Download Vascular Systems

S E C T I O N
14.2
Vascular Systems
E X P E C TAT I O N S
Understand the basic vascular
systems in plants.
Describe the differences and
similarities between xylem
and phloem.
Figure 14.14 Vascular tissue
makes it possible for water to
travel up the 40 m trunk of this
tall conifer and for food to travel
down to the roots.
Vascular plants have specialized tissue for
transporting material from one location to another
within the plant, as in the tall tree in Figure 14.14.
This vascular tissue consists of an internal system
of tubes that transport water and dissolved food
throughout the plant.
Humans and all other mammals also possess a
“vascular system,” which consists of tubes that
transport water, dissolved food, and oxygen
throughout their bodies. In animals, this mixture is
pushed around in the blood by a pump called the
heart. How can material move throughout a plant
with no heart to pump it? Look for the answer in
the following sections.
vascular bundles
Vascular Bundles
If you have ever had a “string” of celery stuck
in your teeth, you have encountered a vascular
bundle. These bundles transport material within
the plant. In dicot stems, the bundles form a
discontinuous ring of vascular bundles. In
monocot stems, the vascular bundles are scattered
throughout the stem of the plant in no discernible
pattern. These patterns are shown in Figure 14.15.
A
B
REWIND
To review transport tissues and vascular bundles, turn to
Chapter 9, Section 9.5.
Figure 14.15 Locate the vascular bundles in the stem
cross-sections of a typical dicot (A) and a typical
monocot (B).
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The vascular bundles in a stem are continuous,
tube-like strands connecting the vascular tissue of
the root to the vascular tissue in the leaves. In the
root, the vascular tissue forms a central cylinder, or
core. In the stem, this cylinder branches out to
form several separate bundles. These bundles form
a continuous connection through the stem, running
up and down between the roots and the leaves.
Xylem Transports Water
The transportation of water is carried out by
specialized tissue called xylem in the vascular
bundles. In angiosperms, xylem consists of long,
mainly hollow tubes formed by the nonliving cell
walls of tracheids or vessel elements (see Figure
14.16.) These structures begin as living cells that
grow end-to-end in the immature stem. When the
cells mature, their living contents die, leaving the
non-living cell walls in place. Fluids pass from one
tracheid or vessel element to the next.
Translocation and Phloem
The term translocation can mean different things.
A few authors use this term as a synonym for
transportation. Others use it only for the transport
of organic molecules. A few reserve it for the
transport of food (specifically sugar) from one plant
part to another. The greater precision of this third
definition allows us to give the transport of food
its own dedicated name. This textbook defines
translocation as the transportation of food from
one region in a plant part to another region.
Translocation is carried out within the specialized
tissue called phloem.
The process of translocation is still poorly
understood. Several main points concerning how
translocation occurs are summarized here:
Sugary tree sap rises in the spring when there are
no leaves from which transpiration can occur. So
transpiration-tension cannot be responsible for
the movement of dissolved food through phloem.
Phloem moves food from regions of low
concentration to regions of high concentration.
Therefore, translocation cannot be explained by
simple diffusion, which can only move solutes
from regions of high concentration to regions of
low concentration.
Phloem is made of living cells, which use oxygen
while they are moving food (see Figure 14.17).
tracheid
vessel element
Figure 14.16 Both tracheids (left) and vessel elements
(right) conduct water. The xylem of gymnosperms contains
only tracheids, while the xylem of angiosperms contains
both tracheids and vessel elements. Water (and anything
dissolved in it) passes from one tracheid to the next through
“pits,” which are thin regions in the adjacent end walls.
sieve tube element
companion cell
sieve plate
REWIND
To find out more about xylem and phloem, turn back to
Chapter 9, Section 9.5.
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MHR • Plants: Anatomy, Growth, and Functions
Figure 14.17 The phloem is made up of a companion cell,
sieve-tube element, and sieve plate.
