Form & Function in Flowering Plants – Structure; Transport Categories: Life Spans: Annuals – go through complete life cycle in one year or less Biennials – go through life cycle over a two year period Perennials – live for many years and typically reproduce yearly Classes: Monocots Dicots http://www.ucmp.berkeley.edu/glossary/gloss8/monocotdicot.html Plant Form and Function: Cells and Tissues: Primary growth: Apical Meristem – transverse in position thus contributes primarily to

an increase in length of the organ ↓ Fundamental tissues Procambium Ground meristem Protoderm Calyptrogen (only in roots) ↓ Primary Permanent Tissues Procambium → primary xylem, primary phloem, vascular cambium,

pericycle (mostly in roots – rare in stems), pith (in some roots) Ground meristem → cortex (a complex tissue consisting of the endodermis (lacking in some stems), cortical parenchyma, and hypodermis, pith (in stems), cork cambium (in some) Protoderm → epidermis Calyptrogen → calyptra or root cap (in roots only)

Secondary growth: Takes place in some but not all flowering plants Originates from cambia – lateral in position thus contributes primarily to

an increase in diameter of the organ Vascular cambium – produces secondary xylem to the inside and secondary phloem to the outside Cork cambium – produces cork to the outside – in some plants a layer or two of phelloderm is produced to the inside

Figure 1 - Diagram of a root tip showing regions of primary growth.

Tissue Systems – found only in plants – animals do not produce tissue systems Dermal – epidermis Vascular – xylem and phloem

Ground – all the rest Fundamental cell types: Parenchyma – produces only a primary cell wall, thus are typically

thin-walled and flexible. Living at maturity

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Figure 2 - Photomicrograph of parenchyma cells.

Figure 3 - Electron micrograph of a parenchyma cell. Note the large central vacuole.

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Collenchyma – the primary cell wall is unevenly thickened with the thickenings found at the angles of the cell. They are living at maturity and provide support while retaining the ability to elongate.

Figure 4 - Collenchyma cells. Note the irregularly thickened walls (the yellowish color seen in the photomicrograph).

Sclerenchyma – typically have very thick secondary and tertiary cell walls

which are either lignified (contain the carbohydrate lignin) or suberized (contain the lipid suberin). May be of two general types; fibers or sclerids. Fibers are elongated with tapering ends while sclerids are irregular in shape. These cells are typically dead at maturity. They primarily add strength and support to the plant. Fibers are the basis for many types of textiles and rope (cordage).

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Figure 5 - The thick walled reddish stained cells in this photomicrograph are phloem fibers (in cross section). Note the very tiny lumen in the center of some of the cells. The layers of cell wall laid down have nearly filled the cell.

Figure 6 - Fibers (toward the bottom) in longitudinal view.

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Complex tissue – composed of a variety of cell types Xylem – contain two types of water and ion transport cells, the tracheids

and the vessels in addition to fibers, ray cells, and parenchyma cells.

Figure 7 - Xylem tissue – the large cells are vessels while the narrow, tapering cells are tracheids.

Phloem – contain two types of food transport cells, the sieve cells and the sieve tube cells in addition to companion cells, ray cells, fibers, and parenchyma cells.

Figure 8 - Phloem tissue with a sieve tube cell and companion cell labeled.

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Vegetative Plant Anatomy: Roots: For our purposes we are most concerned with that region of the root called the stele. The stele is the central core of tissue, most of which has its origin from the procambium. The outmost boundary of the stele is the pericycle. The pericycle is involved in producing lateral roots, and specific cells of the pericycle are involved in producing the vascular cambium. The stele also contains the vascular tissues, xylem and phloem. In monocot roots, but not in most dicot roots, the center of the stele is occupied by the pith. Primary xylem occupies the center of the stele in most dicots. Recall the previous discussion the origin of root tissues.

Figure 9 - The origin of tissues of the root of a dicot.

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Figure 10 – Cross-section through the stele region of a primary root of a typical

dicot. a – primary xylem, b – pericycle (a single row of cells completely surrounding the stele), c – primary phloem, d – endodermis (a single row of cells that forms the innermost layer of the cortex), e – a cell of the cortical parenchyma (the cortical parenchyma is the most extensive tissue of the cortex of a primary root.

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Figure 11 – Cross-section of a monocot root. The tissue in the middle of the

root that is not labeled is the pith. Note that the pericycle is several cell layers thick.

