518 Chapter 13 Understanding Concepts 1. What root cell activity pushes the root tip through the soil? 2. What tissue forms lateral roots? 3. What is the difference between aerial roots and pneumatophores? 4. What are the differences between annual and perennial plants? Applying Inquiry Skills 5. As part of a certain field experiment, the root systems of various types of plants were examined. (a) It was noticed that cactus roots have very shallow, widely spreading root systems. Explain how such a feature would enhance survival in desert conditions. (b) Epiphyte roots were often found exposed to the air, green in colour, and with few or no root hairs. Hypothesize how each of these features might be beneficial to such plants. Hypothesize the climatic conditions in which epiphytes would be successful. (c) Grasses and other monocots often have fibrous root sys- tems, while dicots often have taproot systems. Which root system would you expect to penetrate to greater depths, and what advantage would it provide to the plant? Explain which root system you predict would be better suited for reducing soil erosion. Making Connections 6. A homeowner plants a vegetable garden next to a large walnut tree. The garden receives lots of sunlight and she ensures it is well fertilized and watered. However, the plants fail to thrive. What is the likely cause of this problem? What options does this gardener have? Section 13.4 Questions (f) Based on your observations, what feature(s) could be used to distinguish between monocot and dicot roots? (g) Use the distinguishing criteria you described in your answer to (f) to deter- mine if the plant you observed in step 8 was a monocot or a dicot. 13.5 Stems Stems and leaves make up the plant shoot system. Stems provide support for the plant and serve as a transport link to and from leaves, roots, and reproductive parts such as flowers, fruits, and seeds. Stems may also serve to store water and carbohydrates. Some plants have herbaceous stems. Herbaceous stems are thin, soft, green, short-lived, and contain little or no wood. They carry out photosyn- thesis and thus produce some carbohydrates for the plant. Usually, herbaceous stems do not grow more than one metre tall. Exceptions include palm trees, which are not actually trees at all, but huge herbaceous monocots lacking true woody tissue. Stems that are not herbaceous are woody. Examples of plants with woody stems include grape vines, shrubs, conifers, and dicot trees (eg., oaks). During the first year of growth, woody shoots and stems and herbaceous shoots and stems closely resemble each other (Figures 1 and 2), with growth at the apical meristems increasing the shoot length. However, monocot and dicot herbaceous: describes the fleshy stems of annual plants. These stems usually do not survive more than one year, especially if there is a cold winter. They are also called nonwoody stems. woody: describes stems of perennial plants. They increase in diameter each year as more and more vascular tissue is created. The xylem cells, even after they have died, create the hard, woody tissue called wood.
Transcript
518 Chapter 13
Understanding Concepts
1. What root cell activity pushes the root tip through the soil?
2. What tissue forms lateral roots?
3. What is the difference between aerial roots and pneumatophores?
4. What are the differences between annual and perennial plants?
Applying Inquiry Skills
5. As part of a certain field experiment, the root systems of varioustypes of plants were examined.(a) It was noticed that cactus roots have very shallow, widely
spreading root systems. Explain how such a feature wouldenhance survival in desert conditions.
(b) Epiphyte roots were often found exposed to the air, greenin colour, and with few or no root hairs. Hypothesize howeach of these features might be beneficial to such plants.Hypothesize the climatic conditions in which epiphyteswould be successful.
(c) Grasses and other monocots often have fibrous root sys-tems, while dicots often have taproot systems. Which rootsystem would you expect to penetrate to greater depths, andwhat advantage would it provide to the plant? Explain whichroot system you predict would be better suited for reducingsoil erosion.
Making Connections
6. A homeowner plants a vegetable garden next to a large walnuttree. The garden receives lots of sunlight and she ensures it iswell fertilized and watered. However, the plants fail to thrive.What is the likely cause of this problem? What options does thisgardener have?
Section 13.4 Questions
(f) Based on your observations, what feature(s) could be used to distinguishbetween monocot and dicot roots?
(g) Use the distinguishing criteria you described in your answer to (f) to deter-mine if the plant you observed in step 8 was a monocot or a dicot.
