HortBotany
Lesson Plan #6 – instructor
Roots
 Functions of a root system:
o Absorption: the root hairs near the tips of actively growing
roots absorb water and dissolved minerals. Root hairs are
modified epidermis cells. Root hairs increase the surface
area for absorption.
root hairs on a radish seedling
o Anchorage: the root system of a plant supports and anchors
the plant in the soil.
These large caliper roots on Maple
play an important role in anchoring
the plant. They do not, however,
absorb water and nutrients.
-2o Plants, in general, have one of two types of root systems:
o tap roots; more typical of the dicots. The tap root
develops from the radicle, which is the first thing
to emerge from a germinating seed. Eventually,
smaller branch roots develop on the tap root.
tap root
o fibrous root system; more typical of the monocots.
In monocots the radicle is short-lived and the root
system becomes highly branched.
fibrous root system
 Storage: some plants have root systems that have been modified
for food storage. Plants store extra glucose produced during
photosynthesis in modified roots as starches. It is an advantage,
from the plants perspective, to store excess food underground and
away from browsing animals.
-3o Two major types of root modifications for food storage:
o Fleshy tap root; examples include carrots and beets.
Carrots are fleshy tap roots
o Tuberous roots; examples include sweet potato and
Dahlia.
sweet potato
Dahlia

Conduction: roots contain vascular tissue (xylem and phloem);
water and dissolved minerals, absorbed by the roots, are
transported through the xylem of the roots, into the stem xylem
and eventually into the leaves. In a like manner, sugars are
transported from the leaves down into stems and roots for
storage.
-4 There are 3 main factors that affect the movement of water through
plants. They are:
o Osmosis
o Transpiration
o The properties of water molecules
 Let’s begin by looking at osmosis. Definition of osmosis: The
diffusion (movement) of water across a semi-permeable
membrane.
o Osmosis is occurring in the root hairs found at the tips of
actively growing roots.
o The semi-permeable membrane that water must cross as it
moves from the soil into a root hair is the cell membrane of
the root hair.
 During osmosis water molecules attempt to equalize their
concentration on both sides of the cell membrane of a root hair. The
water typically found in the soil contains small quantities of dissolved
“stuff” like salts (also known as minerals or nutrients). The
protoplasm of a root hair, however, contains water but in that water is
a large amount of dissolved “stuff” (sugars, various nutrients, salts,
and other chemicals).
Compare the concentration of water, and the amount of “stuff
dissolved in the water, between the soil and protoplasm of a root hair:
SOIL
PROTOPLASM
water concentration:
high
low
amount of “stuff”
dissolved in the water:
low
high
-5 So, what makes osmosis work? Why does water move from the soil
into a root hair?
o Water in the soil is in high concentration compared to the
low concentration of water in the protoplasm of a root hair.
o Water always moves from areas of high concentration to
areas of low concentration.
o Water moves from the soil (where it is in high
concentration) into a root hair (where it is in lower
concentration).
 As water moves into a root hair it is stored in the central vacuole. As
the vacuole expands it pushes the cell cytoplasm and membrane
against the cell wall. When a cell is full of water it is said to be
turgid.
 Root Pressure is the force responsible for “pushing” water through
roots and up stems.
 Root pressure is driven by:
o Osmosis
o Turgor Pressure
 How does root pressure work?
-6-
o The vacuoles in the cells of the epidermis and cortex fill
with water.
o As more water enters the root hairs by osmosis, it squeezes
the water out of the cells and into the intercellular spaces
between the cells.
o Once in the intercellular spaces the water quickly moves
across the cortex and into the xylem.
o Root pressure can only move water a few feet up a stem.
-7 The second factor affecting water movement through plants is
transpiration. Transpiration defined: The loss of water vapor
through open leaf stomata.
o Why are the leaf stomata open? During the day the plant is
photosynthesizing, the stomata must be open to allow
carbon dioxide into the leaf.
o Root pressure pushes water up stems. Transpiration “pulls” or
“sucks” water up stems in much the same way that you pull
liquid up through a straw.
o The rate of transpiration is affected by the following 3 things:
 Humidity; during dry, less humid days the rate of
transpiration increases.
 Temperature; the rate of transpiration is greater on hot
days than on cooler, cloudy days.
 Wind; the rate of transpiration is greater on windy days
than calmer days.
o What happens to the plant when water is in short supply?
 When more water is lost to transpiration than the
roots are able to absorb the stomata quickly close to
conserve water.
-8o How does transpiration work?
 On sunny, warm days the sun warms up the leaf surfaces.
 The water in the cells of a leaf begins to evaporate.
 Water vapor collects in the spongy mesophyll tissue of
the leaf.
 The water vapor moves out of spongy mesophyll,
through the open stomata, and into the drier air
surrounding the leaf.
 As water evaporates from the spongy mesophyll, new
water molecules are delivered to the leaf through the
xylem in leaf veins.
o Transpiration is the most important of the 3 factors affecting
water movement in plants.
-9 The third factor affecting the movement of water through plants is the
properties of water molecules.
o Water molecules (H2O) are electrically charged. Having a
positive charge at one end and a negative charge at the other
end of the molecule.
o Because of this water molecules stick to one another like little
magnets.
o Water moves through the xylem as long chains of molecules
sticking to each other.
H+
 To summarize:
H+
O-
o Osmosis is responsible for getting water into a root hair.
o Root pressure pushes water through the intercellular spaces of
the cortex and into the xylem.
o Transpiration pulls water up through the stem xylem and
ultimately into the vein xylem of leaves. Water moves through
xylem as long chains of water molecules sticking to one another
like little magnets.
H:\DATA\PSALADI\Class Folders\HortBotany\HortBot_Curr\HortBot_lesson plan6.doc