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Plant Complexity          Plant Life Span          Method of Growth           Pollination          Levels of Organization

Knowledge of plant structure and function is important when troubleshooting problems or researching growing techniques, such as cloning or propagation. What's a node? What's a meristem? How is a plant structured and how does that structure function? How does this function affect the ways in which we approach growing plants?The more we know about the materials and organisms we're dealing with the more able and successful we will be at it. Knowledge = Power is not a cliché for nothing.

The following section is designed to make the grower more comfortable and informed regarding plant structures. While there are obviously myriad forms of plants and not every one falls neatly into a specific category a generalization will open many doors in our pursuit to maximize plant production. In order not to get bogged down with the specifics of relatively nominal plant types our emphasis hear will lie with flowering plants of production.

Plant Complexity

Non-vascular
Termed bryophytes, non-vascular plants do not have a true vascular system and are unable to pull water and nutrients up from the ground at any significant distance. Lacking this specialized system distinguishes bryophytes from ferns and flowering plants. It is for this reason that they are considered to be rather primitive plants. These plants do not flower and therefore, never produce seeds. They reproduce by way of spores. Examples of bryophytes includes the mosses, liverworts, and hornworts amongst others. There are about 25,000 known species of bryophytes in the world today. Although these plants are small in size, they are one of the largest groups of land plants and can be found almost everywhere in the world. There are more species of bryophytes than the number of conifer and fern species combined. Bryophytes are regarded as transitional between aquatic plants like algae and higher land plants like trees. They are extremely dependent upon water for their survival and reproduction and are therefore typically found in moist areas like creeks and forests. Some bryophytes, however, are able to survive in areas with little or no rainfall.

Seedless Vascular
Ferns are the most familiar plant in this category. They have the vascular system, but do not reproduce via seeds, but spores. Most ferns are leafy plants that grow in moist areas under forest canopy. They are "vascular plants" with well-developed internal vein structures that promote the flow of water and nutrients. Unlike the other vascular plants, the flowering plants and conifers, where the adult plant grows immediately from the seed, ferns reproduce from spores and an intermediate plant stage called a gametophyte.

Seed bearing, Vascular
Gymnosperms

Gymnosperms : non-flowering plants
: Literally meaning "naked seeds", the gymnosperms were the first seed plants and bear their ovules and seeds exposed on the plant surface. This is in marked contrast to the more recent group of seed plants, the flowering plants, which have their ovules sealed within the flower and their seeds contained within a fruit. An example of a gymnosperm would be a conifer, such as pines.

Angiosperms

Angiosperms : a flowering plant
: Angiosperms are flowering plants. They have stems, roots, and leaves. Unlike gymnosperms such as conifers and cycads, angiosperm's seeds are found in a flower. Angiosperm eggs are fertilized and develop into a seed in an ovary that is usually in a flower. The flowers of angiosprems have male or female reproductive organs.

Flowering plants are broken up into two general categories: Monocots (short for monocotyledon, or one cotyledon) and Dicots (or two cotyledons

Cotyledon : The "seed leaves" or "starter leaves" produced by a seed plant embryo. They serve to absorb nutrients packaged in the seed, until the seedling is able to produce its first true leaves and begin photosynthesis.
). General discrepancies are as follows:

          

Plant Life Span

The so-called higher plants (or seed plants) are either herbaceous or woody. Woody plants (shrubs and trees) have significant woody parts above the ground that persist year after year. Herbaceous plants do not have such strong, woody parts that persist year after year, and are rarely as large as the woody plants. Plants can complete their life cycle in from one to several growing seasons.

  • Annuals are plants that germinate, grow, reproduce and die in one growing season.
  • Biennials grow vegetatively in their first growing season and reproduce and die in a second growing season.
  • Perennials grow for several seasons. They may reproduce year after year, once reaching sexual maturity (polycarpic), or in the case of some plants, grow vegetatively for a number of seasons, then reproduce once (monocarpic) and die. Perennials can be herbaceous or woody. Some woody perennials, such as the Bristle cone pine, live for thousands of years.

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Method of Growth

Plant growth is a phenomenon different from animal growth. Animals exhibit a growth pattern called determinate growth. After fertilization, the zygote

Zygote : A cell formed by the union of male and female gametes.
cells rapidly divide into undifferentiated cells. However, after a certain critical stage, the cells differentiate and form tissues. From this point onward, their developmental fate is sealed, however there are exceptions to this (i.e. stem cells in bone marrow). Most animals have a pre-programmed body plan (i.e. barring mutation or accident, most humans have 10 fingers and toes, two eyes, a heart with four chambers, etc.) and quit growing after a certain age.

