Plants ❮ 169
A root is not just a root, for not all root structures are the same. In Chapter 13,
Taxonomy and Classification, two types of angiosperm plants were mentioned: dicots and
monocots. Dicots are known for having a taproot system, while monocots are associated
with fibrous roots. The taproot(e.g., carrot) systembranches in a way similar to the human
lungs; the roots start as one thick root on entrance into the ground, and then divide into
smaller and smaller branches called lateral rootsunderneath the surface, which serve to hold
the plant in place. Fibrous rootsprovide plants with a very strong anchor in the ground
without going very deep into the soil. The root system can be summarized as follows:
Dicots→taproot→thick entry root →division into smaller branches
Monocots→fibrous root →shallow entry into ground →strong anchor effect
Root Structure
Let’s take a look at the structure of a root moving from outside to inside. The root is lined
by the epidermis,whose cells give rise to the root hairs that plants must thank for their abil-
ity to absorb water and nutrients. Moving farther in, we come to the cortex, the majority of
the root that functions as a starch storage receptacle. The innermost layer of the cortex is
composed of a cylinder of cells known as the endodermis.These cells are important to the
plant because the walls between these cells create an obstacle known as the casparian strip,
which blocks water from passing. This is one of the mechanisms by which plants control the
flow of water. Moving in through the endodermis, we come to the vascular cylinder,which
is composed of a collection of cells known as the pericycle.The lateral roots of the plant are
made from the pericycle, and hold the vascular tissue of the root—our friends from earlier,
the xylem and phloem.
Root Growth
Plants grow as long as they are alive as a result of the presence of meristemic cells.Early
on in the life of a plant, after a seed matures, it sits and waits until the time is right for ger-
mination. At this point, water is absorbed by the embryo, which begins to grow again.
When large enough, it busts through the seed coat, beginning its journey to planthood. At
the start of this journey, the growth is concentrated in the actively dividing cells of the apical
meristem.Growth in this region leads to an increase in the length of a plant: primary plant
growth.Later on growth occurs in cells known as the lateral meristems,which extend all
the way through the plant. This growth leads to an increase in the width of a plant and is
known as secondary plant growth.
Regions of Growth
Root cap:protective structure that keeps roots from being damaged during push through soil.
Zone of cell division:section of root where cells are actively dividing.
Zone of elongation:next section up along the root, where cells absorb H 2 O and increase
in length to make the plant taller.
Zone of maturation:section of root past the zone of elongation where the cells differenti-
ate to their finalized form (phloem, xylem, parenchyma, epidermal, etc).
The Shoot System
Now that we have discussed roots—the part of the plant that is inthe ground—let’s take
a look at shoots (leaves and stems), the parts of the plant that are out ofthe ground.
KEY IDEA
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