma
5: fibers associated to a vascular bundle
6: spongy parenchyma
8: epidermis of the abaxial face
In parenchyma cells the chloroplasts are stained red
Below: on the left epidermis of the adaxial face, hypodermis, and palisade parenchyma (chloroplasts in red). On the right part of the section with strands of fibers (F) that are responsible of the leathery aspect of the leaf. We also note the xylem vessels and fibers in both cross and longitudinal section. Cu: cuticle. Pl: spongy parenchyma.
Characteristics of the section: thick cuticle for both epidermis; an hypodermis under adaxial epidermis ; two different parenchyma in the mesophyll ; strands of fibers and stomata on the abaxial epidermis.