Plants, however, exhibit a growth pattern called indeterminate growth. The plant retains areas where rapidly dividing, undifferentiated cells remain all through the life of the plant called meristems. Meristematic tissue continues to rapidly divide producing undifferentiated cells which may eventually differentiate to form the tissue and cell types discussed above. Plants do not have a pre-programmed body plan. There are constants like leaf shape and branching patterns (opposite, alternate, etc.) but you can never predict where a new branch will come about on a tree. Plants also continue to grow throughout their life.

Meristems

The pattern of plant growth depends upon the location of meristems, which regulate indeterminate growth in plants:

     
Lateral meristem
   Lateral meristem

Apical meristems

  • located at the tips of roots and shoots
  • supply cells for the plant to increase in length (grow up for shoots and down for roots)
    • growth in this direction is known as primary growth

Lateral meristems

  • located near the periphery of the plant, usually in the vascular tissue.
  • supply cells for the plant to increase in girth
    • growth in this direction is known as secondary growth
  • Annual rings are a way to visualize the work of lateral meristems

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Pollination

Much is made of plants contribution to the Earth- CO2 regeneration into O2, the fundamental element of the food chain, etc.-, but plants are to an extent dependent on animals and environmental factors for survival. Since plants cannot get up and walk around, fertilization of gametes has to be accomplished via a middleman of some sort. Organisms, such as honeybees or butterflies, and wind play a vital role in the phenomenon of pollination. The male anther produces pollen, which is
transferred to the female stigma where it can fertilize the awaiting egg. The relative position of the stigma determines if the plant is a cross-pollinated type or self-pollinating variety.

Plants such as carrots, corn, cabbage, and cucumbers- amongst others- are very sensitive to cross-pollination. In order to ensure they are going to harvest the product that is planted and ensure there are no hybrids in the crops that they grow, farmers and seed producers require certain distances between potentially

Hybrid : An offspring created by combining two plants of different breeds, variety, or genetic makeup.
cross-pollinating plants. For example, a corn seed producer requires a distance of 1-mile between different varieties of corn so as not to contaminate seed stocks. This phenomenon of pollination sensitivity is an issue now in Canada. The chemical company Monsanato has a lawsuit against local farmers there regarding the cross pollination of canola crops. (See article here.) Monsanato developed a Round-Up ready strain of canola seed and they are accusing local farmers of using their patented seeds. The problem is that most of these farmers are unaware of the patent by Monsanato and, more importantly, do not have control on the ability of plants to cross-pollinate. Courts are deciding, and in many cases have decided, that this constitutes a breach of the patent and are requiring local farmers to compensate Monsanato. It remains an ongoing issue in the realm of GMO's and Big Ag.

Plants such as beans, peas, lettuce, and others are largely self-pollinating. They can be grown relatively close together without fear of cross-pollination. Interestingly this phenomenon can change depending on zone of growth. For example, beans grown in the tropics have an increased sensitivity to cross-pollination than in temperate regions. All of this must be accounted for by the farmer or seed producer.

Plants such as tomatoes and peppers are largely variable. Some types are more sensitive to cross-pollination than others depending on the placement of the stamen

Stamen : The male reproductive part of a flower, the tip of which produces pollen and is called the anther.
. If the stamen is enclosed in the flower it is much easier for the plant to accomplish self-pollination compared to a plant with a protruding stamen that might need some help from others. For example, a seed producer would have specific tomato varieties 100 feet apart (compared with 1-mile for corn!) and unique pepper plants some 500 feet apart. Outside the influence of wind and insects it is a good idea to give your plants a little shake from time to time to ensure self-pollination.

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Levels of Organization in the Plant Body

The Organism         The Organs            Tissues              Cells

(I) The Organism (PLANTOVERALL PIC)

A. The Shoot System

  • Above ground (usually)
  • Elevates the plant above the soil
  • Many functions including:
    • photosynthesis
    • reproduction & dispersal
    • food and water conduction

B. The Root System

  • Underground (usually)
  • Anchor the plant in the soil
  • Absorb water and nutrients
  • Conduct water and nutrients
  • Food Storage




(II) The Organs
Because our interests lie in plants of production, we will be speaking of angiosperms primarily here. Angiosperms are by far the most diverse group of plants known with over 275,000 named species and thought to be at least as many more unknown to science.