Some roots undergo secondary growth by activity of the vascular cambium. The vascular cambium will produce most of its derivatives to the inside. These differentiate as secondary xylem. The derivatives produced to the outside differentiate as secondary phloem. As monocot roots do not have a vascular cambium, they do not produce secondary xylem and phloem. Roots may even produce some cork tissue from a cork cambium. This is particularly true for aerial roots. Stems: Stems typically grow in length through the activity of an apical meristem, although some monocot stems may elongate through the activity of an intercalary meristem, a transverse meristem located just above each node. The vascular tissue of a stem is not arranged radially as in the root, but is in bundles termed vascular bundles. Within the vascular bundles will be found

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primary xylem towards the center and primary phloem towards the outside. A portion of the vascular cambium, called a fascicular cambium because it is located within a fascicle or bundle, is located between the xylem and phloem. Monocot vascular bundles lack a fascicular cambium. In the dicot stem the bundles are located in one or two rings. In the monocot stem they are scattered. The central portion of the dicot stem is pith, unlike the root of that same plant, with extensions of the pith, called pith rays, running between the bundles. In woody plants cells of the pith ray just opposite the fascicular cambium will differentiate as cambium cells. This will stimulate the adjacent pith ray cell to differentiate, and so on, until a row of cambial cells extends from one bundle to the next. The cambium between the bundles is called the interfascicular cambium, and the interfascicular cambium plus the fascicular cambium forms a continuous layer of vascular cambium. As the vascular cambium becomes active it produces considerable secondary xylem (wood) to the inside and some secondary phloem to the outside. A mature woody dicot stem consists of three regions; the pith, the wood, and the bark. The bark is defined as all the tissues lying to the outside of the vascular cambium. The tissues of the stem develop in the following way:

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Figure 12 – Anatomy of a dicot stem with labeled tissues.

Figure 13 – Note the interfascicular cambium between the bundles. 11

Figure 14 – The anatomy of a monocot stem with labeled tissues and cells.

Figure 15. Woody stem. 5-phloem fiber, 6-phloem ray, 7-vascular cambium, 8-xylem ray, 9-spring wood, 10-summer wood, 11- parenchyma, 12-cortex, 13-secondary phloem, 14- secondary xylem, 15-pith

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Figure 16 – Initiation of a cork cambium in the hypodermis (the layer of cells

just beneath the epidermis or outer layer of the stem). Notice the narrow, rectangular cells which is typical of a cambium.

Figure 17 – An older section of the above in which the cork cambium has been active producing a number of layers of cork to the outside (the reddish staining cells).

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Leaves As mentioned earlier, leaves are lateral appendages of the stem. They have their origin from leaf primordia which are formed in an outer layer of the apical meristem of a bud.

Figure 18 – A longitudinal section through the apical region of a Coleus stem.

SAM – shoot apical meristem region the arrows point to leaf Primordia

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Leaves do not typically exhibit apical growth except for a very brief initial period. Monocot leaves, such as grass, elongate by the activity of a meristem located at the base of the leaf. The leaf is composed of three basic regions, epidermis, mesophyll and veins. Most dicot leaves have a differentiated mesophyll, that is it is composed of two distinct regions, a region of columnar shaped cells called the palisade mesophyll and a region of irregularly shaped cells called the spongy mesophyll. The spongy mesophyll is so-called because in cross-section many air spaces can be seen within it. Pores or stomata are found in the epidermis and may be found only in one surface or in both.

Figure 19 – Cross-section of a dicot leaf. Cells of both the palisade and spongy

layer are photosynthetic while those of the epidermis, with the exception of the guard cells, are not.

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Figure 20 – Cross-section of a monocot leaf. Note that all the vascular bundles

are seen in cross-section. This is because the typical monocot leaf has parallel veination, that is the veins all run parallel to each other.

Transport in Plants Lateral Transport of Water and Minerals Uptake from the soil Water and minerals from soil move into the hydrophilic walls of root hair

cells, epidermal cells, and mycorrhizae. Pathway of transport in the root: Water in the cell walls may move through the apoplast across the

cortical parenchyma to the endodermis. The apoplast is a nonliving continuum consisting of cell walls and intercellular spaces. Water and minerals may also move from the walls across the plasma membrane into the cell where it enters the symplast. The symplast is the continuum of cytoplasm connected cell to cell by plasmodesmata. Neither water nor minerals will move exclusively in one or the other, but generally a combination of the two. However, in general apoplastic movement is greater than symplastic movement in the region of root from epidermis to endodermis. When water or minerals reach the endodermis they cannot continue into the stele via an apoplastic route because within the wall of each endodermal cell is a strip of suberin called the Casparian strip. At this point all movement becomes symplastic. Via either diffusion or active transport water and minerals move from the cytoplasm of the endodermal cells into xylem vessels and tracheids. As both of these cells are dead, this represents a movement back into the apoplast.

