- The Stages of the Flower Life Cycle
- Flowering plant life cycles
- New plant grows from seed
- Adult plant produces flowers
- Seeds and fruit
- Vegetative reproduction
- Length of life cycle
- Basic Plant Life Cycle And The Life Cycle Of A Flowering Plant
- General Life Cycle of a Plant
- Seed Life Cycle: Germination
- Basic Plant Life Cycle: Seedlings, Flowers, & Pollination
- Repeating the Life Cycle of a Flowering Plant
- What is germination?
- Structure of a seed
- Starting to grow
- Steps of Germination
- Try Seed Germination Quiz
- Germination facts for kids
- Germination rate and germination capacity
- Baby plant is also called as
- Tree structure and growth
- General features of the tree body
The Stages of the Flower Life Cycle
The plant life cycle starts when a seed falls on the ground. There are many different kinds of plant life, but the flowering plants, or angiosperms, are the most advanced and widespread due to their amazing ability to attract pollinators and spread seeds. Flowers are more than beautiful objects to look at or decorate with; they serve a very important purpose in the reproduction of plants. The major stages of the flower life cycle are the seed, germination, growth, reproduction, pollination, and seed spreading stages.
The plant life cycle starts with a seed; every seed holds a miniature plant called the embryo. There are two types of flowering plant seeds: dicots and monocots. An example of a dicot is a bean seed. It has two parts called cotyledons in addition to the embryo. The cotyledons store food for the plant. Cotyledons are also the first leaves that a plant has-they emerge from the ground during germination. Monocots have only one cotyledon-the corn seed is an example. Both kinds of seeds have the beginnings of a root system as well. The hard outside of the seed is called the seed coat and it protects the embryo. Some seeds are capable of growing even after many years if they are kept cool and dry.
When a seed falls on the ground, it needs warmth and water in order to germinate; some seeds also need light. Dicots have seed coats that soften with moisture. After being planted in the soil for a few days, the seed absorbs water and swells until the seed coat splits. Monocots have harder seed coats that do not split, but stay in one piece. The stem, called the hypocotyl, pushes through the soil along with the cotyledons, or seed leaves; this is called germination, or sprouting. The tiny root pushes down and grows, looking for water and nutrients. Soon the cotyledons fall off and the first true leaves emerge. It is important that the seed is planted in the right place at the right time in order for it to germinate. Some seeds need to go through a fire in order to sprout, such as prairie grasses. Some need to go through the stomachs of animals, or be scraped. Different seeds have different needs!
In order to complete the flower life cycle stage of growth, plants have to produce their own food. This process is called photosynthesis. As soon as the leaves emerge, they start the process of photosynthesis. Plants contain chloroplasts in the leaves which convert the energy from sunlight, carbon dioxide, and water into sugars, which they use as food. The plants store the sugars in the roots and stem. The root system continues to develop, anchoring the plant into the ground and growing root hairs which help the plant to better absorb water and nutrients. The stem grows longer towards the sun and transports water and food between the roots and leaves. Sugars and starches are changed into energy used to make new plant growth. New leaves grow from the top of the stem, or meristem. After a while, flower buds develop. Some plants flower within days while it takes others months or even years.
Inside the bud, a tiny but complete flower forms. The sepals protect the bud before it opens. Over time, the bud opens and blossoms into a mature flower and the sepals look like little green leaves at the base of the flower. The flower is the sexually reproductive part of the plant. The petals of the flower are often very noticeable, brightly colored, and strongly scented in order to attract pollinators. This is a very exciting stage of the plant life cycle!
The female part of the flower is called the pistil and it has four parts– the stigma, style, ovary, and ovules. The male part of the flower is called the stamen and it consists of the long filament and the anther, where pollen is made. In the center of the flower, there is a long slender tube that ends in a rounded oval. The tube is called the style. On the top of the style is the stigma-its job is to catch pollen. It may be sticky, hairy, or shaped in a way that helps it to better trap pollen. Sometimes several stamens surround the pistil. Once the pollen is trapped it travels down the style to the rounded part at the end, called the ovary, where eggs are waiting to be fertilized. The fertilized eggs become seeds in this stage of the flower life cycle. In fruit producing plants, the ovary ripens and becomes fruit.