A. The Root

The root is the lifeline to plant growth and development. While plants can utilize material via the leaf surface via foliar sprays, without roots plants would not be able to import materials sufficient for subsequent growth. Roots also serve as an anchor to a developing plant and a storage mechanism for excess photosynthate

Photsythates : Food products (sugars and starches) created through photosynthesis.
.


(a) The Root Tip

  • Meristem: A region of rapid mitosis, or cell division, which produces the new cells for root growth. Also known as the zone of cell division.
  • Root Cap: A sheath of cells that protects the meristem from abrasion and damage as the root tip grows through the soil or medium.


(b) The Zone of Elongation

  • Region of the root (or shoot) that undergoes elongation in the direction of the developing of the root in response to gravity via gravitropism.

(c) The Zone of Differentiation

  • Epidermis : A single layer of flattened cells at the surface of the root. When first formed, epidermal cells have extensions- root hairs-, which greatly increase the surface area available for water and mineral uptake.
  • Cortex : A band of parenchyma cells that develops beneath the epidermis used for food storage.
  • Endodermis : The inner surface of the cortex that separates it from the stele.
  • Stele : Site of primary growth. Contains the vascular tissue:
    • Pericycle : The outer boundary of the stele. Source of secondary roots.
    • Xylem : Arranged in bundles. Focused on water transport.
    • Phloem : Alternates with xylem. Focused on mineral transport.
  • Cambium : Found in older parts of the root in certain types of plants, another meristem forms between the xylem and phloem. Mitosis in the cambium produces secondary growth.

Root Water Uptake

Water is utilized via osmosis. By ensuring a higher mineral content inside the root, plants utilize the phenomenon of water traveling from a lower to a higher solute concentration, or osmosis, essentially wicking water into itself for transpiration. This explains the response of a plant to too high of a mineral content in the root zone via water stress, or burning.

Water enters the root via root hairs. These extensions of epidermal cells adhere tightly to soil or media particles with a film of moisture. Once within the epidermis, water passes through the cortex, traveling between and within the cells. Water travels into the cytoplasm of root cells, or the symplast, via plasmodesmata. Water travels around the root cells in the non-living portions of the root, called the apoplast, freely.

However, the endodermis is protected by a layer called the casparian strip. The casparian strip is made up of suberin and is impervious to water. In order to enter the stele or vascular area, water must pass through the endodermal cells by plasmodesmata into the cells of the stele.

Once within the stele, water is free again to move between cells as well as through them. In young roots, water enters directly into the xylem. Xylem are nonliving conduits so are part of the apoplast. In older roots, it may have to pass first through a band of phloem and cambium. Once in the xylem, water with the minerals that have been deposited in it move up in the vessels of the plant. At any level, the water can leave the xylem and pass laterally to supply the needs of other tissues. At the leaves, the xylem passes into the petiole and then into the veins of the leaf where it is used for metabolic functions or is lost via transpiration.

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Root Mineral Uptake
One might have expected that minerals would enter the root dissolved in water, but in fact they enter separately. Minerals enter roots via a different mechanism, even entering the root when no water is being absorbed. Minerals can enter the root against their concentration gradient by way of active transport then into the symplast of epidermal cells and move toward and into the stele through the plasmodesmata connecting the cells. They enter the water in the xylem from the cells of the pericycle (as well as of parenchyma cells surrounding the xylem) through specialized transmembrane channels.

Plants absorb minerals in an inorganic form. Ex. Nitrogen as NO3- or NH4+, Potassium as K+, calcium as Ca2+. When you hear the virtues of "organic" fertilizer, remember that such materials meet no nutritional need of the plant until their constituents have been broken down into inorganic forms. A plant doesn't eat guano or kelp, but their inorganic constituents. Organic matter does play an important role in making good soil texture and microbial conditions, but only to the extent that it can yield inorganic ions that can meet the nutritional needs of the plant. This explains why synthetic fertilizers can be utilized for plant growth in recirculating hydroponic scenarios, but are counterproductive for soil growth. The salt-based fertilizers lock out soil biological processes, whereas in a recirculating hydroponic scenario those respective biological processes are not needed to provide the inorganic forms of fertilizers necessary for plant utilization. They are provided directly by synthetic fertilizers. In short, synthetic fertilizers in soil treat the plants, not the soil.

