part four botany basics: stems
Stems support buds and leaves with a vascular system that forms a continuous ‘highway’ inside the stem from the root, through the stem and to the leaves – conduits for carrying water, minerals, and photosynthates (food).
The structure of the vascular system consists of the xylem tubes that conduct water and dissolved minerals; phloem tubes that conduct sugars, proteins, and other organic molecules; and cambium which is a layer of meristematic tissue that separates the xylem and phloem and constantly produces new cells of each.
“Flower will not grow, if the stem doesn’t allow”nayreil
Stems and shoots respond to outside signals like light and gravity, and as growth in plants occurs the stems and roots lengthen. This increase in the length of the shoot and the root is referred to as primary growth. It is the result of cell division in the shoots apical meristem. Herbaceous plants mostly undergo primary growth with very little increase in thickness from secondary growth.
Most primary growth occurs at the apices, or tips, of stems and roots, and is a result of rapidly-dividing cells in the apical meristems at the shoot and root tip. Subsequent cell elongation also contributes to primary growth. The growth of shoots and roots during primary growth enables plants to continuously seek water through the roots or sunlight with shoots.
The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems. Most coniferous trees demonstrate strong apical dominance, producing the typical conical Christmas tree shape. If the apical bud is removed, then the axillary buds will start forming lateral branches.
Removing the apical bud when pruning plants (cutting off the tops of branches) encourages axillary buds to grow out, giving the plant a bushy shape.
Secondary growth is characterized by an increase in thickness or girth of the plant. It is caused by cell division in the lateral meristem. Secondary growth, or “wood”, is noticeable in woody plants. This growth occurs in some dicots, but very rarely in monocots.
The increase in stem thickness that results from secondary growth is due to the activity of the lateral meristems, which are lacking in herbaceous plants. Lateral meristems include the vascular cambium and, in woody plants, the cork cambium. The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form tracheids and vessel elements (secondary xylem) to the inside sieve elements and companion cells (secondary phloem) to the outside. The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength.
In woody plants, cork cambium is the outermost lateral meristem. It produces bark containing a waxy substance known as suberin that can repel water. The bark protects the plant against physical damage and helps reduce water loss. The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the cambium. The cork cambium, cork cells (bark), and phelloderm are together termed the periderm and the periderm substitutes for the epidermis in mature plants. In some plants, the periderm has many openings, known as lenticels, which allow the interior cells to exchange gases with the outside atmosphere. This supplies oxygen to the living- and metabolically-active cells of the cortex, xylem, and phloem.
Some plant parts, such as stems and roots, continue to grow throughout a plant’s life: a phenomenon called indeterminate growth. Other plant parts, such as leaves and flowers, exhibit determinate growth, which ceases when a plant part reaches a particular size.
A node is an area on the stem where buds are located. At this site of cellular activity and growth, small buds will develop into leaves, flowers, or stems. Locating a plants nodes is important when pruning and you will want to make the cut just above and not to close to a node to encourage the bud at the node to develop and eventually become a flower, leaf, or stem.
The area between nodes is called the internode and its length depends on many factors including:
- Reduced soil fertility which will decrease the internode length
- Application of nitrogen fertilizer which can greatly increase the internode length
- Lack of light increases internode length and can cause spindly stems known as etiolation or stretch and often happens with seedlings started indoors or houseplants that do not have enough sunlight.
- Varies with season – Early season growth has long internodes, while late-season growth is usually shorter
- Often if a stems energy is diverted by fruit growth, or divided among three or four side stems, internode length is shortened
- Plant growth regulator substances and herbicides can also influence internode growth.
types of stems
All stems must have buds or leaves to be classified as stem tissue. They may be long such as in the branches of trees or runners on strawberries, or compressed like found in fruit spurs and African violets. While most stems are found growing above ground, some such as rhizomes, tubers, croms, or bulbs grow below.