Some flowers have only male parts, and some have only female parts. In others, the male and female structures are far apart. These plants depend on insects, birds, animals, wind, water, or other pollinators to carry pollen from the male flowers or male parts to the female flowers or female parts. Without pollinators, there would be no seeds or new plants in these plant species. . Even flowers that can self-pollinate benefit from being fertilized by pollen from a different plant, which is called cross pollination, because cross pollination results in stronger plants.
Brightly colored petals, strong smell, nectar, and pollen attract pollinators. Flowers are specially adapted to attract their specific pollinators. For example, the corpse flower smells like rotting flesh in order to attract flies. Pollen sticks to the legs and wings of insects that go from flower to flower for nectar and pollen, which they use as a food. Pollen sticks to the fur of animals and even to the clothes of humans. Wind blows pollen which lands on other flowers.
Seed spreading, or dispersal, is the final stage of the flower life cycle. Seeds are spread in many ways. Some, like dandelion seeds, are scattered by the wind. Others rely on animals-an example of this is the cockleburs that get stuck in the fur of animals and hitchhike to new locations. Water lilies depend on water to spread their seeds. Humans spread many seeds intentionally by planting gardens. Once the seeds fall to the ground, the plant life cycle starts all over again.
For more information on the flower life cycle, please visit these links:
Written by Ava Rose
Flowering plant life cycles
The flowers and fruit of flowering plants come and go as part of their life cycle. Some flowering plants don’t even have stems and leaves all the time. The fruit and vegetables we eat come from different parts of the life cycle of various plants, such as roots, stems, leaves, flowers, fruit and seeds. There is a good botany lesson to be found in food on our plate, which may include a few surprises. For example, if it has seeds in it, to a botanist it is a fruit – that includes tomato, pumpkin and cucumber.
Gardeners need to know about plant life cycles so they can have food crops and colourful gardens all year round. Farmers and fruit growers need to know about plant life cycles so that they can predict when their crops will be ready.
Flowering plants all go through the same basic stages of a life cycle.
New plant grows from seed
When a seed comes to rest in conditions suited to its germination, it breaks open and the embryo inside starts to grow.
Roots grow down to anchor the plant in the ground. Roots also take up water and nutrients and store food.
A shoot grows skyward and develops into a stem that carries water and nutrients from the roots to the rest of the plant. The stem also supports leaves so they can collect sunlight.
Leaves capture sunlight to make food for the plant through the process of photosynthesis.
Adult plant produces flowers
When the plant matures and is ready to reproduce, it develops flowers. Flowers are special structures involved in sexual reproduction, which includes pollination and fertilisation.
Pollination is the process by which pollen is carried (by wind or animals) from the male part of a flower (the anther) to the female part (the stigma) of another flower. The pollen then moves from the stigma to the female ovules.
Pollen has male gametes containing half the normal chromosomes for that plant. After pollination, these gametes move to the ovule, where they combine with female gametes, which contain half the quota of chromosomes for its plant. This process is called fertilisation.
Seeds and fruit
After fertilisation, a combined cell grows into an embryo within a seed formed by the ovule. Seeds are what a plant uses to spread new plants into new places. Each seed contains a tiny plant called an embryo, which has root, stem and leaf parts ready to grow into a new plant when conditions are right.
Another part of the flower (the ovary) grows to form fruit, which protects the seeds and helps them spread away from the parent plant to continue the cycle.
As well as sexual reproduction making seeds, new plants are sometimes made by asexual vegetative reproduction. These new plants have exactly the same genes as the parent. Some plants have stems called stolons that grow out sideways above the soil, and new plants grow up along them. Other plants send out underground stems called rhizomes, which form new plants at a distance from the parent. Tubers (for example, potatoes) and bulbs (for example, onions) are also special underground structures that can grow into new plants.
