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ROOTS
I. Roots
A. Make up the underground portion of the sporophyte
B. Anchor and absorb water and nutrients
C. Some roots are also important storage organs. For example, biennials
(plants which complete their life cycle in two years) such as carrots and
beets accumulate storage products during the first growing season, which is
used the second season to complete their life cycle
D. How far roots penetrate the soil and how far they spread laterally is
dependent upon several factors, including soil moisture, temperature and
compositions
a. Feeder roots, those actively involved in water and
mineral uptake, in trees generally occur toward upper surface (15 cm) of the
soil
E. Most roots spread laterally much more than deeply. However some tap
roots may go to 50 meters deep.
F. As a plant grows it must maintain a certain balance
between the total surface area available for food manufacture (photosynthetic
surface of the leaves) and the surface area available for water and mineral
uptake
1. Early in a plant life cycle, water and mineral uptake surface area
greatly exceeds the photosynthetic surface area
2. However, later in life the photosynthetic surface area will exceed the
root surface area. Therefore, when transplanting older plants you should cut
back the shoots because much of feeder roots are left behind when it is dug
up
II. Root systems
A. There are two types
of root systems:
1. Taproot system
a. Primary root originates in the embryo
and, in dicots and gymnosperms, it becomes a taproot,
growing directly downward and giving rise to lateral roots
b. A taproot and its branches make up the taproot
system
2. Fibrous root system
a. In monocots the primary root is short lived and the root system
arises from adventitious roots that arise from the stem (adventitious
= arising from an unusual place)
b. These roots and their lateral branches make up a fibrous
root system
c. Fibrous root systems don't go as deep as taproots, but they spread
laterally and hold the soil together, therefore they are good ground
covers to prevent erosion. This is why grasses make good lawns
III. Origin and growth of primary root tissues
A. Root growth is essentially a continuous process from embryo until death,
unless stopped by conditions such as drought or low temperatures
B. The root tip is covered by a root cap, a mass of cells that protects the
apical meristem behind it
1. As the root pushes downward these cells are sloughed off such that
they form a slimy layer (of polysaccharides secreted by outer cells) which
lubricate passage through the soil
2. The sloughed off ones are replaced by the apical meristem
C. The root apical meristem is composed of small many sided cells, the
initials, and their immediate derivatives. It gives rise to three
primary meristems:
1. Protoderm - gives rise to the outer, protective coating
of the plant
2. Procambium - gives rise to the vascular tissue
3. Ground meristem - gives rise to ground
tissue
D. Regions of the root (these
regions are not sharply delimited but intergrade into each other):
1. Region of cell division - the apical meristem plus
nearby portions in which cell division occurs.
2. Region of elongation - immediately behind the region
of cell division. It results in most of the increase in root length
3. Region of maturation - region in which most of the
cells of the primary tissues mature
a. Root hairs are also produced in this area
IV. Root primary structure
A. The internal anatomy of roots is
relatively simple compared to stems
1. Recall that most roots are protosteles
with a solid cylinder of vascular tissues
2. However, a few are siphonosteles,
with a hollow cylinder around a pith
a. A few monocots, including corn, have a central
pith
B. Epidermis - usually
composed of a single layer of parenchyma cells closely packed together
1. In young roots water and mineral absorption is facilitated by
numerous root hairs, extensions
of the epidermal cells
2. Root hairs greatly increase the surface area for water absorption,
e.g. a 4 month old rye plant has 14 billion root hairs with an absorbing
surface of 401 square meters and total length of 10,000 km
3. The epidermal surfaces of many roots are often covered by a slime
sheath called the mucigel
a. The mucigel provides a favorable environment for beneficial bacteria
and it may also influence the availability of ions to the root and also
provide some protection from desiccation
4. Epiphytic orchids have their roots exposed
so they have a multiple epidermis called a
velamen
C. Cortex - composed
of parenchyma cells, occupies the greatest area of the primary root
1. The individual c ells lack chloroplasts and they store starch and
other substances
2. Cortical tissues contain many intercellular
air spaces for aeration of the cells and they have numerous contacts
with each other
a. Substances may move between them by way of their protoplasts and
plasmodesmata or in the spaces between adjacent cell walls
3. The innermost layer of the cortex is compactly arranged to form the endodermis
a. Casparian strips
impregnated with suberin, making them impermeable to water, occur between
the adjacent cells of the endodermis
b. This prevents passage of material between the cell walls. Therefore,
materials must pass through plasmodesmata connecting endodermal cells or
across their plasma membranes. This allows cells more control over what
goes into the vascular cylinder
D. Vascular cylinder -
consists of vascular tissues (xylem and phloem) and one or more layers of
cells which comprise the pericycle.
1. The pericycle completely surrounds the vascular tissues. It has
several functions:
a. It gives rise to lateral roots
and it also contributes to the vascular cambium in plants that have
secondary growth. It also generally gives rise to the first cork cambium
V. Secondary growth in roots
A. In roots that undergo secondary growth the vascular cambium arises by
divisions of procambial cells that remain meristematic and occur in between
the primary xylem and phloem
B. Cells in the pericycle also divide and the inner cells contribute to the
development of the vascular cambium
C. Shortly thereafter a ring of vascular cambium is formed around the
primary vascular cylinder and it gives rise to secondary xylem and secondary
phloem
D. Pericycle cells also give rise to the first cork cambium producing the periderm,
which replaces the epidermis
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