Growth & Development
Trees, turfgrasses, shrubs, flowers and vines make up the majority of plant types on your grounds. Common aspects of these plants are that they are all highly evolved and that they have similar characteristics of growth and development. Most notably, they are all vascular seed plants, they all conduct photosynthesis, and they all derive nutrients and water from their surroundings. All seed plants have flowers that are made up of reproductive structures (stamens and pistils) and non-reproductive structures (petals and sepals). Flowers bear the seed that contains the embryo that can develop into a new plant.
Three vegetative organs—the root, stem and leaf—are easily recognized. These organs are responsible for extracting water and nutrients from the soil and transporting them through a plant, providing plant structure and anchorage, and conducting photosynthesis.
Fundamental to an understanding of plant anatomy and function is an understanding of the interrelationship and interdependence of plant parts. Seed plants start as rudimentary embryos and develop roots, leaves and flowers initially in that order. Each plant part develops from undifferentiated cells and becomes a more distinct organ. Consequently, the separation of the plant into discrete, defined organs is only approximate. You won’t find a line dividing leaves from stems or roots from stems, but you will find a gradual change in cell structure and organization where two organs join.
Plants are made up of many individual basic units called cells. Cells are the smallest biological unit having characteristics of life. They have the ability to extract substances from their environment and continually adapt to their environment; and they have a unique chemical composition, structure, metabolism, growth, reproduction and organization.
Although plant cells differ depending on their function, you can visualize a generalized, undifferentiated, unspecialized plant cell (see Figure 1). On the outside of the cell is the primary cell wall, which encloses the cytoplasm in which you find deposits called inclusions (oil and fat droplets, spherosomes, protein bodies, starch grains and crystals) and metabolic bodies called organelles (nucleus, plastids, mitochondria, Golgi bodies and ribosomes). The cytoplasm is a semifluid substance that is pressed against the cell wall by a central vacuole. The vacuole is filled with a watery solution of dissolved inorganic and organic molecules, as well as some insoluble material. The plasma membrane separates the cytoplasm from the primary cell wall. Another cell membrane, the tonoplast, envelopes the vacuole and separates it from the cytoplasm. The last generalized plant-cell feature is the plasmodesmata, which are strands that extend through cell walls and connect the cytoplasm of adjoining cells.
The organelles in the cytoplasm have distinct functions. Chloroplasts are the bodies in which photosynthesis takes place. The nucleus contains all the genetic material needed for cell reproduction. Respiration takes place in the mitochondria.
A seed plant is made up of many individual cells, which are cemented together. You will find several different types of cells in plants, and when you group them together as a distinct functional and structural unit, you call them tissue. The major tissues of vascular plants are the dermal, vascular and fundamental (ground) tissues. The dermal tissues (epidermis and periderm) make up the protective structures of a plant. Vascular tissues are a plant’s conducting tissues. Vascular tissues include the xylem (water conducting) and phloem (food conducting) tissues. Fundamental or ground tissues make up the basic substance of plants and include three distinct types: parenchyma, col-lenchyma and sclerenchyma (see Figure 2). Parenchyma cells are living and capable of growth and division. They are responsible for photosynthesis, wound healing, storage and new growth. Collenchyma cells also are living and capable of producing new growth but consist of thick-walled cells and mainly serve as supporting tissue in leaf veins and stems. Sclerenchyma are thick-walled cells that may or may not be living at maturity. Sclerenchyma function in a structural role and are made up of fibers (slender elongated cells) and sclereids (cells that vary in form from branched to elongated to relatively concentric).
GROWTH AND GROWING POINTS
Growth is defined as the increase in size by cell division or cell enlargement. This increase in size results from the plant taking up air, water and nutrients and incorporating them into its structure. Light energy is the driving force behind growth.
As mentioned above, parenchyma and collenchyma cells are living and capable of dividing to produce new tissue. Tissue that is actively dividing to produce new tissue is called a meristem. Meristems are actually the growing points on a plant (see Figure 3). Buds on branches of trees or the apical meristem on the top of turfgrass crowns are examples of growing points. Growing points give rise to new leaves, flowers, branches and roots.
Keeping these growing points alive and active is the key to growing plants.
Roots are responsible for extracting moisture and nutrients from the soil and anchoring the plant in the soil (see Figure 4). Roots originate from the growing embryo and are the first structure to emerge from the seed during germination. This first root developing from the embryo is called the primary root, or taproot, and all other roots are called adventitious roots—roots arising from any organ other than the embryo. Roots consist of a growing point at the tip, which is protected by a root cap; the epidermis on the outside of the root; the cortex, which makes up the flesh of the root; root hairs, which absorb moisture and minerals; and the vascular cylinder, which includes the xylem, phloem, cambium and pericycle. Roots add new growth via the growing points at their tips.
