170 ❯ STEP 4. Review the Knowledge You Need to Score High
Structure of a Leaf
Leaves are protected by the waxy cuticleof the epidermis, which functions to decrease the
transpiration rate. Inside the epidermis lies the ground tissue of the leaf, the mesophyll,
which is involved in the ever-so-important process of photosynthesis. There are two impor-
tant layers to the mesophyll: the palisade mesophylland the spongy mesophyll.Most of
the photosynthesis of the leaf occurs in the palisade mesophyll, where there are many
chloroplasts. Inside a bit farther is the spongy mesophyll whose cells provide CO 2 to the
cells performing photosynthesis. Important structures to successful photosynthesis are
stomata,which are controlled by the guard cells that line the walls of the epidermis.
Extending a bit farther inside the leaf, we find the xylem,the supplier of water to photosyn-
thesizing cells, and the phloem,which carries awaythe products of photosynthesis. In C 4
plants, a second type of cell called a bundle sheath cellsurrounds the vascular tissue to make
the use of CO 2 more efficient and allow the stomata to remain closed during the hot day-
time hours. These cells prevent excessive transpiration.Structure of Stems
Again, let’s travel from the outside in and discuss the basic structure. The epidermis for
the stem provides protection and is covered by cutin,a waxy protective coat. The cortex
of a stem contains the parenchyma, collenchyma, and schlerenchyma cells mentioned
earlier in this chapter. You’ll notice that there is no endodermis in the stem because this
portion of the plant is not involved in the absorptionof water. As a result, the next struc-
ture we see as we move inward is the vascular cylinder and our friends the xylem and
phloem.
A term to know is the vascular cambium,which extends along the entire length of the
plant and gives rise to secondary xylem and phloem. Over time, the stem of a plant will
increase in width because of the secondary xylem produced each year.
Another term to know is the cork cambium,which produces a thick cover for stems
and roots. This covering replaces the epidermis when it dries up and falls off the stem during
secondary growth, forming a protective barrier against infection and physical damage.
The growth of plants is not a continuous process in seasonal environments. There are
periods of dormancy in between phases of growth. Have you ever seen the rings of a tree
after it has been cut down? These rings produced each year are a window into the past, and
give insight into the amount of rain a tree has encountered in a given year. The wider the
ring, the more water it saw.Plant Hormones
Hormones perform the same general function for plants that they do for humans—they are
signals that can travel long distances to affect the actions of another cell. There are five main
plant hormones you should study for this exam.- Abscisic acid.This is the “babysitter” hormone. It makes sure that seeds do not germi-
nate too early, inhibits cell growth, and stimulates the closing of the stomata to make
sure the plant maintains enough water. - Auxin.This is a popular AP Biology exam plant hormone selection. Auxin is a hormone
that performs several functions—it leads to the elongation of stems, and plays a role in
phototropism and gravitropism, which we will discuss a bit later. - Cytokinins.Hormones that promote cell division and leaf enlargement. They also
seem to have an element of the “fountain of youth” in them, as they seem to slow
KEY IDEA