Length of life cycle
Flowering plants all go through the same stages of a life cycle, but the length of time they take varies a lot between species. Some plants go though their complete cycle in a few weeks – others take many years.
Annuals are plants that grow from a seed, then flower and make new seeds, then die, all in less than a year. Some go through this cycle more than once in a year.
Biennials are plants that take 2 years to go through their life cycle. They grow from a seed, then rest over winter. In spring, they produce flowers, set seeds and die. New plants grow from the seeds.
Perennials are plants that live for 3 or more years. Some, such as trees, flower and set seeds every year for many years. Some others have stems and leaves that die away over winter but the plant continues to live underground. In the spring, new stems grow, which later bear flowers.
Did you know that the life cycle of ferns is different from other land plants as both the gametophyte and the sporophyte phases are free living, find out more in this interactive on the fern life cycle.
Basic Plant Life Cycle And The Life Cycle Of A Flowering Plant
While many plants can grow from bulbs, cuttings or divisions, the majority of them are grown from seeds. One of the best ways to help kids learn about growing plants is by introducing them to the basic plant life cycle. Bean plants are a great way to do this. By allowing kids to both examine and grow their own bean plant, they can develop an understanding of the plant’s seed life cycle.
General Life Cycle of a Plant
Learning about the life cycle of a flowering plant can be fascinating, especially for kids. Start by explaining what a seed is.
All seeds contain new plants, called embryos. Most seeds have an outer cover, or seed coat, which protects and nourishes the embryo. Show them examples of the various types of seeds, which come in many shapes and sizes.
Use handouts, which can be filled out and colored, to help kids with seed and plant anatomy. Go on to explain that seeds remain dormant, or sleep, until certain growing conditions are met. If kept cool and dry, this can sometimes take years.
Seed Life Cycle: Germination
Depending on the type of
seed, it may or may not require soil or light to germinate. However, most all plants need water in order for this process to occur. As water is absorbed by the seed, it begins to expand or swell, eventually cracking or splitting the seed coat.
Once germination occurs, the new plant will gradually begin to emerge. The root, which anchors the plant to the soil, grows downward. This also enables the plant to take up water and nutrients required for growth.
The shoot then grows upward as it reaches for light. Once the shoot reaches the surface, it becomes a sprout. The sprout will eventually take on a green color (chlorophyll) upon developing its first leaves, at which time the plant becomes a seedling.
Basic Plant Life Cycle: Seedlings, Flowers, & Pollination
Once the seedling develops these first leaves, it is able to make its own food through photosynthesis. Light is important for this process to occur, as this is where the plant gets its energy. As it grows and becomes stronger, the seedling changes into a young adult plant, with many leaves.
Over time, the young plant will begin to produce buds at the growing tips. These will eventually open up into flowers, which is a good time to introduce kids to the different types.
In exchange for food, insects and birds often pollinate the flowers. Pollination must occur in order for fertilization to happen, which creates new seeds. Take this opportunity to explore the pollination process, including the various methods plants have for attracting pollinators.
Repeating the Life Cycle of a Flowering Plant
After pollination has occurred, the flowers transform into fruiting bodies, which protect the numerous seeds that are inside. As the seeds mature or ripen, the flowers will eventually fade away or drop.
Once the seeds have dried, they are ready to be planted (or stored), repeating the life cycle of a flowering plant all over again. During the seed life cycle, you may want to discuss various ways seeds are dispersed, or spread, as well. For example, many seeds are passed through animals after ingesting the seeds. Others are spread through water or air.
What is germination?
The growth of a seed into a young plant or a seedling is called germination.
In this lesson we are going to learn about the growth of a seed into a young plant which is called germination. Learn the lesson and try the ‘Quiz’ at the end of the lesson to check your knowledge.
Example of the steps of germination
Structure of a seed
First of all, let’s learn the three main parts of a seed.
- Food Store (Stored food)
- Seed coat
Example of the structure of a seed
This is the tiny plant inside the seed which will develop into the adult plant. It consists of the young root and shoot of the plant.