Researchers have concluded that translocation is
an active process. That is, the living cells of the
phloem somehow provide the energy for
translocation through their own cellular
respiration. Exactly how the phloem moves the
dissolved sugars around the plant is not yet
understood, and is an ongoing topic of investigation
by scientists.
Figure 14.18 illustrates how the structure of
phloem differs from that of xylem. Note especially
that phloem consists of living cells.
You may recall from Chapter 1 that glucose is
produced by photosynthesis and is immediately
converted to starch and stored in tiny grains within
the cell’s chloroplasts. You have also learned that
starch grains are stored in the cortex cells of the
root, and that the phloem transports dissolved food
between the leaf and the root. However, starch is
insoluble in water, which raises an unavoidable
question: what “dissolved food” is being
transported by the phloem? The answer: sucrose.
PAUSE
companion cell
sieve
plate
RECORD
Recall from Chapter 1 that the structure of sucrose differs
from the structure of glucose, even though both are
classified as sugars. Glucose is a single sugar or
monosaccharide. Why is sucrose often called a double sugar
or disaccharide? Why is starch called a polysaccharide?
Glucose, Sucrose, or Starch
The food stored in a tree’s roots is starch, but this
food cannot be transported through the stem in the
form of starch. It must be broken down chemically
into sucrose so it can dissolve in water. The sap
that floods upward through the phloem of maple
trees in spring contains large amounts of sucrose.
It also contains small amounts of the organic
molecules that give maple syrup its distinctive
flavour. Once the maple sap is delivered to the
immature buds, the sucrose is further broken down
into glucose. The cells in the buds need this
glucose to provide the energy they need to divide
and grow in order to produce full-grown leaves.
The growing leaves then begin to make their own
glucose by photosynthesis.
xylem vessel cells
phloem sieve-tube cells
Figure 14.18 The structure of xylem and phloem.
Once the leaves are producing more glucose than
they require, the excess glucose is converted into
starch grains in the chloroplasts. Then the starch is
broken down to form sucrose. Dissolved sucrose
will travel from the leaves to the root by way of the
phloem tissue in the stem. It is converted to starch
in the root and stored mainly in the root cortex.
Most plants, large or small, handle the
translocation of food in the same way as the maple
tree, as shown in Figure 14.19 on the next page.
That is, the food is transported up and down the
stem in the form of dissolved sucrose molecules.
PLAY
Go to your Electronic Learning Partner to enhance your
learning about plant transport systems.
BIO
FACT
What makes honey sweet? Bees make honey from flower
nectars that consist mainly of dissolved sucrose.
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used by buds and
leaves are formed
glucose
leaves produce starch
through photosynthesis
starch
sucrose
sucrose
sucrose
starch
starch
early
spring
late spring
summer
Figure 14.19 The formation and use of maple sap in a maple tree.
PAUSE
RECORD
Use the information presented on the previous page, along
with the diagram shown here in Figure 14.19, to summarize
how food is transported through a tree. You may use
diagrams, a flow chart, or a written description for your
summary. Keep this information in your notebook.
SECTION
1.
K/U Compare and contrast the function of the
phloem and xylem in vascular plants.
2.
What are the structural differences between the
phloem and xylem?
3.
K/U If you analyzed the liquid that is found in the
xylem, what would be its largest component?
4.
If the xylem or phloem of a plant were damaged,
what would happen to the plant? Give explanations
for your answer.
5.
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REVIEW
6.
I In an experiment, a researcher interrupted the
flow of materials through a tree by placing a metal
plate part of the way through the trunk. One day
later, a chemical analysis revealed that the sugar
concentration in the tree tissue was higher above the
plate than below the plate. As well, the concentration
of minerals and water was greater below the plate
than above the plate. Explain the results.
7.
Maple syrup consists mainly of sucrose and
water. Could the procedures for making this sweet
treat be used with other tree saps, such as oak sap,
willow sap, or birch sap? What types of information
would you need to test this hypothesis?
K/U
C
MC Would the rate of maple sap retrieved from the
trees be greater during the day or during the night?
Why?
MHR • Plants: Anatomy, Growth, and Functions
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