Roots of most woody species primarily are concentrated in the top 3 feet of soil and can spread two or three times the diameter of the spread of branches, also referred to as the drip line of a tree. But surprisingly, the most active portion of a woody-plant root system is about 6 inches deep and only reaches out to the drip line of the tree. By contrast, the most active zone of grass-root water absorption is only about 1 inch deep.
Root systems are broadly divided into either fibrous-root or tap-root systems. Fibrous roots are multibranched and brushlike and are typical of mature grasses and certain trees and shrubs. Taproot systems consist of one main root that grows deep in the soil. Under dry conditions, plants with tap roots are able to extract water from deep in the soil. However, tap-rooted plants are more difficult to transplant because they lack the extensive branching close to the soil surface that fibrous-root systems have. This branching tends to hold the root ball together and produce more small roots for extracting moisture from the soil.
Stems are the connecting structures between the leaves and roots. It is through the stem that water and nutrients are transported to the leaves from the roots, and carbohydrates are conducted from the leaves throughout the plant. Stems consist of nodes separated by internodes with buds developing at the nodes. Buds can give rise to leaves, flowers and lateral stems. A shoot is a general term that refers to the stem and its associated leaves.
In the case of grasses, three types of stems exist: the crown, flowering culm and lateral stems (see Figure 5). The crown is the primary stem of grasses; it is from the crown that leaves, flowering culms and other stems originate (see Figure 6). The crown consists of a series of nodes with unelon-gated internodes. The crown is the key to survival of turfgrasses. Leaves grow from the crown and envelope the growing point. You can cut the leaves off turfgrass plants or injure them, but the plants will live as long as the crowns stay alive.
At certain times of the year, turfgrasses produce a flowering culm. A culm originates from the top of the crown, consists of nodes and internodes, and terminates with a flower or florets.
Alternately appearing on either side of the crown is a series of axillary buds that give rise to lateral stems, such as tillers, rhizomes and stolons. Lateral stems are elongated stems with nodes and elongated internodes. When the lateral stem grows up within the leaf that lies under the node, it is called a tiller. All grasses produce tillers that generally do not spread far from the mother plant. Some grasses vegetatively spread by tillering alone, and these are called bunch grasses. Perennial ryegrass is an example of a bunch grass. When a lateral stem grows horizontally through the leaf that lies below on the crown, it is called a rhizome or stolon. Rhizomes (see Figure 7) grow horizontally under the ground, may branch or root at nodes and produce a new plant at their tip(s). Stolons are similar to rhizomes but grow on top of the soil. Examples of rhizomatous grasses are Kentucky bluegrass and creeping red fescue. Rough bluegrass and bentgrass are stoloniferous grasses. Some grasses, such as zoysiagrass and bermudagrass, spread by rhizomes, stolons and tillers. It’s important to reinforce the fact that both rhizomatous and stoloniferous grasses also produce tillers, but bunch grasses vegetatively spread only by tillering.
Because the leaf is the chief site of photosynthesis, it is critical that you understand its structure before going on to learn about photosynthesis. The major tissues of a leaf are epidermis, mesophyll and vascular bundles (see Figure 8). The epidermis is like the skin on a leaf. It is a single layer of cells and is covered with a cuticle—a wax-like layer that seals the leaf from movement of gases and water into and out of the leaf. One peculiar structure of the epidermis is the stomate—the pore through which water and gases, such as carbon dioxide and oxygen, flow into and out of the leaf. The unique geometry of guard cells on either side of the stomate allows them to open and close the stomate in response to fluctuations in their turgor pressure. Stomates generally are open during the day and close at night or when moisture stress occurs. When stomates are open, water can leave the plant in the form of vapor, thus cooling the plant. This water vapor loss is called transpiration.
In grasses, leaves are composed of two parts—the blade and the sheath—that are connected by the collar region. The collar region is especially important in identifying turfgrasses. Structures, such as ligules (a membranous or hairy tissue at the base of the blade), auricles (appendages at the margins of the leaf in the collar region) and collars (the back sides of the leaves in the collar region) may vary among the turfgrasses.
Flowers are the reproductive organs of plants. They consist of male and/or female parts (see Figure 10). Some plants produce separate male and female flowers, and other plants produce flowers with both male and female parts. Some entire plants are either male or female. This can be important for ornamental species. For example, many fruitless trees are merely male plants.