specialized above ground stems
A plant crown is the area where the stems join the root and is sometimes referred to as the plant base. On trees, the plant crown is the area where branches grow from the trunk and grafted trees are usually grafted below the crown. With the exception of non-vascular plants like moss or liverwort, most plants have a crown that functions as a transfer area for energy and nutrients between roots and stems. When planting the crown is at or just above the soil level to avoid crown rot, and with a few exceptions; trees, because the crown is above the trunk and plants such as clematis, asparagus, potatoes, tomatoes, and peonies benefit from having the crown planted below soil level. Bulbous and tuberous plants are also planted with the crowns below the soil. In cool climates, tender plants with crowns will benefit from having mulch placed over the crown to protect it from frost damage.
specialized below ground stems
Potato tuber, iris rhizomes, and tulip bulbs are all underground stems that store food for the plant. Sometimes it is difficult to distinguish between roots and stems – always look for nodes. Stems have nodes…roots do not.
Rhizomes resemble stolons because they grow horizontally from plant to plant. Some rhizomes are compressed and fleshy as in the iris, while others are slender and have elongated internodes like in bentgrass.
Rhizomes differ from other storage structures by growing horizontally under the surface of the soil. On some plants, this type of rooting structure can be very invasive.
The lily-of-the-valley rhizome spreads horizontally underground and produces pips which develop into new plants. This plant is increased in the landscape by digging in the fall and removing pips with developed roots for replanting.
Tulips, onions, daffodils and lillies have shortened , compressed underground stems. The true bulb has five major parts. It contains the basal plate, fleshy scales, tunic, the shoot (consisting of developing flower and leaf buds), and lateral buds (develop into bulblets or offsets). Many plants such as daffodils form new bulbs around the original bulb. These bulbs, called offsets, develop from buds within the base of the mother bulb and produce new plants. When these bulbs become overcrowded, the flowers start to diminish in size. This is an indication that it is time to dig up and divide the bulbs.
There are two kinds of bulbs:
truncate bulbs – has a paper-like covering or tunic that protects the scales from drying and from mechanical injury. examples of tunicate bulbs include: tulips, daffodils, hyacinths, grape hyacinths (muscari), and alliums
imbricate bulbs – does not have the tunic (papery covering) to protect the fleshy scales. Imbricate bulbs must be kept constantly moist before planting so they are not injured by the scales drying out. An example of the imbricate bulb is the lily.
A corm is a swollen stem base that is modified into a mass of storage tissue. A corm does not have visible storage rings when cut in half. This distinguishes it from a true bulb.
The corm contains a basal plate, thin tunic and a growing point. Examples of plants that develop from corms include: gladiolus, crocus, and autumn crocus.
glossary of terms
Anther – The pollen sac on a male flower
Apex – The tip of a root or shoot
Apical dominance – The tendency of an apical bud to produce hormones that suppress growth of buds below it on the stem
Apical meristem – the growth region in plants found within the root tips and the tips of the new shoots and leaves. Apical meristem is one of three types of meristem, or tissue which can differentiate into different cell types. Meristem is the tissue in which growth occurs in plants.
Axil – The location where leaf joins the stem
Basal plate – bottom of bulb from which roots develop
Bulb – is structurally a short stem with fleshy leaves or leaf bases that function as food storage organs during dormancy
Bolting – plants produce a flowering stem in a natural attempt to produce seeds as a means of survival when under stress.
Cambium – A layer of growing tissue that separates the xylem and phloem and continuously produces new xylem and phloem cells
Chlorophyll – The green pigment in leaves that is responsible for trapping light energy from the sun
Chloroplast – A specialized component of certain cells; contains chlorophyll and is responsible for photosynthesis
Cold hardy – generally measured by the lowest temperature a plant can withstand
Cortex – Cells that make up the primary tissue of the root and stem
Cotyledon – The first leaf that appears on a seedling. also called a seed leaf.
