Page 18 - 新思维科学学生用书9 样章

1.2 More about photosynthesis

The diagram on the right shows what
a leaf looks like if you cut it across, and A waxy layer on the leaf The upper epidermis
then look at the cut edge. surface stops the leaf protects the cells
cells from drying out. inside the leaf.

The palisade layer
Leaves are so thin that it is di cult to contains cells that
imagine they contain several layers of do most of the
cells. It is the cells in the middle of the photosynthesis.
leaf that carry out photosynthesis.

The spongy layer
has lots of air
A vein carries spaces. The cells
water to the in the spongy layer
cells in the leaf.
do a small amount
of photosynthesis.

The lower epidermis protects A stoma is a tiny hole in the lower epidermis.
the cells inside the leaf. These holes let carbon dioxide from the air get into the leaf.

Activity 1.2.1

Which surface of a leaf has most stomata?
Work with a partner for this activity.
Put some warm (not hot) water into a beaker or glass.
Take a fresh leaf, and push it down under the water.
Can you see any bubbles coming out of the leaf?

Which surface of the leaf do most of the
bubbles come from?
Discuss these questions with your partner.

• What do you think the bubbles contain?
• If most bubbles came from one surface of
the leaf, why do you think this happened?
• Think about what you know about the
effect of increased temperature on gases.

Can you suggest why the bubbles came out
of the leaf when you put it into warm water?

17