The male flower parts consist of the stamen composed of an anther and a filament. The anther holds the pollen grains, which give rise to sperm. The female portion of the flower is the pistil. It is commonly flask shaped, with a swollen basal portion called the ovary connected to a stalk-like style topped off with a swollen portion at the tip called the stigma. In most cases, pollen must be transferred from the anther to the stigma for the pollen tube to germinate and transfer the sperm to the ovary for fertilization to occur and embryo and seed to develop. One exception occurs in Kentucky bluegrass, where an embryo is developed in the flower from cell division of the vegetative tissue in the ovule. This process is called apomixis and is the reason why most seeds of Kentucky bluegrasses are genetically identical, like a clone.
We mention that light energy is the driving force behind growth, and plants accomplish this through the process of photosynthesis. Photosynthesis involves activation of chlorophyll (the green pigment in leaves) molecules by light combined with the assimilation of carbon from the air as carbon dioxide to form glucose sugar (a carbohydrate). Carbohydrates are the actual food plants rely on for growth.
Photosynthesis is actually a combination of two separate but related processes—a light reaction and a dark reaction (see Figure 11). In the light reaction, the chlorophyll molecule changes to an excited state when exposed to blue or red light. During this light phase of the reaction, water is split into oxygen (which the plant expels), electrons (which are transferred to produce photochemical energy in the bonds of adenosine triphosphate [ATP]) and hydrogen ions (which are used to create another high-energy molecule called the reduced form or nicotinomide adensine diphosphate [NADPH]). The high-energy bonds in ATP and NADPH are then used to drive the second reaction called the dark reaction.
In the dark reaction, carbon dioxide from the atmosphere enters the plant through openings— stomates—in the leaves. In most plants, carbon dioxide immediately reacts with a five-carbon molecule called ribulose-bisphosphate (RuBP) to form a three-carbon compound called 3-phosphoglycerate (3-PGA) in a reaction catalyzed by the enzyme RuBP carboxylase. Because the first molecule formed after carbon dioxide is fixed is a three-carbon molecule, we call plants with this carbon-fixation system C3 plants.
One problem encountered by C3 plants is that the same enzyme that catalyzes the fixation of carbon dioxide from the air also can fix oxygen from the air and lead to photorespiration (see Figure 12) and a net loss of carbon dioxide.
Some plants (primarily monocots) have a different way of fixing carbon dioxide from the air. They get around the problems of photorespiration through their unique plant anatomy and additional enzyme systems for fixing carbon dioxide (see Figure 13). In these species, a three-carbon molecule called phosphoenol pyruvate (PEP) reacts with carbon dioxide from the air to form a four-carbon molecule called oxaloacetic acid (OAA)—thus the name C4 plants. The enzyme that catalyzes the carbon-dioxide fixation is called PEP carboxylase, and it only fixes carbon dioxide and not oxygen. Eventually OAA is converted to malic acid, which moves out of the cytoplasm—or mesophyll cells—and into the chloroplasts of tightly packed bundle-sheath cells. There the malate splits off a carbon-dioxide molecule, which is picked up by RuBP carboxylase, the same enzyme that fixes carbon dioxide from the air in C3 plants. Because the RuBP carboxylase is compartmentalized in bundle-sheath cells and not exposed to oxygen in the air, no photorespiration takes place in C4 plants.
ATP and NADPH from the light reaction are then used in the next two reactions. Eventually these phosphorylated sugars transform into glucose, fructose and other simple sugars.
Because photorespiration is more of a problem in warmer temperatures, C4 plants tend to be better adapted to warmer climates, where they evolved the C4 process to avoid photorespiration. Further, species that possess the C4 mechanism and anatomy mostly (though not entirely) are monocots, especially grasses and sedges. Therefore, turfgrass scientists have found it useful to distinguish C4 turfgrasses from C3 turfgrasses with the terms warm-season species (C4 plants) and cool-season species (C3 plants). Virtually all trees and shrubs grown in North American landscapes are C3 plants, so the distinction is not as useful in the ornamental industry, where these terms are not used in this regard.
Once simple sugars are produced through photosynthesis, they can be converted to more complex sugars, such as sucrose, starch, fructosans or structural carbohydrates such as cellulose. In the case of cool-season grasses, the primary storage carbohydrate is fructosan—a long chain of fructose with a terminal glucose. Warm-season grasses store sugars as starch.
The sugars that the plant stores then are broken down by the plant and used for growth. This process is called respiration. Both plants and animals carry on respiration and use the same biochemical pathways. Oxygen is required for respiration, and carbon dioxide is expelled (the opposite).
WARM-SEASON VS. COOL-SEASON GRASSES