Download 5 VEGETATIVE PLANT MORPHOLOGY

VEGETATIVE PLANT MORPHOLOGY
Even the embryo of a new plant has distinctive parts, which contribute later to the overall growth and organization
of the plant. Let's start with the seed and trace what happens to its embryo. The typical embryo consists of these
parts: radicle (the future root) hypocotyl (the future lower part of the stem)
• cotyledon(s) (the first seed leaves) epicotyl
(the future growing tip and shoot)
A seed is said to germinate when the embryo starts to grow by pushing its roots through the seed coat. This is
accomplished by water uptake, allowing the cells in the embryo to expand and thus grow. Soon after the radicle
(root) finds its way into the soil, the hypocotyl elongates to push the rest of the embryo above the soil, often carrying
with it the remains of the seed coat.
As the cotyledons and epicotyl shed the now useless seed coat, the cotyledons have the chance to expand and start
functioning as leaves. Note that in many plants (for example, peas and beans), the cotyledons stored food to nourish
the embryo while still inside the seed, but now the stored food is used up and the cotyledons turn green in order to
photosynthesize. Note also that the number of cotyledons is an indicator of which of the two great subgroups of
flowering plants monocots or dicots--the seedling belongs to. Monocots (short for monocotyledons) have a single
coytledon; dicots (short for dicotyledons) have two.
Soon the epicotyl--the growing tip just above the cotyledons--begins its active growth. Growth here is a result of
both numerous cell divisions and uptake of water from the soil via the roots. We call this region the shoot, and from
this develops ALL the stems and leaves which the plant will make the rest of its life! Because the shoot grows in a
coordinated way to develop new stem and leaves at the same time, we can consider the shoot an integral unit. Under
the microscope, we'd find that the end of the shoot is a delicate growing point--the meristem with actively dividing
cells--and that below this point, new stem is being added downwards and new leaves around the sides. The young
preformed leaves (leaf primordia) arch over and around the growing point to protect it from the outside
environment.
As the epicotyl grows in earnest, the development sequence of the shoot is obvious. Each species has a different
plan for the spacing and distribution of parts in the shoot, resulting in a unique physiognomy or growth shape. All
flowering plants, however, share the following structures:
node: the point on the stem where each leaf is attached to the stem.
internode: the region of the stem between any two leaves. axillary
bud: the bud just above (in the axil of) the node.
terminal bud: the bud at the end of the shoot or stem. This bud is not obvious when the shoot is actively growing,
but is apparent when the plant is dormant, often in the winter.
The buds are miniature growing points or shoot tips, which may start their own shoots when they grow actively.
Thus, axillary buds result in branches, with their own set of buds, leaves, and stem. But not all buds grow, and their
growth cycle depends also on environmental factors, such as day length, temperature, and moisture. Many axillary
buds remain dormant their whole lives, while others become new side branches right away. Each plant is different in
this respect--which of its buds grow and which do not. This is determined by complex interactions of growth
hormones called auxins, which inhibit or promote growth of buds, and the concentration of these auxins is
determined by the genetic makeup of the plant! Since auxins are produced by the terminal bud of each shoot,
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pruning (removing the shoot tip) effectively removes the auxins, and allows the axillary buds directly below to start
growing.
Of course, no description of the shoot would be complete without talking about the leaves themselves: how they're
arranged, their overall structure, shape, color, texture, and size. All leaves consist in the main of a broad, flattened
blade, the part which absorbs carbon dioxide and photosynthesizes, producing sugars for the growth and
metabolism of the plant. Leaves may also have:
petiole. A stem or stalk-like portion at the base of the leaf.
stipules. A pair of (usually) tiny leaf-like appendages attached to the base of the leaf at its node.
Leaves may be arranged in the following patterns:
alternate. A single leaf at the node. opposite.
A pair of leaves at the node.
whorled. Three or more leaves at the node.
fascicled. A dense cluster of leaves on one side of the node.
basal. Leaves all clustered at the base of the plant.
We can also speak about how long leaves stay on the plant, and when they die and are shed, whether this happens all
at once or over a period of time. The two main terms to know are:
deciduous. Leaves which are all shed from the plant about the same time, usually in fall (to avoid the cold
winter), also sometimes in early summer (to avoid the drought of summer).
evergreen. Leaves which remain on the plant for some time, and are not all shed simultaneously. Note that
There are many more terms concerning leaves, but we won't attempt to cover them here.
Note that there are dozens of terms to describe leaf shape, apex (end), base, margin (edge), vein pattern, color,
presence and kinds of hairs, and more. Many of these relate to genetic influences as well as environmental
adaptations to the particular climate the plant grows in.
