13.3: Eudicot Leaves

Macroscopic Features

Eudicot leaves tend to have netted venation, with a larger central vein (the midrib or midvein) that branches off into a network of smaller veins. In the image below, you can see this branching pattern in a skeletal leaf.

Figure \(\PageIndex{1}\): A skeletal holly leaf shows the network of vascular tissue. The lignified xylem and phloem fibers break down far more slowly than the parenchyma cells of the leaf. As the rest of the leaf tissues decompose, the lignified vascular tissue is left behind. This reveals the netted pattern of increasingly smaller side branches found in eudicot leaves. Photo by Maria Morrow, CC BY-NC.

Microscopic Features

Eudicot leaves can usually be distinguished by netted venation at the macroscopic level, but they also differ at the microscopic level. Note the difference in organization between the tissues in the leaf below and the leaves shown in the monocot section.

Figure \(\PageIndex{2}\): A cross section through a eudicot leaf. The upper epidermis is a single layer of parenchyma cells. There are no stomata present in the upper epidermis of this leaf. Below the epidermis, cells (appearing pink due to staining of the nuclei and chloroplasts) are arranged in columns, forming the palisade mesophyll. Beneath the palisade mesophyll is the spongy mesophyll. The cells are approximately the same size as the palisade mesophyll, but there are large intercellular spaces between them. The lower epidermis is another single layer of parenchyma cells, but several stomata (flanked by guard cells) are visible in this epidermal layer. A large vascular bundle is in the center of the leaf. The xylem (stained pink) is on the top and the phloem is on the bottom. Photo by Maria Morrow, CC BY-NC.

Cuticle

You will often see a waxy cuticle coating the surface of most plant tissues. In leaves, the location and thickness of the cuticle can give you clues about the environment that the plant has adapted to.

Figure \(\PageIndex{3}\): A cross section through the upper epidermis and palisade mesophyll. On top of the upper epidermis of this leaf, a transparent layer of cuticle is visible, sealing the top of the leaf. This waxy layer protects the leaf and forms a barrier to the movement of water. Image is in the Public Domain, sourced from Berkshire Community College Bioscience Image Library.

Vascular Bundles

Seeing vascular bundles of eudicots in cross sections can be confusing. The organization of tissues in the much larger midrib vascular bundle is often spread out into a semicircle, still with xylem on the top and phloem on the bottom, but they can be difficult to distinguish. In addition to this, the smaller veins are not oriented in the same direction, as they are in monocots.

In the image below, the vascular bundle just to the left of the midrib is coming more or less straight at us, so it is easy to distinguish the tissues. In contrast, the vascular bundle to the right of the midrib was moving diagonally and so was caught in an oblique section and looks more like a smear. Often with these oblique sections, you can distinguish the xylem cells by their strange secondary wall thickenings -- they look a bit like coiled springs.

Figure \(\PageIndex{4}\): A cross section through the midvein of a eudicot leaf. The xylem tissue within the large vascular bundle is arranged in an arcing semi-circle, with phloem tissue in an arc traveling just below it. There are layers of collenchyma cells under the epidermis both above and below the midvein. What would be the function of these collenchyma cells? Image is in the Public Domain, sourced from Berkshire Community College Bioscience Image Library.

Figure \(\PageIndex{5}\): Just to the right of the center of this image, you can see the coiled spring appearance of the xylem vessel elements that have been caught in an oblique section. Image is in the Public Domain, sourced from Berkshire Community College Bioscience Image Library.