Root Gas Exchange

Older parts of roots are sheathed in layers of a waxy, waterproof substance called suberin, which primarily acts to prevent water loss. Actual gas exchange in roots occurs in a relatively small permeable region consisting of lenticels. These permit the exchange of oxygen INTO the root and carbon dioxide OUT of the root.

B. The Stem

A stem is the above ground axis of a vascular plant providing support and a pathway for fluid transport throughout. The surface cells differentiate and mature into a protective epidermal layer. A few cells differentiate as collenchyma, providing support to the young stem. The remainder of the stem consists of parenchyma cells. Lenticels are also found on most stems providing gas exchange.


C.The Leaf
Although chloroplasts are found in the cells of young stems and some fruits, leaves are the real photosynthetic factories in plants. They allow for food production, gas exchange, and even food implementation via foliar sprays. A leaf is connected to the stem and supported by the petiole.
(a) Upper epidermis: This is a single layer of cells containing few or no chloroplasts. The cells are quite transparent and permit most of the light that strikes them to pass through to the underlying cells. The upper surface is covered with a waxy, waterproof cuticle, which serves to reduce water loss from the leaf. This illustrates the logic behind a leaf-wash. Cleaning the leaf effectively allows maximum light to reach the chlorophyll and ensures that there are no particulates that can block gas exchange.

(b) Palisade layer: This consists of one or more layers of cylindrical cells oriented with their long axis perpendicular to the plane of the leaf. The cells are filled with chloroplasts (usually several dozen of them) and carry on most of the photosynthesis in the leaf.

(c) Spongy layer: Lying beneath the palisade layer, its cells are irregular in shape and loosely packed. Although they contain a few chloroplasts, their main function seems to be the temporary storage of sugars and amino acids synthesized in the palisade layer. They also aid in the exchange of gases between the leaf and the environment. During the day, these cells give off oxygen and water vapor to the air spaces that surround them. They also pick up carbon dioxide from the air spaces. The air spaces are interconnected and eventually open to the outside through pores called stomata.

(d) Lower epidermis: Typically most of the stomata (thousands per square centimeter) are located in the lower epidermis. Although most of the cells of the lower epidermis resemble those of the upper epidermis, each stoma is flanked by two crescent cells called guard cells. These differ from the other cells of the lower epidermis not only in their shape but also in having chloroplasts.

Leaf Gas Exchange

In order to carry on photosynthesis, green plants need a supply of carbon dioxide and a means of disposing of oxygen. In order to carry on cellular respiration, plant cells need oxygen and a means of disposing of carbon dioxide (just as animal cells do).

The exchange of oxygen and carbon dioxide in the leaf (as well as the loss of water vapor in transpiration) occurs through pores called stomata (singular = stoma). The immediate cause is a change in the turgor of the guard cells. The plant increases the amount of Potassium (K+) in the guard cell, effectively utilizing osmosis to regulate turgor. The inner wall of each guard cell is thick and elastic. When turgor develops within the two guard cells flanking each stoma, the thin outer walls bulge out and force the inner walls into a crescent shape. This opens the stoma. When the guard cells lose turgor (or the plant decreases the relative amount of K+), the elastic inner walls regain their original shape and the stoma closes.

Leaf Photosynthesis

While some photosynthesis occurs in stems, most occurs in the leaf. Plants have evolved to use certain aspects of the Sun's wavelength energy to drive photosynthesis. Plant calibrated grow lamps use the knowledge of this phenomenon to replicate the correct photosynthetic spectrums in the technology of their lamps, as seen in the corresponding chart.

D. The Flower and Fruit
Flowers have many different shapes and sizes, and there are many variations in color, number of flower parts, and corresponding arrangements of these parts. Flowers are the reproductive parts of plants that are responsible for the production of gametes. After fusion of the male and female gametes a zygote is produced which develops into an embryo within the seed, which is its ultimate goal. This seed gives rise to a new flowering plant of the same kind. The purpose of this website is not to provide exquisite detail regarding plant structure, but a general ballpark so that the grower can obtain general knowledge in order to maximize plant production. Generally, plants consist of four whorls:

(a) Calyx: The calyx is the outermost whorl of a flower. It consists of sepals, which are green. The sepals may be free from each other in the same flowers of some plants or fused to form a cup in flowers of other plants. The calyx encloses and protects the inner whorls in the bud stage. Since the sepals contain chlorophyll, they can also synthesize food.

