Stems and their function:
A stem is a collection of
integrated tissues arranged as nodes and internodes. Nodes are locations
where leaves attach to stems, and internodes are the parts of stems
Nodes and internodes
perform several important functions:
- Support leaves-Turgor
pressure in stems provides a hydrostatic skeleton that supports young
plants. Leaves are also supported by a stem's internal structure of
collenchyma and sclerenchyma.
carbohydrates-Stems of plants are green and photosynthetic. Although
photosynthesis in stems is usually not significant compared to leaves,
in some plants such as cacti it accounts for most of the plants carbon
materials-Parenchyma cells in stems store large amounts of starch and
water. Water accounts for as much as 98% of the weight of many cactus
- Transport water and
solutes between roots and leaves-The vascular system of stems
maintains an aquatic environment in leaves and transports sugars and
other solutes between leaves and roots. Stems link leaves with he
water and dissolved nutrients of the soil.
Control of Stem Growth:
The growth of stems is
controlled by many factors, leaves being the most important. Leaf
primordial control the differentiation of procambium in stems. There is no
procambium above the youngest primordium, and vascular tissues in
developing leaves along and connect with mature vascular tissues of the
stem. Light also controls stem growth.
Stems elongate in subapical
regions in response to auxin and gibberellins. These hormones elongate
stems by stimulating cellular elongation and division. In a lot of plants,
elongation occurs throughout the internode; while in some others it
continues as a wave originating at the base of the internode.
Grasses such as bamboo
elongate at meristems intercalated between mature tissues at the bases of
their nodes and leaf sheaths. These meristems are called intercalary
meristems. These remain important because they reform the stem or leaf of
a grass when its tip is torn off by an animal or lawn mower. Intercalary
meristems can produce high rates of growth in many plants. They also have
a direct impact on us-no matter how many afternoons we spend behind the
lawn mower, we are assured that the grass blades will grow back quickly
because of intercalary meristems.
Plants whose stems do not
elongate are called rosette plants; these plants have short internodes
with tightly packed leaves.The stems of rosette plants are short and made
almost entirely of overlapping leaf bases. Most rosettes fail to elongate
because they do not make enough physiologically active gibberellins.
Treating rosettes with gibberellins usually causes internodal elongation.
Structure of Stems:
Stems are composed of
- Epidermal tissue-Stems
are encased by a transparent epidermis which is usually about one cell
thick and often has trichomes. The trichomes serve various purposes;
some plants secrete juices to attract insects, which others have hoo-shaped
trichomes which often entangle the insects and keep them from feeding
while they struggle to free themselves.
- Vascular tissue-As I
mentioned before, there is no procambium above the youngest leaf
primordium, and removing a plant's leaf primordia stops vascular
differentiation. So, substances coming from young leaves control the
differentiation of procambium and vascular tissues in stems. Xylem and
phloem in stems occurs in vascular bundles. Phloem forms before the
xylem and it differentiates on teh outside of the bundle. Xylem forms
on the inside of the bundle. Vascular bundles are often enclosed by
sclerenchyma fibers that differentiate after the internode has
finished elongating. But, there is a layer of cells between the xylem
and phloem, which remains meristematic. In woody plants and some
herbaceous dicots, this layer of cells later becomes part of the
vascular cambium, and produces secondary growth. The absence of a
vascular cambium in monocots, is an important feature. Vascular
bundles in stems are arranged differently in different plants.
- Ground tissue-Between
the epidermis and the ring of vascular tissue in dicots is the cortex.
Most cells of the cortex are parenchyma. Cortical cells are
photosynthetic in plants and often store starch. In dicots, the ground
tissue with the parenchyma cells in the center of the stem is
specialized for storage and is called pith. Pith cells are often
lignified, arranged loosely, and the pith may contain secretory
structures such as laticifers. Because monocots have vascular bundles
throughout their ground tissue, their stems do not have a discernible
pith; the parenchyma cells in monocot stems are referred to simply as
Buds and Branching:
Early in leaf
development, a small island of meristematic cells forms in the axil where
a leaf attaches to the stem. These cells quickly form an axillary bud,
that undergoes a dormant period controlled by hormones made by teh shoot
apex. In most plants, axillary buds near the shoot apex stay dormant,
while those farther away from the tip start to grow. This dominating
effect of the shoot apex on growth of axillary buds is called apical
dominance and influences the symmetry of the shoot. Plants with weak
apical dominance have a shrub shape. Axillary buds are important because
they are a shoot's insurance policy: they are inactive cells that can form
a branch or flower.
Plants often modify their
stems for special functions. These modifications can have unusual and even
bizarre shapes, and can grow above as well as below the ground.
Modified stems that grow
- Stolons or
runners-are horizontally oriented stems that grow along the soil
surface. Their function is vegetative production.
- Tendrils and twining
shoots-coil around objects and help support the plant.
- Searcher shoots-are
stems with long internodes that move in circles through the air
seeking a support.
- Cladodes or cladophylls-are
flat, leaflike stems modified for photosynthesis.
- Thorns-are modified
stems that protect plants from grazing animals.
- Short and long
shoots-long shoots have long internodes, while short shoots are
rosettes and have short internodes.
- Succulent stems-of
plants have a low surface to volume ration and store large amounts of
water and are common in desert plants.
Modified stems that grow
- Bulbs-are rosette stems
surrounded by fleshy leaves that store nutrients. When the nutrients
are removed, the leaves collapse.
- Rhizomes-are underground
stems that grow near the soil surface. They typically have short
internodes and scale leaves, and produce roots along their lower
surface. They store food for renewing growth of the shoot after
periods of stress.
- Corms-are stubby,
vertically oriented stems that grow underground, have only a few thin
leaves, and store nutrients.
- Tubers-are swollen
regions of stems that store food for subsequent growth.