Let's turn now to the functions of our major plant parts: roots, stems, and leaves. Roots not only anchor the plant to
its chosen place in the soil, they are the main arteries for bringing in water and minerals. Lining the
roots just above their growing tip are hundreds of microscope root hairs, which are outgrowths of the surface
layer or epidermis of the root. These outgrowths have tremendous surface area because of their minuscule diameter
and tremendous number. These surfaces come in contact with soil particles, where charged mineral ions are
present, and these are absorbed through the root hairs along with water. These dissolved minerals then pass across
the root into the vascular tissue, where specialized cells carry the water upwards into all other parts of the plant.
Vascular tissue consists of two types: xylem, the core which consists mostly of long, tapered, empty but reinforced
cells that act like pipes to efficiently transport water upwards. These tiny tubes are called vessels. The other kind of
vascular tissue is phloem, which differs by having more delicate, living, tubular cells--the sieve tubes--which carry
food from one part of the plant to another.
As the water moves up the root, it soons enters the stem, where additional xylem allows further movement upwards.
"Ihe main function of the stem is to provide a framework for the leaves and to hold them rigidly in place. The stem
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is also the conduit for moving food, water, and minerals from one part of the plant to another, and may also store
food during dormancy (as does also the root). From the stem, strands of vascular tissue diverge outwards into each
leaf. In the leaf, these vascular pathways multiply in a complex pattern, which to the naked eye is the veins. In
monocot leaves, the main veins usually run parallel to one another, while in the dicots, they form a network. Each
leaf has its own special vein pattern, and once the eye has seen a certain pattern often enough, it is remembered as
special to that species of plant. Ultimately, the water carried in the xylem of the veins ends up being evaporated
(transpired) from the leaf surface through tiny holes in the leaf surface called stomates. Some water is used to add
water to rapidly expanding new cells in growth, and some water is needed during the process of photosynthesis, but
most of the water is lost through transpiration!
Let's now look at how the leaf is put together to ftnction as the manufacturing center of food and energy for the
plant. The thiness of leaves is to allow maximum surface area to sunlight for maximum efficiency during
photosynthesis. You'll note that many leaves have reduced this surface area on plants growing in hot or dry climates
where minimizing water loss is also important. The leaf is surrounded on all sides by a skin--the epidermis--which
protects it from drying out by producing a waxy covering--the cuticle. Despite this, the underside (sometimes also
the topside) is perforated by hundreds of tiny holes or stomates, which are essential to allowing carbon dioxide
(C02) to enter the leaves from the outside air. It's through these stomates that most water is lost. Guard cells around
each stomate may inflate or deflate to open or close the stomate itself. Normally, stomates are open during the day
and closed at night.
Inside the leaf, the upper half is covered with long, narrow, column-shaped cells--the palisade parenchyma-which
are full of tiny green bodies or chloroplasts, which contain the green pigment chlorophyll for photosynthesis. As
carbon dioxide diffuses into these cells, and they're struck by sunlight, photosynthesis takes place. Simply stated,
photosynthesis is a process which takes carbon dioxide from the atmosphere, combines it with water from the soil
to produce simple sugars (glucose) and release oxygen to the air. Actually this is a very complex set of processes,
which we don't have time to delve into. What's important is that this allows the plant to make organic substances
(components of living cells) from simple, common substances such as carbon dioxide and water. Only plants have
this remarkable ability, which is at the base of the food chain for all life on earth. To do this, the plant needs the
green chlorophyll to absorb light energy from the sun, and this is why leaves must be green to function. Plants are
also the major source of all atmospheric oxygen; without plants, we would not have breathable air!
What happens to the glucose sugar? It is usually transported via the phloem to wherever it's needed in the plant. If
the plant is actively growing, it is taken to the shoot tips. If the plant is flowering or making seeds, the sugars travel
there. If the plant is dormant (in a resting stage), the sugars get stored for future use. Some sugars are burned
immediately to produce energy for metabolism. We call this process respiration. Other sugars may be built into larger,
more complicated substances, which add to the structure or help build new cells. The main building blocks of cells
are fats and proteins, and it takes many steps to convert simple glucose into one of these essential substances. This is
where the other minerals absorbed with water by the root come into play. Nitrogen in the form of nitrate is needed to
make such essential substances as proteins and the DNA (genetic material) of the cell- Other minerals necessary for
complex building blocks, or for helping the actual manufacture of these substances, include iron, phosphorous,
potassium, sulfur, calcium, and magnesium among many others.
Any time the cell does work by making more of itself, it requires energy. As mentioned above, this energy comes
from the respiration or breaking apart of glucose. There are two kinds of respiration: anaerobic, respiration in the
absence of air (without oxygen) and aerobic, respiration with oxygen. Although plants can survive a short while
without oxygen through anaerobic respiration, this process if inefficient, and results in the build-up of poisonous
substances, such as lactic acid and alcohol. (This is how such primitive organsims as yeasts produce alcohol for the
production of wine and beer). On the other hand, aerobic respiration is highly efficient, providing much energy,
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and breaking the glucose down completely into the starting materials of carbon dioxide and water. Most animals as
well as plants utilize this process for their energy; this is why we must breathe oxygen.
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