These are food stored by the parent plant. Also, known as stored food, which is starch. Young plant uses this stored food until it is large enough to make its own food by the process of photosynthesis. (Click to read the lesson Photosynthesis)
This is the hard protective outer covering around the embryo and the food store. Seed coat protects the embryo and the food store.
The embryo rests inside the seed until the conditions are right for it to start to grow.
E.g. – Similarly, some seeds can stay in this resting state for hundreds of years.
Starting to grow
The growth of a seed into a young plant or a seedling is called germination.
Examples of the conditions (factors) that plants need to germinate
- Water – Helps the seed to swell up, so that the embryo can start growing
- Warmth – Speeds up and improves the process of germination
- Air (oxygen) – Releases energy for the embryo to germinate
Watch the video of seed germination and read the steps of germination below to understand about how germination takes place.
Steps of Germination
- When conditions are right the seed starts to take in water.
- As water is taken in, the seed swells bigger and bigger until the coat splits apart.
- Air can then get to the seed. So, the oxygen in the air helps the baby plant burn the food packed inside the seed.
- Burning the food produces energy. As a result, the baby plant uses the energy to grow.
- A tiny root grows downwards whereas a shoot begins to grow upwards.
- The shoot develops and reaches toward the light while the root system develops deep in the soil.
- The cotyledon later become the first leaves of the seedling when the seed germinates.
- Tiny leaves sprout at the end of the shoot letting Photosynthesis to take place. These are called foliage leaves. They give the baby plant energy, until it gets its own green leaves to photosynthesise.
- The primary root grows longer and thicker together with the secondary roots. The leaves grow larger.
- Finally, More and more leaves grow and the stem becomes thicker and stronger.
Diagram of Seed Germination
Example of the steps of germination
Try Seed Germination Quiz
Written By : K8School 9:23 am
Germination facts for kids
Germination occurs when a spore or seed starts to grow. It is a term used in botany. When a spore or seed germinates, it produces a shoot or seedling, or (in the case of fungi) a hypha. The biology of spores is different from seeds.
A spore germinates if and when conditions are right. It has a very limited lifespan. The method of spore-bearers, which are lower plants such as mosses, ferns and also fungi, is to produce vast numbers of spores, of which only a small percentage germinate.
Seeds contain an embryo, a store of food (the endosperm), and a protective coat. Seed plants include Gymnosperms (such as conifers) and Angiosperms (such as flowering plants). Seeds can survive much longer than spores, sometimes for hundreds of years. The strategy of seed-bearing plants is to invest energy and material in the substance of seeds, and they have evolved more sophisticated methods of dispersal than just wind.p98
Seeds do not germinate until their requirements are met, and these needs differ from species to species.
- The dormancy period must be over. Dormancy is governed by changes inside the seed. Each species has its own dormancy period, and will not germinate until that period is over.
- Their hibernation must be over. The hibernation is ended when events in the environment trigger germination. Sometimes this is just temperature and water, sometimes fire, sometimes the seed must go through a long cold spell.
The requirements for fruits is the same as for seeds. A fruit is just a seed with one or more extra layers derived from parts of the flower.
Germination is usually the growth of a plant contained within a seed; it results in the formation of the seedling, it is also the process of reactivation of metabolic machinery of the seed resulting in the emergence of radicle and plumule. The seed of a vascular plant is a small package produced in a fruit or cone after the union of male and female reproductive cells. All fully developed seeds contain an embryo and, in most plant species some store of food reserves, wrapped in a seed coat. Some plants produce varying numbers of seeds that lack embryos; these are called empty seeds and never germinate. Dormant seeds are ripe seeds that do not germinate because they are subject to external environmental conditions that prevent the initiation of metabolic processes and cell growth. Under proper conditions, the seed begins to germinate and the embryonic tissues resume growth, developing towards a seedling.
Seed germination depends on both internal and external conditions. The most important external factors include right temperature, water, oxygen or air and sometimes light or darkness. Various plants require different variables for successful seed germination. Often this depends on the individual seed variety and is closely linked to the ecological conditions of a plant’s natural habitat. For some seeds, their future germination response is affected by environmental conditions during seed formation; most often these responses are types of seed dormancy.
