You might have observed a leaf of Hibiscus plant. How does it appear? Did you find any difference between the upper and lower surface of the leaves? Now think of a bamboo leaf. How different is the Hibiscus leaf from the bamboo leaf? Well, external observation can help us reach the conclusion that while the upper and lower surfaces are identical in a bamboo leaf, they vary a lot in the Hibiscus leaf. Do you know why? It is because bamboo is a monocot leaf and Hibiscus is a dicot leaf.
So are there any more differences in the internal structure or anatomy of these leaves? Here we are going to discuss the anatomy of a dicot leaf and find out why its upper and lower surfaces vary. We will also find out how the anatomy of dicot leaves helps in the process of photosynthesis.
The flowering plants or angiosperms in which the seeds possess a pair of seed leaves or cotyledons are called dicots. Anatomy of the leaves of dicot plants is different from monocot plants. So the anatomical differences can be studied from the transverse section of leaves.
Dicotyledonous leaf is also called dorsiventral leaf as it has two surfaces (dorsal and ventral). It shows reticulate venation.
Fig: Dorsal and ventral surface of leaf
Types of tissues in dicot leaf
The transverse section of a leaf exhibits many layers. Three main tissues of a dicot leaf are Epidermal tissue system, Mesophyll tissue system and Vascular tissue system.
An epidermal tissue system of dicot leaves possesses tissues like Epidermis and Stomata.
Epidermis is the outermost layer of the leaf and it gives protection to the leaves. It is formed of a single layer of epidermal cells. It is of two types. One is adaxial epidermis and other is abaxial epidermis. Adaxial epidermis covers the upper surface and abaxial epidermis covers the lower surface. Cuticle is present on the epidermis and it is conspicuous on the adaxial epidermis.
Fig: Epidermal tissue system
In the epidermis of tree leaves and needles, there are tiny pores called stomata that play a role in the exchange of water and respiratory gases between plants and the atmosphere. Stomata is present on the epidermis. In dicot leaves, stomata are more in the abaxial epidermis and very less or absent in adaxial epidermis. Hence a dicot leaf is known as a hypostomatic leaf. Each stoma has cells called guard cells around it. They assist in controlling the rate of transpiration by opening and closing the stomata. Guard cells of dicot stomata are bean shaped or kidney shaped.
Fig: Stomatal apparatus
Ground tissue system
All the tissues except the epidermal tissues and vascular tissues can be included under the ground tissue system. In a dicot leaf, the ground tissue system is composed of the mesophyll cells.
Mesophyll cells are present between upper and lower epidermis. If we observe the mesophyll cells we can find some cells with chloroplast in it. Such chloroplasts containing parenchyma cells are called chlorenchyma cells. These cells perform photosynthesis. Two types of mesophyll cells are present in the leaf and they are palisade parenchyma and spongy parenchyma. How does palisade parenchyma differ from spongy parenchyma? Let’s check it out.
Palisade parenchyma is present on the adaxial surface. They possess columnar cells. They are arranged vertically and parallel to each other. They possess chlorophyll and perform photosynthesis.
Spongy parenchyma are oval or round cells. Between the palisade parenchyma and the lower epidermis, they are loosely organised. They have a lot of intercellular space as well as air cavities. It allows air circulation inside the leaf. They possess few chloroplasts.
Fig: Ground tissue system
Vascular tissue system
Vascular tissue system of a dicot leaf includes the vascular bundles with xylem and phloem.
Vascular bundles of dicot leaves have some particular characteristics. Vascular bundles are present in veins and midribs and as a result size of vascular bundles varies with the size of the vein. They are surrounded by bundle sheaths. Bundle sheath is a sheath of compactly arranged parenchyma cells.
Some of the dicot plants follow the C4 pathway for carbon dioxide fixation during photosynthesis. Hence the leaf anatomy of C4 plants is adapted and modified into a special kind of anatomy called Kranz anatomy. In Kranz anatomy, undifferentiated mesophyll cells are arranged in a ring-like fashion around the bundle-sheath cells. The bundle-sheath cells have thicker walls and a greater number of chloroplasts than the mesophyll cells.
The position of the vascular tissues are different from stem and root. In dicot leaves xylem is present on the upper side and phloem on the lower side. It is conjoint, collateral and closed. In conjoint vascular bundles the xylem and phloem are arranged in a single bundle along the same radii. Collateral vascular bundles are those in which the xylem and phloem tissues are arranged in the same radii with xylem on the upper side and phloem on the lower side. Collateral vascular bundles can be open or closed. Closed vascular bundles do not have a cambial layer for secondary growth. This indicates that there is no secondary growth happening in the dicot leaves.
Fig: Vascular tissue system
Q 1. Typically, mesophyll is well-differentiated in
a. monocot leaf
b. isobilateral leaf
c. dorsiventral leaf
d. both (a) and (b)
Answer: Parenchymatous cells called mesophyll cells are found below the epidermis of leaves and are responsible for photosynthesis since they contain chloroplast. Monocot plants have isobilateral leaves, which lack differentiation in the mesophyll cells found in between the epidermal layers. Dicot plants have separate mesophyll tissues, and they have dorsiventral leaves. Palisade parenchyma, which is present in the adaxial epidermis, and spongy parenchyma, which is present in the abaxial epidermis, are formed from the differentiation of the mesophyll cells. While spongy parenchyma is made up of loosely arranged oval or circular mesophyll cells, palisade parenchyma is composed of elongated cells that are parallel to one another.