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Figure 21 – Diagram of lateral transport across the cells of a root. Vertical transport of water and minerals: Root Pressure – the development of water pressure in root cells can

generate a positive pressure that forces fluid up the transport cells of the xylem. The development of such root pressure occurs only under certain circumstances, does not occur at all in some plants, and can account for, at most, vertical movements of water only a few meters up the stem.

The Transpiration-Cohesion-Tension Mechanism – Transpiration is the evaporation of water from cell surfaces. There

are three plant structures associated with transpiration but stomatal transpiration accounts for ~90-95% of total water loss by a plant. The process occurs as a consequence of a pressure gradient from root to leaf.

Environmental effects: Water concentration – in general the greater the concentration of

water within the leaf the greater the rate of transpiration. Atmospheric humidity – the lower the atmospheric humidity relative to that of the leaf the greater the rate of transpiration. Temperature – in general the higher the air temperature the greater the rate of transpiration. Light intensity – insolation can produce several effects which

influence the rate of transpiration. It can warm the leaf thus increasing the rate of transpiration and it can increase the rate of photosynthesis which induces stomatal opening and thus increase the rate of transpiration. Air movement – air currents blowing across the surface of the leaf can have two opposing effects on the rate of transpiration. In can cool the leaf surface thus decreasing the rate of transpiration or it can remove water vapor from the immediate vicinity of the stomate thus increasing the rate of transpiration. Soil moisture – in general, the greater the water content of the soil

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transpiration. However it is not simple the amount of water in the soil that is important but rather the amount of available water, that is water that is not bound to soil particles.

Mechanism Transpiration rate depends on the generation of negative

pressure in the leaf. This is a consequence of the particular properties of water molecules to cohere to each other and to adhere to cell walls. As transpiration takes place and water is lost from air spaces within the leaf, water will evaporate from the thin film coating the walls of mesophyll cells surrounding the air spaces. Water will then move from cell to cell and onto the surfaces of those cells losing water. This causes a pathway of water movement back to the ends of the xylem vessels and tracheids in the vascular bundles of the leaf. As water is drawn from the surface of mesophyll cells immediately surrounding the vein, water will be drawn from the ends of the xylem transport elements into these cells which will have the effect of drawing a continuous column of water all the way up those vessels from the root. Thus water is being pulled along a gradient of water potential from the -0.3 MPa of the soil to a – 0.6 MPa of the root xylem to a -0.8 MPA of the trunk (stem) xylem to a -1.0 MPa of the leaf cell walls to a -7.0 MPa of leaf air spaces to a -10 to a -100 MPA of the outside air. Thus the movement of water and minerals from root to leaf represents a bulk flow movement, that is not only does the water move but anything in solution in the water moves along with it. No metabolic energy is expended in the process. If this process, as we have described it, is appropriate any break in the water column would bring the process to a halt. The introduction of an air bubble into the xylem stream is called cavitation and indeed does seem to halt transpiration pull in that particular vessel. However water and minerals can move laterally across to other vessels and tracheids and thereby continue their upward movement.

Figure 22 – General diagram of water transport in a plant.

Translocation of Food Organic substances, particularly in the form of sucrose, move through

the phloem sieve cells and sieve tube cells in a source to sink movement. The source is any photosynthetic region of the plant where sugar is being formed while the sink is any part of the plant where such compounds are being used, either for storage or for energy. Thus, while transport in the xylem is upward only, transport in the phloem can occur in either direction. In dicots the sugar is loaded into the sieve elements from companion cells or transfer cells. This process requires energy. Once in the sieve elements the phloem sap (mostly sugar and water) moves by bulk flow driven by the difference in osmotic pressure between source and sink. As a consequence of high solute concentration at the source water osmoses into the sieve member causing an increase in hydrostatic pressure. The hydrostatic pressure is relieved at the sink by water osmosing out of the sieve member. This causes the water to flow in the direction of source to sink carrying the sugar along with it.

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Figure 23 – Diagram of source to sink movement of sucrose.

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