7th Grade Science Student ebook

14.2 Roots, Stems, and Leaves.

CHAPTER 14: PLANTS

Leaves

The function of

leaves

The main function of leaves is to use sunlight to make food

during the process of photosynthesis. Leaves take in carbon

dioxide from the air and absorb sunlight. During photosynthesis,

cells in leaves produce carbohydrates and oxygen. Oxygen is

released into the air through the leaves. Carbohydrates are

transported to other parts of the plant for later use.

The structure of

a leaf

The structure of leaves is related to their function. Many leaves are

broad and flat so they can absorb the maximum amount of

sunlight. The diagram in Figure14.14 shows a cutaway view of a

leaf. As you can see, a leaf consists of many layers of tissue. The

outer surface of the leaf is covered by the cuticle which protects the

leaf. Next, is a single layer of cells called the epidermis. Light can

easily pass through the epidermis and into the palisade and spongy

layers where photosynthesis happens.

The palisade and

spongy layers

The palisade layer is made up of elongated cells. Palisade cells

contain many chloroplasts, the organelles where photosynthesis

occurs. Cells in the spongy layer are spaced further apart. The

air spaces between the cells allow carbon dioxide to diffuse

throughout the leaf. The veins of the leaf contain xylem and

phloem. Xylem carries water and minerals throughout the leaf.

Phloem carries the carbohydrates made during photosynthesis

to the rest of the plant.

Stomata The lower epidermis contains tiny pores called

stomata (singular, stoma). Stomata allow carbon

dioxide to enter the leaf and oxygen and water

vapor to exit. Each stoma is opened and closed by

guard cells. The picture (right) shows two open

stomata and their guard cells.

Figure 14.14: A cutaway view of a

leaf showing its structures.

stomata - tiny pores that allow

carbon dioxide to enter a leaf and

oxygen and water vapor to exit.