Corms – bulbo-tuber, or bulbotuber is a short, vertical, swollen underground plant stem that serves as a storage organ that some plants use to survive winter or other adverse conditions such as summer drought and heat
Cuticle – A relatively impermeable surface layer on the epidermis of leaves and fruit
Dermal tissue – covers the outer surface of herbaceous plants. It is composed of epidermal cells, closely packed cells that secrete a waxy cuticle that aids in the prevention of water loss
Dicot – having two seed leaves
Herbaceous – vascular plants that have no persistent woody stems above ground
Epidermis – The outermost layer of plant cells
Fibrous roots – a network of feeding lateral roots found on most plants
Ground meristem – an area of primary meristematic tissue, emerging from and immediately behind the apical meristem, that develops into the pith and the cortex
Guard cell – Epidermal cells that open and close to let water, oxygen and carbon dioxide pass through the stomata
Internode – the space between nodes on a stem
Lateral root – roots that branch from larger primary roots
lenticel: small, oval, rounded spots upon the stem or branch of a plant that allow the exchange of gases with the surrounding atmosphere
Marginal meristems – the meristem located along the margin of a leaf primordium and forming the leaf blade. The apical meristem or growing tip, is a completely undifferentiated meristematic tissue found in the buds and growing tips of roots in plants.
Meristem – Specialized groups of cells that are a plant’s growing points.
Meristematic zone – located at the tip of a root and manufactures cells: it is an area of cell division and growth
Mesophyll – A leafs inner tissue, located between the upper and lower epidermis; contains chloroplasts and other specialized cellular parts (organelles)
Monocot – having one seed leaf
Mycorrhizae – symbiotic association between certain fungi and roots of a plant
Node – an area on a stem where a leaf, stem, or flower bud is located
Ovary – The part of a female flower where the eggs are locate
Periderm – the outer layer of plant tissue: the outer bark
Petiole – The stalk that attached a leaf to the stem
Phloem – Photosynthate-conducting tissue
Pistil – The female flower part; consists of a stigma, style, and ovary
Pith – or medulla, is a tissue in the stems of vascular plants. Pith is composed of soft, spongy parenchyma cells, which store and transport nutrients throughout the plant. … In trees pith is generally present in young growth, but in the trunk and older branches the pith often gets replaced – in great part – by xylem
Primary root – originating at the lower end of a seedlings embryo and continues to elongate downward. It may or may not persist into plant maturity, and has limited branching – it is called a tap root
Procambium – is a meristematic tissue concerned with providing the primary tissues of the vascular system; the cambium proper is the continuous cylinder of meristematic cells responsible for producing the new vascular tissues in mature stems and roots
Protoderm – The primary meristem in vascular plants that gives rise to epidermis. Also called dermatogen
Respiration – the process of converting sugars and starches to energy
Rhizomes – is a modified subterranean plant stem that sends out roots and shoots from its nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from axillary buds and grow horizontally. The rhizome also retains the ability to allow new shoots to grow upwards
Root cap – group of cells protecting the apical meristem at the root tip
Root hairs – delicate, elongated epidermal cells that occur in a zone behind the root’s growing tip with the function of increasing the roots surface area and absorptive capacity
Root plate – That part of the root system (excluding the small outermost roots) needed to keep a tree wind-firm.
Stamen – The male flower part; consists of an anther and a supporting filament
Stigma – The top f a female flower part; collects pollen
Stoma (pl. stomates, stomata) – tiny openings in the epidermis that allow water, oxygen, and carbon dioxide to pass into and out of a plant
Suberin – a waxy material found in bark that can repel water
Style – The part of a female flower that connects the stigma to the ovary. Pollen travels down the style to reach the ovary, where fertilization occurs
Tap root – see Primary root
Transpiration – the process of losing water (in the form of vapor) through stomata
Tuber – enlarged structures in some plant species used as storage organs for nutrients. They are used for the plant’s perennation, to provide energy and nutrients for regrowth during the next growing season, and as a means of asexual reproduction. Stem tubers form thickened rhizomes or stolons
Tunic – skin-like covering that protects the fleshy scales
Turgor – Cellular water pressure; responsible for keeping cells firm
Vascular tissue – Water, nutrient, and photsynthate-conducting tissue (xylem and phloem)
Vegetative structures – The vegetative (somatic) structures of vascular plants include two major organ systems: (1) a shoot system, composed of stems and leaves, and (2) a root system
Xylem – Water and nutrient-conducting tissue
Zone of elongation – located behind the meristem. Cells in this area increase in size through food and water absorption. As they grow, they push the root through the soil
Zone of maturation – located directly beneath the stem. Cells in this zone become specific tissues such as epidermis, cortex, or vascular