Cracking the AP Biology Exam

10

Plants

PLANT GROWTH

How do plants grow? Plants have unspecialized, actively dividing cells called meristems. The meristems of a plant make it grow in two ways. These two means of growth are called primary growth and secondary growth.

• Primary growth increases the length of a plant. The tissues that cause primary growth are the apical meristems and are located in the tips of roots and stems.
• Secondary growth is carried out by the lateral meristems. These dividing cells increase the girth, or width, of a plant, and are located on the sides of stems and roots. The lateral meristem produces two types of cells: vascular cambium and cork cambium. The vascular cambium produces secondary xylem and secondary phloem, which replace primary xylem and primary phloem. The cork cambium produces the tissues of the outer bark. Plants also have lenticels, which allow for gas exchange through the bark.

When it comes to plant growth, make sure you know the difference between the two types of growth and which tissues are responsible for each one.