- Water is required for germination. Mature seeds are often extremely dry and need to take in significant amounts of water, relative to the dry weight of the seed, before cellular metabolism and growth can resume. Most seeds need enough water to moisten the seeds but not enough to soak them. The uptake of water by seeds is called imbibition, which leads to the swelling and the breaking of the seed coat. When seeds are formed, most plants store a food reserve with the seed, such as starch, proteins, or oils. This food reserve provides nourishment to the growing embryo. When the seed imbibes water, hydrolytic enzymes are activated which break down these stored food resources into metabolically useful chemicals. After the seedling emerges from the seed coat and starts growing roots and leaves, the seedling’s food reserves are typically exhausted; at this point photosynthesis provides the energy needed for continued growth and the seedling now requires a continuous supply of water, nutrients, and light.
- Oxygen is required by the germinating seed for metabolism. Oxygen is used in aerobic respiration, the main source of the seedling’s energy until it grows leaves. Oxygen is an atmospheric gas that is found in soil pore spaces; if a seed is buried too deeply within the soil or the soil is waterlogged, the seed can be oxygen starved. Some seeds have impermeable seed coats that prevent oxygen from entering the seed, causing a type of physical dormancy which is broken when the seed coat is worn away enough to allow gas exchange and water uptake from the environment.
- Temperature affects cellular metabolic and growth rates. Seeds from different species and even seeds from the same plant germinate over a wide range of temperatures. Seeds often have a temperature range within which they will germinate, and they will not do so above or below this range. Many seeds germinate at temperatures slightly above 60-75 F (16-24 C) , while others germinate just above freezing and others germinate only in response to alternations in temperature between warm and cool. Some seeds germinate when the soil is cool 28-40 F (-2 – 4 C), and some when the soil is warm 76-90 F (24-32 C). Some seeds require exposure to cold temperatures (vernalization) to break dormancy. Some seeds in a dormant state will not germinate even if conditions are favorable. Seeds that are dependent on temperature to end dormancy have a type of physiological dormancy. For example, seeds requiring the cold of winter are inhibited from germinating until they take in water in the fall and experience cooler temperatures. Cold stratification is a process that induces the dormancy breaking prior to light emission that promotes germination . Four degrees Celsius is cool enough to end dormancy for most cool dormant seeds, but some groups, especially within the family Ranunculaceae and others, need conditions cooler than -5 C. Some seeds will only germinate after hot temperatures during a forest fire which cracks their seed coats; this is a type of physical dormancy.
Most common annual vegetables have optimal germination temperatures between 75-90 F (24-32 C), though many species (e.g. radishes or spinach) can germinate at significantly lower temperatures, as low as 40 F (4 C), thus allowing them to be grown from seeds in cooler climates. Suboptimal temperatures lead to lower success rates and longer germination periods.
- Light or darkness can be an environmental trigger for germination and is a type of physiological dormancy. Most seeds are not affected by light or darkness, but many seeds, including species found in forest settings, will not germinate until an opening in the canopy allows sufficient light for growth of the seedling.
Scarification mimics natural processes that weaken the seed coat before germination. In nature, some seeds require particular conditions to germinate, such as the heat of a fire (e.g., many Australian native plants), or soaking in a body of water for a long period of time. Others need to be passed through an animal’s digestive tract to weaken the seed coat enough to allow the seedling to emerge.
Some live seeds are dormant and need more time, and/or need to be subjected to specific environmental conditions before they will germinate. Seed dormancy can originate in different parts of the seed, for example, within the embryo; in other cases the seed coat is involved. Dormancy breaking often involves changes in membranes, initiated by dormancy-breaking signals. This generally occurs only within hydrated seeds. Factors affecting seed dormancy include the presence of certain plant hormones, notably abscisic acid, which inhibits germination, and gibberellin, which ends seed dormancy. In brewing, barley seeds are treated with gibberellin to ensure uniform seed germination for the production of barley malt.
