Practical botany – how plants work, the key functions of roots, stems, leaves, flowers and fruits – is the underlying knowledge that informs us regarding the “culture” of plants (appropriately, horticulture). Basic, yes. But a thorough understanding of the basics can be a key to your success in the green industry, whether you’re just starting out or you’re the CEO.
“A tree’s leaves
may be ever so good,
so may its bark,
so may its wood;
But unless you put
the right thing to its root,
it never will show
much flower or fruit.”
Roots, though out of sight and poorly understood by the casual observer, are critical to the plant. The primary functions of plant roots are structural support (anchoring the plant in the soil) and absorption (bringing in needed water and minerals from the soil or other growing media). We often think of large, structural woody roots, including taproots, which are common on some plants, but it is the young, fibrous network of “feeder” roots that are active in the uptake of needed minerals and water from the soil. This network of feeder roots is much shallower than is typically supposed.
Roots need certain things to grow and properly metabolize.
- Roots need oxygen, with some plants being more tolerant of low soil oxygen than others.
- Roots need minerals such as the macronutrients N, P, K (nitrogen, phosphorous and potassium) and micronutrients such as iron and manganese, which are supplied by the soil and sometimes supplemented by fertilizers.
- Roots also need food. Food – the carbon-based energy source by which plants function – is not produced by roots. Food is mainly produced in plant leaves and, to a limited extent, in photosynthesizing bark.
They can be variegated, lobed, green, red or purple. They provide shade. They flutter in the breeze. They sell the plant when the plant’s not in bloom. The primary function of plant leaves, however, is food production. This occurs through the wondrous process of photosynthesis.
Light energy from the sun (or another source) is absorbed by chlorophyll in leaf cells, and with adequate carbon dioxide from the air and water from the soil, the result is – drum roll, please – sugar. This carbohydrate energy source is the food that fuels the rest of the plant processes, including protein synthesis and respiration.
We see here the second flush of growth in a good growing year for this upright sugar maple. This will be reflected in a big-time annual ring.
Photosynthesis is a wondrous thing, because it starts the food chain that allows life on Earth as we know it, including our own survival. The food chain starts with plants harnessing the energy of the sun and converting it to plant growth, which is eaten by animals, and those animals are eaten by other animals, and so on. This photosynthesis is performed by trees and shrubs, herbaceous plants and grasses, and a number of other primitive plants, such as phytoplankton.
As author and New York Times science journalist Natalie Angier stated: “The core feature of planthood is autotrophy, that is, the happy ability to make one’s own food. Plants essentially eat the sun, transforming solar energy into sugars and starch through the stepwise enzymatic stitchery of photosynthesis. Animals, by contrast, are heterotrophs, defined by their need to devour other organisms – the hard-won fruit and fiber of the suneaters, or the once-removed flesh of herbivores.” Or as Robert DeFeo, chief horticulturist for the National Park Service, said, “The most important chemical reaction on earth is photosynthesis … We are all parasites upon it.”
Pretty important stuff.
Sun shining through these katsuratree leaves reminds us of how plants start the food chain. Carbon dioxide and water and the energy of the sun-voila. Carbohydrates for all!
In addition to carbohydrate production, photosynthesis also results in the byproduct of oxygen, making plants’ role in purifying our air critical. Of course, oxygen is used up by the plant in its respiration activities and when plant materials decompose, but the overall process has resulted in the oxygen concentration we have in our air. There are many factors that affect photosynthesis rates, including temperature, light intensity, carbon dioxide, water and nutrient availability. Careful manipulation of these factors, such as the addition of nitrogen fertilizers in the soil and enhancing carbon dioxide and light levels in greenhouse production, are important horticultural tools in regulating plant growth.
Another important function of plant leaves is the transpiration of water. Water vapor is lost through the stomates of leaves, and the pull of this evaporating water creates pressure that helps bring water into the roots from the soil solution. But wait: How does this all connect and function inside of the plant? How do water and soil minerals get pulled into the roots and moved all the way up to the leaves at the top of the plant? How does the food produced in the leaves get all the way to the roots to feed them?
How? Through the plant’s vascular transport system, which traverses the entire length of the plant via the stem.
Stems provide mechanical support for the plant and the all-important food producing leaves, and also are the living connector between the roots and leaves. Stem growth consists of an increase in both length and girth.
The vascular system consists of xylem, phloem and vascular cambium tissues and conducts or moves water, minerals and food. The xylem consists mostly of dead cylindrical cells that conduct water and minerals upward from the roots through the stems and to the leaves. Old, inner xylem becomes wood and eventually is incapable of active liquid transport. The phloem consists of living cells that conduct food produced in the leaves – sometimes upward, but mostly downward – as a continuous system through the leaves, stems and roots.
Between the xylem and the phloem is an area of new cell development called the vascular cambium. The vascular cambium produces new xylem cells to the inside and new phloem cells to the outside. There are additional types of stem tissues such as the inner pith and the outer bark, but the key functional components are these vascular transport tissues.
In the background photo, nursery production is thriving at Bailey Nursery’s Yamhill, Ore., location. In the end, this is what our basic botany knowledge is all about. Inset, ‘Green Gleam’ pachysandra soaks in the sun.
Photos courtesy of Jim Chatfield unless otherwise noted.
