Primary Growth in plant

I. Primary Growth

Primary growth in plants is longitudinal growth, or growth in length.  All vascular plants,
whether herbaceous or woody, undergo primary growth.  Growth requires plant meristems, areas
of cells capable of mitotic cell division.  Growth consists of three stages:
1) Cell division via mitosis: Daughter cells (new cells) are produced from meristematic cells
(apical initials, or mother cells).
2) Cell expansion:  Daughter cells grow to a functional size.
3) Cell differentiation and maturation: Cells develop the physical and physiological
characteristics necessary to perform specialized functions.
Meristems are undifferentiated, perpetually juvenile plant tissues which are capable of dividing
mitotically, producing plant growth.  Apical meristems are areas of actively dividing cells at the
tips of all roots and shoots.  Apical meristems produce three types of primary meristems:
protoderm, ground meristem and procambium.  Each of these primary meristems produces
different types of primary tissue systems: dermal tissue, ground tissue, and vascular tissue.

II. Primary Root Growth

1. Root Function

The structure of roots relates to the many functions that they must perform.  Roots must be able
to penetrate soil, which is often hard and potentially damaging to a growing root via friction.
They must be able absorb water and dissolved mineral nutrients from the soil.  They then need to
be able to "filter" substances absorbed to exclude undesirable compounds (e.g., salt) from thiswater.  They must transport water from the roots to the shoot of the plant.  Conversely, roots
must be able to transport photosynthate (carbohydrates produced by photosynthesis) from the
shoot to the roots where it is distributed to support respiration and growth.  Roots often also
serve as storage organs for photosynthate.  Finally, roots anchor and support the above-ground
portion of plant.  These functions are accomplished by specialized tissues with specific
physiological functions.

2. Root Trivia

Roots are, of course, critical to the survival and growth of most plants, but tend to be unseen and
therefore forgotten.  Roots can penetrate to depths that exceed tree height in some arid
environments.  For example, Acacia roots in Egypt were found at 30 m depth, while mesquite
(Prosopis juliflora) roots in Arizona reached 53.3 m deep.  Roots can also be very numerous and
have a large collective length and surface area.  A 6 month old Cornus florida seedlings had
~2,600 roots totalling 51.4 m in length, while a 6 month old Pinus taeda seedling had 767 roots
totalling 3.87 m in length.  Roots can grow anywhere from <25 mm per day.

3. Anatomy

The first root originating from the embryo as the radicle is called the primary root.  This
primary root can become a taproot in gymnosperms and eudicots, growing directly downwards
and giving rise to lateral roots.  Monocots typically have a dense fibrous root system rather than
a taproot system, with a short-lived primary root and new adventitious roots arising from the
base of the stem.  The total surface area of feeder roots, actively engaged in uptake of water and
minerals, needs to be balanced with the total above-ground photosynthetic surface area (leaves
and sometimes stems).  The end of the root is protected by the root cap, which consists of
loosely-packed, mucilaginous cells which protect the root apical meristem and lubricate the root
for growing between soil particles.  As the root cap is pushed into the soil by the elongating root,
cells slough off as a result of soil friction.  The meristematic region is the area of active
longitudinal cell division at the root apex (behind the root cap).  Behind the meristematic region
is a zone of elongation, and behind that a zone of differentiation and maturation.  The root apical
meristem produces new cells of three types:  1) Protoderm cells (on the outside) will produce
new epidermal tissues; 2) The procambium in the middle will produce cells that develop into a
primary vascular (=conducting) system; 3) Ground meristem cells will produce the root cortex.
Several root tissues are produced by the protoderm and the ground meristem.  The outermost
tissue is the epidermis (="skin" of the new root).  Some epidermal cells also expand laterally to
become root hairs, which absorb moisture and dissolved nutrients from the soil.  Root hairs are
short lived and usually found only in the zone of maturation. The cortex is largely a storage
tissue but it also affects lateral water movement across the root.  The endodermis regulates the
flow of materials into the center of the root.  The endodermis consists of cells with a band of
suberin on radial cell walls called the Casparian strip.  This impermeable layer forces most
substances to go through the protoplasm of endodermal cells rather than moving through
intercellular spaces.  The Casparian strip thus acts as a filter to exclude undesirable substances,
e.g., salt in mangroves.  The pericycle is a tissue that appears similar in structure to the cortex;
but areas of the pericycle will later become meristematic, form new apical meristems and root
caps, and produce lateral roots that will push their way out into the soil.  The procambiumproduces vascular tissues for transport of water and nutrients.  It produces primary phloem for
the transport of food and other things (including hormones).  It also produces primary xylem,
which transports water and dissolved mineral nutrients up to the shoot.  Primary xylem consists
mainly of cells that die during maturation in order to function in water transport (tracheids in
gymnosperms; fibers and vessels in angiosperms).  The procambium also produces a
meristematic tissue called the vascular cambium.  The vascular cambium is initially
discontinuous (only found in vascular bundles).

4. Gravitropism

Most cell growth occurs in the zone of elongation, well behind the root cap.  Cells approximately
triple in size in this zone.  Root elongation pushes the root tip further into the soil, and this is
lubricated by the mucigel of the root cap.  Primary roots are positively gravitropic, and if placed
horizontally, will curve and grow downwards by elongating cells more on top than on bottom.
Only the tap root grows straight down.  Primary laterals, on the other hand, usually grow at an
angle, while secondary laterals are indifferent to gravity.  Gravity sensing cells in root cap
contain leucoplasts filled with starch grains (=amyloplasts) that settle to 'bottom' of cell and
orient growth of root accordingly.

III. Primary Shoot Growth

Shoots are negatively gravitropic and  positively phototropic.  The meristematic region for shoot
primary growth, called the shoot apical meristem, is in the shoot apex.  Leaf primordia
develop on the sides of the meristematic region and are arranged in a specific pattern – spirally
arranged in Douglas-fir and many gymnosperms and angiosperms, and in pairs on opposite sides
of the apical meristem in some species such as maples (Acer).  Leaf primordia will develop into
leaves.  The points of leaf attachment are called nodes, while the distance along a shoot between
two nodes is called an internode Internodes vary widely in average length, for example, they are
2-3 mm in Douglas-fir and 5-10 cm in red alder.  New apical meristems form in leaf axils (i.e., at
nodes).  These new apical meristems will either form buds, or will produce a new shoot.  In
Douglas-fir ~1 in 20 leaf axils develop apical meristems, while in red alder and most hardwoods,
every leaf axil gets a new meristem. A leaf plus its node and the internode below constitutes a
repeating unit called a phytomere.
The shoot apical meristem gives rise to the three primary meristems (protoderm, ground
meristem and procambium) and these in turn produce all shoot tissues.  As in roots, the
outermost layer of a primary shoot is the epidermis, which arises from the protoderm.  The
epidermis may contain stomata or lenticels for gas exchange, or may have a waxy or hairy
surface to prevent dessication.  The shoot cortex arises from ground meristem.  The primary
vascular tissues, the primary phloem, vascular cambium and primary xylem, arise from the
procambium.  These three tissues are organized into separate vascular bundles.  Finally, the
innermost shoot tissue is the pith.  The pith is produced by the central ground meristem, and
functions for a short period as a storage tissue.