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Chapter 27

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Western University
Biology 1225
Michael Butler

Chapter 27: Plant Form and Function (18) Overview Most multi-cellular organisms depend ultimately upon plants as a food source, either directly or indirectly. Plants differ from animals in many profound ways. Plants make their own "food", they are autotrophs, this word literally means they are "self feeders". Plants do not use complex organic materials as nutrients as we must do, they take simple inorganic materials such as carbon dioxide and water and use sunlight energy to convert them into complex organic compounds. The cells of plants are unique; they have no bones or similar internal skeletal structure, yet trees can support branches weighing many tons due to the presence of cell walls - rigid or semirigid walls (absent in animal cells) that provide strength and support for plants. Furthermore, plant cells contain organelles called chloroplasts (these, too, are absent in animals). These are structures that contain a green pigment called chlorophyll that is responsible for trapping and harnessing energy in sunlight via a process called photosynthesis. Unlike most animals, plants are sessile - that is they remain rooted to a particular location, and, therefore, have much more limited movement than do animals. Also, plants exhibit a unique form of growth called modular or clonal growth. This simply means that most plants can grow indeterminately or throughout their lives, whereas most animals do not. In Chapter 27, you will learn about the structure and function of plants and their particular aboveground and belowground parts. Note also that plants do not grow as we do; for us, cells can reproduce at many different locations, in the skin, gut, organs etc. This is not the case with plants, they only grow at certain restricted regions such as root and shoot tips, called meristematic tissue. Meristematic cells are constantly being generated, and they then differentiate into the various specialized plant cells and tissues Responsibilities Most of the chapter is relevant. Be sure to know how monocots (like grasses and lilies) differ from dicots (like roses and oak trees). Be sure to understand the differences between xylem and phloem. For the remainder of the chapter, you need to learn the forms and functions of roots, leaves, and stems, and, in particular, how these types of tissues contribute to the acquisition and movement of water and nutrients. Plant structure, function, and reproduction Plant types The Kingdom Plantae is divided into about 11 major phyla (though, just to be annoying, plant biologists use the term Division instead of Phylum). The most primitive (i.e., of the most ancient evolutionary origin) of these Phyla lack vascular tissue (veins or “piping” to move water and sugars around) and they do not reproduce by producing pollen and seeds (mosses are an example). A few other plant Phyla have vascular tissue but still do not reproduce by forming seeds or pollen, a good example is the ferns. The most complex plants have vascular tissue and reproduce by forming pollen and seeds, the largest and most well known of these are the gymnosperms which produce seeds in cones, and the angiosperms (aka anthophyta) which produce their seeds in flowers. Plant scientists have detected definite evolutionary trends in plants that adapted them away from a dependence on a life in water to a life on land. These include the development of vascular tissues, and the development of pollen and seeds which can be dispersed by wind and other means than by water. More primitive plants (those that are evolutionarily the most ancient in origin) like mosses and ferns retain sperm that swim and this needs free water, In higher plants one finds structures such as flowers and cones that do not need water for fertilization and in which the sperm do not therefore swim to reach and fertilize an egg. In flowering plants the sperm and egg are still present, but the sperm is contained within the pollen grain, and the “egg” the ovule- is protected deep within the female components of the flower structure, or it is in the cone if it is a gymnosperm like a fir tree. The sperm within the pollen that lands on the female structure of a flower has to germinate to generate a growing tube that physically reaches and fuses with the egg, no free water is needed, the sperm within a pollen grain no longer has to swim, pollen grains can be carried to the flower by wind and insects instead. Some so called primitive plants also do not have vascular tissue, the system of several different kinds of conductive “pipes” that transport water, sugar and other materials around the plant and take water up from the soil. There is a clear pattern of adaptation to a life on land, where access to water is more difficult, by the appearance of vascular tissues. Mosses (which are bryophytes) do not have any vascular tissue, no xylem or phloem, they do not have true leaves (which are defined as containing vascular tissue or “veins”), or true roots (which also have vascular tissue in them) or true stems (which also contain vascular tissue). Water “wicks” its way up a moss by traveling along the outside of the “leaves” and “stems” which soak it up like paper towels. The sperm of mosses swim and must have free water to do so, thus mosses are generally found