1.
Answer question numbers 1.1-1.4 on the basis of your understanding of the
following paragraph and the related studied concepts.
The leaf is the main photosynthetic organ in a plant. It controls gas exchange in
plants, controls the amount of water loss in plants. Upper epidermis cells contain
no chloroplasts – which is not true for the guard cells. They form layers on the
upper and lower surface of the leaf. Their function is to prevent water from getting
out and stopping unwanted substances/organisms getting in. The palisade
mesophyll layer is where most of the photosynthesis occurs in the leaf. The
palisade cells contain a lot of chloroplasts to help them perform this
photosynthesis. Lower epidermis is the bottom layer of the leaf, and is one cell
thick. They may not contain a cuticle within the lower epidermis, there are some
holes found in leaves called stoma. These holes allow gases to diffuse in and out
of the leaves. The stoma are formed by two highly specialized epidermis cells,
called guard cells. Guard cells are the only epidermis cells that contain
chloroplasts.
1.4 List one structural and one functional difference between upper and lower
epidermis
Answers
Answer:
Key Points
The epidermis consists of the upper and lower epidermis; it aids in the regulation of gas exchange via stomata.
The epidermis is one layer thick, but may have more layers to prevent transpiration.
The cuticle is located outside the epidermis and protects against water loss; trichomes discourage predation.
The mesophyll is found between the upper and lower epidermis; it aids in gas exchange and photosynthesis via chloroplasts.
The xylem transports water and minerals to the leaves; the phloem transports the photosynthetic products to the other parts of the plant.
Plants in cold climates have needle-like leaves that are reduced in size; plants in hot climates have succulent leaves that help to conserve water.
Key Terms
trichome: a hair- or scale-like extension of the epidermis of a plant
cuticle: a noncellular protective covering outside the epidermis of many invertebrates and plants
mesophyll: the inner tissue (parenchyma) of a leaf, containing many chloroplasts.
Leaf Structure and Function
The outermost layer of the leaf is the epidermis. It consists of the upper and lower epidermis, which are present on either side of the leaf. Botanists call the upper side the adaxial surface (or adaxis) and the lower side the abaxial surface (or abaxis). The epidermis aids in the regulation of gas exchange. It contains stomata, which are openings through which the exchange of gases takes place. Two guard cells surround each stoma, regulating its opening and closing. Guard cells are the only epidermal cells to contain chloroplasts.
The epidermis is usually one cell layer thick. However, in plants that grow in very hot or very cold conditions, the epidermis may be several layers thick to protect against excessive water loss from transpiration. A waxy layer known as the cuticle covers the leaves of all plant species. The cuticle reduces the rate of water loss from the leaf surface. Other leaves may have small hairs (trichomes) on the leaf surface. Trichomes help to avert herbivory by restricting insect movements or by storing toxic or bad-tasting compounds. They can also reduce the rate of transpiration by blocking air flow across the leaf surface.
image
Figure 30.4C.1 : Trichomes: Trichomes give leaves a fuzzy appearance as in this (a) sundew (Drosera sp.). Leaf trichomes include (b) branched trichomes on the leaf of Arabidopsis lyrata and (c) multibranched trichomes on a mature Quercus marilandica leaf.
Below the epidermis of dicot leaves are layers of cells known as the mesophyll, or “middle leaf.” The mesophyll of most leaves typically contains two arrangements of parenchyma cells: the palisade parenchyma and spongy parenchyma. The palisade parenchyma (also called the palisade mesophyll) aids in photosynthesis and has column-shaped, tightly-packed cells. It may be present in one, two, or three layers. Below the palisade parenchyma are loosely-arranged cells of an irregular shape. These are the cells of the spongy parenchyma (or spongy mesophyll). The air space found between the spongy parenchyma cells allows gaseous exchange between the leaf and the outside atmosphere through the stomata. In aquatic plants, the intercellular spaces in the spongy parenchyma help the leaf float. Both layers of the mesophyll contain many chloroplasts.