In some definitions, the appearance of the radicle marks the end of germination and the beginning of “establishment”, a period that utilizes the food reserves stored in the seed. Germination and establishment as an independent organism are critical phases in the life of a plant when they are the most vulnerable to injury, disease, and water stress. The germination index can be used as an indicator of phytotoxicity in soils. The mortality between dispersal of seeds and completion of establishment can be so high that many species have adapted to produce huge numbers of seeds
Germination rate and germination capacity
Germination of seedlings raised from seeds of eucalyptus after 3 days of sowing.
In agriculture and gardening, the germination rate describes how many seeds of a particular plant species, variety or seedlot are likely to germinate over a given period. It is a measure of germination time course and is usually expressed as a percentage, e.g., an 85% germination rate indicates that about 85 out of 100 seeds will probably germinate under proper conditions over the germination period given. The germination rate is useful for calculating the seed requirements for a given area or desired number of plants. In seed physiologists and seed scientists “germination rate” is the reciprocal of time taken for the process of germination to complete starting from time of sowing. On the other hand, the number of seed able to complete germination in a population (i.e. seed lot) is referred as germination capacity.
Baby plant is also called as
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Tree structure and growth
General features of the tree body
As vascular plants, trees are organized into three major organs: the roots, the stems, and the leaves. The leaves are the principal photosynthetic organs of most higher vascular plants. They are attached by a continuous vascular system to the rest of the plant so that free exchange of nutrients, water, and end products of photosynthesis (oxygen and carbohydrates in particular) can be carried to its various parts.
Growth regions of a tree(A) Longitudinal section of a young tree showing how the annual growth rings are produced in successive conical layers. (B) Shoot apex, the extreme tip of which is the apical meristem, or primary meristem, a region of new cell division that contributes to primary growth, or increase in length, and which is the ultimate source of all the cells in the aboveground parts of the tree. (C) Segment of a tree trunk showing the location of the cambium layer, a secondary meristem that contributes to secondary growth, or increase in thickness. (D) Root tip, the apex of which is also an apical meristem and the ultimate source of all the cells of the root system.From (A) W.W. Robbins and T.E. Weier, Botany, an Introduction to Plant Science,; © 1950 by John Wiley & Sons, Inc. (B,D) Biological Science, an Inquiry into Life,; 2nd ed. (1968); Harcourt, Brace, Jovanovich, Inc., New York; by permission of the Biological Sciences Curriculum Study; (C) E.W. Sinnott, Botany: Principles and Problems, 4th ed., copyright 1946; used with permission of McGraw-Hill Book Co.
The stem is divided into nodes (points where leaves are or were attached) and internodes (the length of the stem between nodes). The leaves and stem together are called the shoot. Shoots can be separated into long shoots and short shoots on the basis of the distance between buds (internode length). The stem provides support, water and food conduction, and storage.
Roots provide structural anchorage to keep trees from toppling over. They also have a massive system for harvesting the enormous quantities of water and the mineral resources of the soil required by trees. In some cases, roots supplement the nutrition of the tree through symbiotic associations, such as with nitrogen-fixing microorganisms and fungal symbionts called mycorrhizae, which are known to increase phosphorous uptake. Tree roots also serve as storage depots, especially in seasonal climates.
As is true of other higher vascular plants, all the branches and the central stem of trees (the trunk or bole) terminate in growing points called shoot apical meristems. These are centres of potentially indefinite growth and development, annually producing the leaves as well as a bud in the axis of most leaves that has the potential to grow out as a branch. These shoot apical growing centres form the primary plant body, and all the tissues directly formed by them are called the primary tissues. As in the stems, the growing points of the roots are at their tips (root apical meristems); however, they produce only more root tissue, not whole organs (leaves and stems). The root meristem also produces the root cap that covers the outside of the root tip.