in wet environments on land. Raindrops dislodge and pick up sperm from moss plants and splash them onto neighboring mosses where they swim to the female structures and fertilize the eggs. Ferns have true vascular tissue, roots (called rhizomes- and not as efficient as roots in higher plants), true stems, and true leaves that contain xylem and phloem, BUT they still have swimming sperm and rely on water for sexual reproduction. Flowering plants (angiosperms) and cone bearing plants (gymnosperms) are fully adapted for life on land free of standing water, they have true vascular tissue and non swimming (immotile) sperm held in pollen grains. There are two categories of flowering plants, monocots and dicots (the short forms of monocoyledons and dicotyledons). Monocots have only one seed leaf (a cotyledon) that is seen when a seed first germinates, Dicots have two seed leaves when the seed first germinates. A cotyledon it is a small leaf or 2 leaf’s that appear when a seed first germinates, it is there to quickly generate new food and energy from sunlight, to give the new growing plant a “kick start”. The cotyledons are not the same as the normal leaves that form after the germinated seedling gets into rapid growth, and they also store food resources used by the young plant before it begins to photosynthesize, cotyledons wither and die once new, true leaves form. Grasses are typical monocots, carrot and tomato plants are typical dicots. Dicots and monocots have clear differences in basic structure. Monocots have leaves in which the veins are parallel, dicot leaves have veins in numerous orientations to each other, like net or apparently random patterns. Monocots do not have their leaves attached to a stalk by a petiole (a smaller leaf stalk), dicot leaves are attached to the main stalk by petioles. Monocot flower petals are arranged in threes or multiples of three, dicot petals are in groups of four or five. Monocots have a fibrous root system that does not have a single larger root from which other roots radiate, dicots have such a main root, called a taproot (as in carrots for instance). A cross section of the stem of a monocot will show bundles of xylem and phloem (vascular bundles) that are randomly distributed through the stem, but in dicots the vascular bundles are ordered in a definite circle towards the periphery of the stem. Dermal tissues of plants form a single cell depth outer layer called the epidermis, that on leaves usually exudes a surface layer of a waxy waterproof material called cutin. Ground tissues ( Collenchyma, Parenchyma, sclerenchyma) surround the vascular system, where they form the support fibres and stiffening materials, and also undertake many of the metabolic activities of the plant. Parenchyma tissue forms soft new primary growth of stems, roots, leaves and flowers. Parenchymal cells have thin walls, are alive and still able to divide when they are mature. Parenchyma cells form repair tissues when plants are wounded. The mesophyll cells of leaves are of parenchymal origin. Collenchyma tissue cells are elongated and thick walled, and include the carbohydrate polymer pectin, which gives collenchyma tissue flexibility. Collenchyma fibres form support strands in stems, celery is a good example. Collenchyma cells are alive when mature. Sclerenchyma tissues are thick walled, and contain a strengthening polymer called lignin. Lignin also provides waterproofing and can resist microbial attack. Sclerenchymal cells are dead at maturity and form the strong fibrous columns that allow stems to be rigid and plants to grow tall. Leaves are “sugar synthesizing factories”, this is mostly done by the photosynthesizing mesophyll cells in the leaf. Vascular tissue constitutes the veins of the leaves, the veins contain xylem and phloem. The vascular tissue is closely associated with the mesophyll cells, where its xylem delivers water and ions to the leaf and its phloem removes the sugars produced. Leaves have waterproofed surfaces to minimize water loss, yet water has to be lost in order to allow the transpiration process that drives xylem function to occur. To allow their functions to occur, while conserving water, the leaves usually have a waterproofing cutin layer excreted by the epidermal layer, but they also have many thousands of tiny pores called stomates (usually on the lower leaf surface). These stomates have a system that allows them to open and shut in response to ongoing requirements. When it is sunny and there is maximum photosynthesis, requiring carbon dioxide, the stomates are open. At night when there is no photosynthesis, the stomates close, minimizing water loss. Roots provide support and anchoring for the plant, but here we are interested in their absorptive properties. Roots contain vascular tissues, extensions of the vascular bundles of xylem and phloem down into the root from the leaves and the stem. The epidermis of roots is obviously not waterproof. Water can travel through the tissues of the roots, freely, until it reaches the outer surface of a vascular bundle in the root. The bundle is enclosed by a single cell layer called the endodermis. Roots also function in nutrient storage, as with carrots for instance. The specific mechanism of water absorption by roots is discussed below. Roots also function in food storage, as with carrots for instance. Vascular tissue: More details of Transport in plants The vascular tissue that moves