Hence the correct option is c.
Q 2. Which of the following statements is wrong about the anatomy of a dicot leaf?
I) Adaxial epidermis covers the upper surface and abaxial epidermis covers the lower surface
II) Stomata is more in adaxial epidermis and very less in abaxial epidermis
III) Vascular bundles are present in veins and midribs
IV) Xylem is present on the inner side and phloem on the outer side
Answer: Epidermis is the outermost layer of the leaf and it gives protection to the leaves. It is formed of a single layer of epidermal cells. It is of two types. One is adaxial epidermis and other is abaxial epidermis. In dicot leaves adaxial epidermis covers the upper surface and abaxial epidermis covers the lower surface. Stomata are present on the epidermis. Stomata are more in abaxial epidermis and very less or absent in adaxial epidermis. Vascular bundles are present in veins and midribs and as a result size of vascular bundles varies with the size of the vein. The position of the vascular tissues are different from stem and root. In dicot leaves xylem is present on the upper side and phloem on the lower side.
Hence the correct option is b.
Q 3. What are the components of the epidermal tissue system of a dicot leaf?
Answer: An epidermal tissue system of dicot leaves possesses tissues like Epidermis and Stomata. Epidermis is the outermost layer of the leaf and it gives protection to the leaves. It is formed of a single layer of epidermal cells. It is of two types. One is adaxial epidermis and other is abaxial epidermis. Adaxial epidermis covers the upper surface and abaxial epidermis covers the lower surface. Cuticle is present on the epidermis and it is conspicuous on the adaxial epidermis. Stomata is present on the epidermis. It is more in abaxial epidermis. Stomata will be very less or we can say it is not present in adaxial epidermis. Hence a monocot leaf is known as a hypostomatic leaf. Guard cells of stomata are in bean shape.
Q 4. Differentiate between palisade and spongy parenchyma.
Answer: Two types of mesophyll cells are present in the leaf and they are palisade parenchyma and spongy parenchyma.
Present just below the adaxial or upper epidermis
Present between the palisade layer and abaxial or lower epidermis
Possess columnar cells
Possess oval or round cells
Arranged vertically and parallel to each other
Loosely arranged between palisade parenchyma and lower epidermis
Possess more chloroplast
Possess few chloroplasts
Permits air to circulate throughout the leaf
Q 1. What role does spongy mesophyll play in photosynthesis?
Answer: A thin coating of water covers the spongy mesophyll cells. Gases dissolve in this water as they flow into and out of the cells. When the plant is photosynthesising during the day, these qualities allow carbon dioxide to infiltrate into the spongy mesophyll cells and oxygen to diffuse out of them.
Q 2. Why are all the vascular bundles not of the same size in dicot leaves?
Answer: The vascular bundles of a dicot leaf are of different sizes. This is because of the reticulate venation. The size of the veins are different in different regions of the leaf. Since the vascular bundles are present on the veins of the leaves, the size of vascular bundles is also different.
Q 3. What is the difference between the anatomy of leaves of C3 and C4 plants?
Answer: C3 plants are those that produce 3-phosphoglycerate, which has three carbon atoms, as the first byproduct of the assimilation of carbon dioxide during photosynthesis. C4 plants instead produce a 4-carbon compound (Oxalo acetic acid) as their first product. The mesophyll cells of C3 plants contain chloroplasts are generally differentiated into palisade and spongy parenchyma and the bundle sheath cells lack chloroplasts.
C4 plants have an undifferentiated mesophyll layer and one or more layers of mesophyll cells is arranged like a wreath around the bundle sheath cells. The leaves of C4 plants have two types of chloroplast: mesophyll chloroplast and bundle sheath chloroplast, also known as dimorphic chloroplast. The number of chloroplasts in the bundle-sheath cells is larger than the mesophyll cells. The bundle-sheath chloroplasts are larger compared to mesophyll chloroplasts and also lack grana.
Q 4. Why CAM plants have a different leaf anatomy?
Answer: CAM plants are those that fix carbon dioxide through the Crassulacean acid metabolism or CAM pathway. Cacti and other plants in arid climates, as well as tropical epiphytes like orchids, have developed this photosynthetic response to periodic water supply. Anatomical similarities between CAM species may represent functional restrictions. Leaf succulence, characterised by massive, undifferentiated mesophyll cells dominated by enormous vacuoles, is a common aspect of CAM anatomy. The fact that CAM plants must fix CO2 at night, when their stomata are open, explains why they have a distinct leaf structure than C3 plants. By carboxylating phosphoenolpyruvate to oxaloacetate, which is ultimately reduced to malate, CAM plants store CO2 mostly as malic acid.
YOUTUBE LINK: https://www.youtube.com/watch?v=Vrg9wf5gQd8 (53:20-1:08:52)
Epidermal tissue system: Epidermis, Stomata, Epidermal appendages, Practice problems and FAQs
Tissue system: Ground tissue system, Practice problems and FAQs
Tissue system: Vascular tissue system, Practice problems and FAQs
Anatomy of monocotyledonous leaf, Practice problems and FAQs