(b) Corolla: The corolla is found on the inside of the calyx and is the most conspicuous part in the flower because it is usually brightly colored. This whorl is made up of petals that are much larger than sepals. The petals may be separate from each other or become partly/ completely fused. The brightly colored corolla attracts agents of pollination such as insects and birds and also encloses and protects the stamens and pistil.

(c) Stamens: The stamens form the third whorl on the inside of the corolla. Each stamen is made up of a slender flexible filament that supports the anther at its end. The anther is the male reproductive organ in a flower. It produces pollen grains that contain the male reproductive cells. The stamen bears and supports the anther in the most suitable position for pollen transfer to take place.

(d) Pistils: The pistil is the female reproductive organ (sometimes called a carpel), and the fourth and the innermost whorl of the flower. It consists of a basal, swollen portion, the ovary, from which a long and slender style arises. The stigma is found at the tip of the style. The ovary is a hollow cavity that contains the ovules. Each ovule contains an egg cell. The elongated style bears the stigma in the most suitable position for receiving pollen during pollination. The sticky stigma at the tip of the style can receive or trap the pollen grains for fertilization.

Fruit Production
Flowering is a prerequisite for fruit development. However, flowers do not always produce fruit. Generally, fruit production can be divided into four stages- Flower bud formation, pollination, fruit set, and fruit development. The grand design of fruit production is to proliferate its seed. The plant does not grow the fruit because it tastes good, but because it tastes good to others, i.e. animals. Fruit production is simply a more advanced development of flower production. Flowers attract insects, etc. to attract pollinators, whereas fruit producing plants attract insects, etc. for pollination and then animals that will consume their progeny and proliferate it over large areas via digestion and excretion. The overall dynamic involving plant reproduction and where 100% of their energy is focused is for others to help them proliferate their progeny.
As a convenient segway, this is the essence of hydroponics. By taking the barriers away from plant growth the plant can concentrate on what it really wants to do- produce fruit and reproduce. In other words, a plant doesn't want to grow roots, it has too. By idealizing the root zone, more effective energy can be placed upwards instead of downwards.

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(III.) Tissue Organization in Angiosperms

A. Dermal Tissue

  • Generally a single layer of cells
  • The "skin" of the plant
  • Primarily parenchyma cells
  • Main role is protection of the plant

B. Ground Tissue

  • Makes up the bulk of the plant
  • Mostly parenchyma, but collenchyma and schlerenchyma cells are found
  • Diverse functions including photosynthesis, storage, and support

C. Vascular Tissue

  • Involved in the transport of water, ions, minerals, and food
  • Also has a secondary role in support
  • Composed of xylem, phloem, parenchyma, schlerenchyma

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(IV) Cell Types in the Plant Body

A. Parenchyma Cells

  • Least specialized plant cells
  • Thin and somewhat flexible cell walls
  • Living at maturity
  • Carry on most of the plant's metabolic functions
  • Generally have a large central vacuole
  • Most parenchyma cells have the ability to differentiate into other cell types under special conditions
    • During repair and replacement of organs after injury

B. Collenchyma Cells

  • Thicker primary cells walls (usually with uneven thickness)
  • Living at maturity
  • Role in support of herbaceous plants
    • Example - the "strings" of celery
C. Schlerenchyma Cells
  • Thick secondary cell walls
  • Dead at functional maturity
  • Cannot increase in length - occur in parts of the plant which have quit growing in length
  • Two types - fibers and schlerids
    • Fibers - long, slender cells with a more or less regular secondary cell wall
      • Example - hemp fibers for making rope
    • Schlerids - shorter cells with an irregular shape
      • Example - stone cells in hard nut and seed shells

D. Xylem

  • Thick secondary cell walls, often deposited unevenly in a coil-like pattern so that they may stretch
  • Dead at functional maturity.
  • Involved in conduct of water and ions in the plant
  • Two types - tracheids and vessels
    • Tracheids - long, slender cells connected to each other by pits. Found in all vascular plants
    • Vessels - shorter, larger diameter cells with completely perforated cell wall ends. Found only in Angiosperms

E. Phloem

  • Involved in transport of sucrose, other organic compounds, and some ions
  • Living at functional maturity
    • Protoplast may lack organelles and nucleus, though
  • Endwalls connect to each other via sieve-plates
  • Two types of cells in the phloem - sieve-tube members and companion cells
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