168 ❯ STEP 4. Review the Knowledge You Need to Score HighAnatomy of Plants
The anatomy of a plant in its most simplistic form can be divided into the roots and the
shoots.Rootsare the portions of the plant that are below the ground, while shootsare the
portions of the plant that are above the ground. The roots wind their way through the terrain,
working as an anchor to keep the plants in place. In addition, the roots work as gatherers,
absorbing the water and nutrients vital to a plant’s survival.Tissue Systems
There are threeplant tissue systems to know: ground, vascular, and dermal.Ground Tissue
The ground tissue, which makes up most of the body of the plant, is found between the dermal
and vascular systems and is subdivided into three cell types: collenchyma cells,live cells that
provide flexible and mechanical support—often found in stems and leaves; parenchyma
cells,the most prominent of the three types, with many functions—parenchyma cells found
in leaves are called mesophyll cells,and allow CO 2 and O 2 to diffuse through intercellular
spaces (owing to the presence of large vacuoles, these cells play a role in storage and secre-
tion for plants); and sclerenchyma cells,which protect seeds and support the plant.Vascular Tissue
Plant vascular tissue comes up often on the AP Biology exam. The two characters you need
to be familiar with are the xylem and the phloem.
Xylem.This structure has multiple functions. It is a support structure that strengthens the
plant and functions as a passageway for the transport of water and minerals from the soil.
One interesting (and sad) note about xylem cells is that most of them are dead and are
simply there as cell walls that contain the minerals and water being passed along the plant.
Xylem cells can be divided into two categories: vessel elementsandtracheid cells.They
both function in the passage of water, but vessel elements move water more efficiently
because of structural differences that are not pertinent to this exam. ☺
Phloem.This structure also functions as a “highway” for plants, assisting in the movement
of sugars from one place to another. Unlike the xylem, the functionally mature cells of the
phloem,sieve-tube elements,are alive and well.Dermal Tissue
Dermal tissue provides the protective outer coating for plants. It is the skin, or epidermis.
This coating attempts to keep the bad guys (infectious agents) out, and the good guys
(water and nutrients) in. Within the epidermis are cells called guard cells,which control
the opening and closing of gaps called stomatathat are vital to the process of photosynthe-
sis as was discussed back in Chapter 8, Photosynthesis.Roots
Root Systems
How do plants get their nutrients? Through the hard work of roots, whose tips absorb nour-
ishment for the plant (minerals and water) via root hairs. Most of the water and minerals are
absorbed by plants at the root tips, which have root hairsextending from their surface. These
hairs create a larger surface area for absorption in much the same way as the brush border
does in the human intestines—improving the efficiency of nutrient and water acquisition.Joscelyn (12th
grader): “Know
these two and
what their dri-
ving forces are.”BIG IDEA 2.D.2
Homeostatic mech-
anisms (e.g., plants
obtaining CO 2 and
eliminating O 2 )
reflect common
ancestry and diver-
gence due to adap-
tation to different
environments.
BIG IDEA 2.A.3
The high surface
area of root hairs
helps plants
exchange matter
with the
environment.