It should be noted that trees differ in the type of their water-conducting xylem. For example, maples and birches are termed “diffuse-porous” because both water and dissolved minerals are conducted in several of the most recently formed annular rings of xylem. Meanwhile, ash and oak are termed “ring-porous,” where most of the water and dissolved minerals are conducted in only the current annular ring of xylem. This helps explain some of the devastating nature of emerald ash borer (Agrilus planipennis) on ash compared to bronze birch borer (Agrilus anxius) on birch. Emerald ash borer is more deadly, at least partly due to the fact that it damages the current active ring of xylem that is transporting significant amounts of water; consequently, the plant is less able to overcome the damage.
Also, different types of landscape plants have different types of overall stem structure. Dicots, such as all our deciduous trees and shrubs (those that lose leaves in fall) and many herbaceous plants have continuous rings of xylem, phloem and vascular cambium all the way up the stem. This is what you’re observing when you see growth rings on trees, and it is the vascular cambium, producing xylem to the inside and phloem to the outside, that is increasing the stem girth over the years.
Anything that damages the phloem on the inner portion of the bark and the vascular cambium just inside the phloem can potentially kill the tree by damaging this living ring that connects the food-producing leaves to the roots. Monocots, such as grasses, lilies and palm trees, do not have organized continuous rings of vascular tissue, but rather have vascular bundles, to create areas of new cellular growth, scattered throughout the stem tissue. Thus a girdling cut or obstruction around the stem is not as devastating to a monocot stem, as it is to a dicot stem.
Crabapples are in the rose family (Rosaceae) related to apple and strawberry and rose. It’s not too hard to mistake its relatedness to rose, as seen in this flower of ‘Brandywine’ crabapple.
Flowers, fruits and seeds
The German philosopher and plant lover Goethe once said, “A flower is a leaf mad with love.” This was his way of reminding us that flower parts are really modified leaves, modified to perform the function of plant reproduction. This process is essential to the ultimate survival of a plant species, and it does in fact come at some cost. When vegetative growth converts to reproductive growth, there are energy costs to the plant.
This reproductive growth starts with a flower bud. There are many modifications of flowers, but one common form involves four whorls of modified leaves that emerge from the flower bud once it is initiated by environmental factors. Sepals are often green and surround the petals in the bud. The petals are typically the showy parts of the flowers, an important factor in attracting pollinators. In the center of the flowers are the male (stamens) and female (pistils) parts. The stamens have stalk-like filaments, atop which are anthers, containing pollen sacs. The pistils have ovaries at the base, stalk-like styles and stigmas at the tips.
Pollination involves pollen being transferred by pollinators (the birds and the bees) or blowing with the wind (for example, ashes) to the receptive stigmas at the tips of the styles. The pollen grains then germinate and grow down the style, delivering sperm cells to the eggs in the ovaries. The result is fertilization and the development ultimately of seeds, which are protected by the ovary that ripens around the seeds. That ripened ovary is defined botanically as a fruit, such as an apple or a holly berry, or a winged samara of maple or a tomato or green bean. (What? Yes, what you’ve heard is true: Botanically the tomatoes we eat are fruits, since they are seeds surrounded by the ripened ovary. Commercially, tomatoes are vegetables, because in 1886 the U.S. tomato growers pushed through protective tariff legislation defining tomatoes as vegetables, since vegetables – but not fruits – could be assessed such tariffs, and the foreign tomato growers wept.)
But we digress.
There are many variations of flowers and of fruits; sometimes male and female flowers are on separate plants (dioecious = two houses). For example, with most hollies you need to have male and female plants if you want showy fruits.
Sometimes male and female flowers are on the same plant (monoecious = one house), but may be separated by location or time of maturation. For example, pines often have male cones toward the bottom of the tree and female cones up in the top of the tree. Sometimes a tree may have both male and female flower parts in the same blossom (these are termed “perfect” flowers.) Some plants have elaborate physical and physiological techniques for insuring cross-pollination and cross-fertilization, but some plants (dandelions, for example) self-fertilize.
There are plenty of details, but it is important to remember the basics and that flowers and fruits play many everyday roles in horticulture; for instance, from the small leaves (vegetative growth) that develop on crabapples in heavy flowering (reproductive growth) years to the “unwanted fruits” on many plants: think ginkgoes and sweetgum balls.
Plants are marvels of growth and development, from the interlocking actions of their root, stem, leaf and reproductive organs, to their actively dividing and elongating cells in their meristematic regions at their tips and internally with the vascular cambium, along with their ability to take raw materials and make food (thank heavens for us).
As Natalie Angier writes: “In addition to their caloric self-sufficiency, plants can be envied for their eternal youthfulness. A plant elongates itself through constant cell growth in two zones of its body, at the very tips of the roots, which grow down into the soil or other surface to which the plant clings, and the outer tips of the shoots, from which new leaves, flowers and fruit sprout. Whereas an animal, upon reaching maturity, has almost no young cells left in its body … Dr. [Peter] Raven said … ‘in plants the ends of the roots and shoots are always juvenile, always growing, always babies.'”
What a privilege – and joy! – it is that we spend our daily lives as plant lovers and plant growers, making new plants, making plants thrive, and learning how to do it better and better. As English metaphysical poet Andrew Marvell wrote:
What wondrous life is this I lead!
Ripe apples drop about my head;
The luscious clusters of the vine
Upon my mouth do crush their wine;
The nectarine and curious peach
Into my hands themselves do reach;
Stumbling on melons as I pass,
Insnared with flowers, I fall on grass.