The shoot apical meristems do not appear different between long and short shoots, but the lower part of the meristem does not produce as many cells in short shoots. In some cases, it may be totally inactive. Shoot meristems in some species may interconvert and change the type of shoot they produce. For example, in the longleaf pine, the seedlings enter a grass stage, which may last as long as 15 years. Here the terminal bud on the main axis exists as a short shoot and produces numerous needle-bearing dwarf shoots in which there is little or no internode elongation. Consequently, the seedling resembles a clump of grass. This is probably an adaptation to fire, water stress, and perhaps grazing. The root volume, however, continues to grow, increasing the chance of seedling survival once the shoot begins to grow out (i.e., the internodes start to expand). This process is called flushing.
The outermost layer of cells surrounding the roots and stems of the primary body of a vascular plant (including the leaves, flowers, fruits, and seeds) is called the epidermis. The closely knit cells afford some protection against physical shock, and, when invested with cutin and covered with a cuticle, they also provide some protection from desiccation. Stomata (pores) are interspersed throughout the epidermal cells of the leaves (and to some extent on the stems) and regulate the movement of gases and water vapour into and out of the plant body.
A transverse slice of tree trunk, depicting major features visible to the unaided eye in transverse, radial, and tangential sections.Encyclopædia Britannica, Inc.
Immediately adjacent is a cylinder of ground tissue; in the stem the outer region is called the cortex and the inner region the pith, although among many of the monocotyledons (an advanced class of angiosperms, including the palms and lilies) the ground tissue is amorphous and no regions can be discerned. The roots of woody dicots and conifers develop only a cortex (the pith is absent), the innermost layer of which comprises thick-walled wall cells called endodermal cells.
The final tissue system of the primary plant body is the vascular tissue, a continuous system of conducting and supporting tissues that extends throughout the plant body. The vascular system consists of two conducting tissues, xylem and phloem; the former conducts water and the latter the products of photosynthesis. In the stems and roots the vascular tissues are arranged concentrically, on the order of a series of cylinders. Each column, or cylinder, of primary vascular tissue develops the primary xylem toward the inner aspect of the column and the primary phloem toward the outer aspect. The multiple vascular cylinders are arranged throughout the cortex, either in an uninterrupted ring between the cortex and pith or separated from each other by ground tissues. In some monocotyledons the vascular cylinders are scattered throughout the stem. Regardless of their arrangement, however, the multiple vascular columns form strands from the leaves to the roots, moving water and nutrients where they are most needed.
Cells of the (left) phloem and (right) xylem.Encyclopædia Britannica, Inc.
All plants, including trees, start life as seedlings whose bodies are composed wholly of primary tissues. In this respect, young trees are structurally analogous to the herbaceous plants. It is the conversion of a seedling from an herbaceous plant to a woody plant that marks the initiation of tree-specific structures. In dicotyledonous and coniferous (i.e., woody) trees and shrubs, the defining structure that permits this conversion is a layer of meristematic cells, called the vascular cambium, that organizes between the primary xylem and primary phloem of the vascular cylinders. The cambium forms the wood and the inner bark of the tree and is responsible for thickening the plant, whereas the apical meristems are responsible for forming and elongating the primary plant body. A vascular cambium forms in conifers and dicotyledons and to a lesser extent in some monocotyledons and cycads. Tree ferns do not develop a vascular cambium; hence, no secondary thickening of the trunk takes place in the usual sense.
The formation of the vascular cambium is initiated when cells between the columns of vascular tissue connect the cambia inside the columns of vascular tissue to form a complete cylinder around the stem. The cells formed toward the inside are called secondary xylem, or wood, and those formed toward the outside of the cambium are called secondary phloem. The bark and the wood together constitute the secondary plant body of the tree. The woody vascular tissue provides both longitudinal and transverse movement for carbohydrates and water.
The vascular cambium consists of two types of cells, which together give rise to the secondary xylem and phloem: fusiform initials and ray initials. The fusiform initials are long cells that give rise to the axial (longitudinal) system of vascular tissue. The cells of the axial system are arranged parallel with the long axis of the tree trunk. The ray initials form the radial system of the bark and wood. These initials are more squat in shape and produce cells oriented perpendicular to the axial cells.