13.2: Monocot Leaves

Macroscopic Features

Monocot leaves tend to have parallel venation, as opposed to the branching patterns seen in eudicots.

Figure \(\PageIndex{1}\): These two monocot leaves both have parallel venation. It is more obvious in the leaf on the right. However, if you look closely at the leaf on the left, you'll see that those veins do not cross each other. Instead, they travel in the same direction without overlapping, just as in the leaf on the right. Photos by Maria Morrow, CC BY-NC.

Microscopic Features

The model organism for monocots in botany is usually corn (Zea mays). Below, you'll see examples of corn leaf cross sections to demonstrate monocot leaf anatomy. Note that there are approximately the same number of stomata on either side of the leaf, that the vascular bundles are all facing you in cross section (because they run parallel to each other), and that the mesophyll is not divided into two distinct types. Note: There are exceptions. Many monocots will have a more specialized mesophyll arrangement.

Figure \(\PageIndex{2}\): A cross section of a corn (Zea mays) leaf. See the caption in Fig. 13.2.3 for a detailed description of the features present. Photo by Maria Morrow, CC BY-NC.

Figure \(\PageIndex{3}\): A cross section of a section of a corn leaf, labeled. The upper epidermis is composed of parenchyma cells that appear empty. There are two clusters of enlarged cells within the upper epidermis. These are bulliform cells and are not present in the lower epidermis. Stomata occur in approximately even numbers in both the upper and lower epidermis. Eight vascular bundles can be seen, the one on the far right is much larger than the others. In the larger vascular bundle, it is easier to distinguish the large, open vessel elements (stained red). Within the vascular bundle, the xylem tissue is closer to the upper epidermis and the phloem tissue is closer to the lower. Each vascular bundle is surrounded by larger cells with darkly-stained contents. These make up the bundle sheath. The tissue around the vascular bundles is the mesophyll. Photo by Maria Morrow, CC BY-NC.

Bulliform Cells

The bulliform cells present in the upper epidermis are not common to all monocots. This is an adaptation you can find in many grasses that are adapted to hot or dry environments. To avoid water loss, the bulliform cells can contract, causing the leaf to roll up and reduce surface area.

In the image below, you can see a leaf from the beach grass Ammophila rolled in on itself. Can you find the bulliform cells?

Figure \(\PageIndex{4}\): This European beach grass (Ammophila arenaria) leaf has rolled up due to shrinking of the bulliform cells. The upper epidermis is now highly invaginated and located on the inside of the rolled leaf. Image from the Public Domain, sourced from Berkshire Community College Bioscience Image Library.

Figure \(\PageIndex{5}\): This is a close up of the same European beach grass (Ammophila arenaria) leaf as above. The lower epidermis has a thick hypodermis (stained red). A thick cuticle can be seen coating the epidermis. On the right side of the image, there is a fold in the upper epidermis (which has many trichomes). At the folding point, several slightly larger cells can be seen. This is a region of bulliform cells, which allowed the leaf to fold and roll inward. Image from the Public Domain, sourced Berkshire Community College Bioscience Image Library.

Vascular Bundles

In the vascular bundle, the xylem will be on the top (adaxial side) and the phloem will be on the bottom (abaxial side). If you think about the way a leaf emerges from the plant, this matches the way the xylem and phloem are oriented in the stem, with the xylem toward the center of the stem and the phloem closer to the outside/epidermis.

The vascular bundle is often surrounded by inflated parenchyma cells that form a structure called a bundle sheath. In C4 plants, like corn, this is where the Calvin Cycle would take place.

Figure \(\PageIndex{6}\): A vascular bundle of a corn (Zea mays) leaf. There are two vascular bundles in this image. The one on the left is difficult to distinguish and most of what you see are the enlarged bundle sheath cells. The larger vascular bundle on the right has less prominent bundle sheath cells, though they still form a distinct border between the vascular tissue and the mesophyll. The xylem tissue is located closer to the upper epidermis. You can locate it by searching for the large, open cells (vessel elements) with red-stained secondary walls. Below the xylem is the phloem tissue, which encompasses a smaller area. The larger cells in the phloem are sieve tube elements and the smaller ones are companion cells. Image from the Public Domain, sourced from Berkshire Community College Bioscience Image Library.

Below is an image of a vascular bundle in another monocot, Yucca. This vascular bundle has large groups of sclerenchyma cells within it, a smaller group above the xylem and a much larger group below the phloem. How might you distinguish the sclerenchyma cells from the parenchyma cells? Consider the thickness of the cell wall(s) and how each cell type reacts to stains.

Figure \(\PageIndex{7}\): A vascular bundle from a Yucca plant (a monocot). The large groups of thick-walled, red-stained cells are sclerenchyma. These provide rigid support in the region of the vascular bundle. Image from the Public Domain, sourced from Berkshire Community College Bioscience Image Library.