water and dissolved ions and minerals around a plant is called xylem, it consists of cells (two types; tracheids and vessel elements) that stack on top of each other to form pipes. Since there are pits (holes) in the end walls of the cells of the xylem, the water column in the xylem is continuous, and when xylem cells are mature and performing their function they are dead. The “tension cohesion” model of water transport in plants: Loss of water from the leaves by evaporation (transpiration) exerts a “pull” on the column of water in the xylem pipes that extend from the roots to the leaves. Water is “sticky” (the fancy word is cohesive), the water molecules are hydrogen bonded to each other, and they also tend to bond to the wall of the xylem pipes that contain them. Water is pulled up the xylem columns by the tension force exerted by the loss of water from the leaves. Essential ions and minerals from the soil are also dissolved in the water that is transported in the xylem. Phloem is a system of vascular columns in plants that transports sugars made by photosynthesis in the leaves, to other parts of the plant. This transport of sugars is described by the pressure-flow model (aka the source-sink model): While xylem conducts water (and ions) in an upward direction (and it is composed of dead cells), phloem conducts sugar solution from the site of manufacture (leaves) to any place where it is being stored or used, and phloem cells are alive. Sugar produced in the leaves by photosynthesis is moved by active transport, into the neighboring sieve element cells of the phloem, the sieve cells stack to form long columns in the leaf veins. The sieve cells are alive, but they lack a nucleus (it degenerates and breaks up as the cell matures) and only have a thin layer of cytoplasm plastered against the cell membrane. Each sieve cell has a companion cell that is right next to and touching it, and this companion cell has a nucleus and controls its adjacent sieve cell. Picture a sieve cell at the top of a phloem column in a leaf, loaded up with sugar. This loading is done by the companion cell, when it removes the sugar, by active transport from mesophyll cells. Loading of sugar into the sieve cell makes it hyperosmotic, this causes water to follow the sugar into that sieve cell, by osmosis, and this causes the sieve cell to be under high pressure. This process is controlled by the companion cell. Elsewhere in the phloem column, way down, say, in the roots, the sugar is being removed from the phloem column by active transport, and this causes water to leave the sieve cell as a result, by osmosis and that cell loses pressure as a result. Thus high pressure in the sieve cells of the phloem column portion that is in the leaf (the source region) is “transmitted” to the distant (sink) region of the phloem column, at places like roots, and this drives the movement of sugar along the column of sieve cells. This pressure is constantly being “removed” at distant sites because the pressure is relieved where the sugar is being offloaded from the sieve cells. There is thus a constant process whereby high pressure is being generated at the point where sugars are actively transported into the phloem column, the source, in the leaves) and pressure is being removed at the distant sink end where the sugar is being removed by active transport (at the roots for instance). This pressure gradient drives the sugar along the sieve cells of the phloem to a distant point where it is used or stored. Plant growth Hormones In animals and many other life forms, growth occurs “all over the place”, liver cells reproduce liver cells, skin cells make more skin cells and so on. This generally does not happen in plants, growth by mitosis occurs at definite restricted sites and the assemblage of cells undergoing mitosis at these sites is called meristematic tissue. Meristematic tissue is found at the root tips (this is root apical meristem) and at the shoot tips (called shoot apical meristem), apical means “tip”. These meristematic cells form new cells that will then differentiate into the various functional cells of the mature plant such as xylem, phloem, storage cells, fiber cells and so on. This form of tip located growth that makes a plant grow longer (up into the air and down into the soil) is called primary growth. Some plants can also undergo secondary growth, a good way to differentiate primary growth from secondary growth is to think of secondary growth as “growing thicker” or increasing in girth, whereas primary growth increases the length of a plant. Secondary growth involves another form of meristematic tissue that is found in the stems or trunks of plants, what is called lateral meristem, which means that this kind of tissue produces “sideways” growth, the kind of growth that occurs in stems and branches to make them thicker, not longer). Monocots do not undertake secondary growth in this organized fashion. Dicot plants DO - many common trees are dicots, and secondary growth is seen also in gymnosperm trees like fir trees. A little more about Roots, Stems and leaves. Roots have a primary absorption function. Root epidermis, unlike the plant covering above ground is not waterproof, obviously, or it would not be able to absorb water and ions from the soil, which is the primary function of roots. Roots have many thousands of tiny root hairs wh
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