13.5 StemsStems and leaves make up the plant shoot system. Stems provide support for theplant and serve as a transport link to and from leaves, roots, and reproductiveparts such as flowers, fruits, and seeds. Stems may also serve to store water andcarbohydrates. Some plants have herbaceous stems. Herbaceous stems are thin,soft, green, short-lived, and contain little or no wood. They carry out photosyn-thesis and thus produce some carbohydrates for the plant. Usually, herbaceousstems do not grow more than one metre tall. Exceptions include palm trees, whichare not actually trees at all, but huge herbaceous monocots lacking true woodytissue. Stems that are not herbaceous are woody. Examples of plants with woodystems include grape vines, shrubs, conifers, and dicot trees (eg., oaks).
During the first year of growth, woody shoots and stems and herbaceousshoots and stems closely resemble each other (Figures 1 and 2), with growth atthe apical meristems increasing the shoot length. However, monocot and dicot
herbaceous: describes the fleshy stemsof annual plants. These stems usually do notsurvive more than one year, especially ifthere is a cold winter. They are also callednonwoody stems.
woody: describes stems of perennialplants. They increase in diameter each yearas more and more vascular tissue is created.The xylem cells, even after they have died,create the hard, woody tissue called wood.
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13.5
(a) monocotVascular bundlesare distributed randomlythroughout the stem.
(b) dicotVascular bundles inthe stem are arrangedin a ring.
(i) (ii) (iii)
Figure 1
The very young stem structure of (a) a monocot and (b) a dicot(i) Drawings to show the distribution of vascular bundles in longitudinal stem sections(ii) Photomicrographs of young stem cross sections(iii) Highly magnified photomicrographs of vascular bundles as seen in cross sections
vascular bundles: collections of xylemand phloem tissue, separate from other col-lections, running longitudinally through stems
epidermissclerenchyma
epidermiscollenchymaparenchyma
ground parenchyma
phloem
phloem
xylem
pith
stele(b)
(a)
cortex
pith ray
cambium
xylemvascularbundle
vascularbundle
Figure 2
Diagrams of a small portion of the crosssections of very young stems of (a) Zea,a monocot, and (b) Ranunculus, a dicot
herbaceous plants do have different arrangements of vascular tissues within thestem. Remember, these stems seldom survive more than a year. Any lateralgrowth during that first year is still called primary growth.
However, just like their roots, the stems of many dicots, most notably trees,continue to grow year after year. Secondary growth occurs after year one andinvolves the vascular and cork cambia. As in roots, cells of the vascular cambiumdivide by mitosis to produce secondary phloem to the outside and secondaryxylem to the inside. The secondary phloem cells tend to crush the somewhatfragile phloem cells of the previous years as pressure is exerted outwards. Thesecondary xylem cells do not crush earlier xylem cells because they have thickwalls. Remember that the oldest xylem is nearest the centre of the stem; theyoungest is next to the cambium (Figure 3).
The secondary xylem thickens and forms tissue known as wood. Each year anew layer of xylem is added to this thickening core. This adds size, hardness, andstrength to the aging stem or trunk. Xylem cells produced and maturing in thespring tend to grow very large because of the moisture available. As the season gets
Figure 3
Diagrams showing a longitudinal section of atwo-year-old dicot shoot and five cross sec-tions through different regions of that shoot.Note the labels carefully. Be aware that inspite of the size of the drawing, this shootcould be very small.
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13.5
corkcork cambiumcortexpericycle
phloem
cambium
annual ring
summer wood
spring wood
pith
xylem (wood)bark
Heartwood helpsto support thetree but does notconduct waterand minerals.
Cork protects the tree.
Phloem transportssugars and other
solutes.
Vascular cambiumproduces secondary
phloem and xylem.
Sapwood conductswater and dissolved
minerals.
later and later, conditions often get much drier. The result is that the xylem cellsthat get produced and mature later are much smaller. One zone of spring xylemplus one zone of summer-fall xylem form an annual ring (Figures 4 and 5). Invery wet years, the rings are wider than in very dry years. By counting the numberof rings, we can learn the age of a woody plant. In tropical zones, growth in woodis somewhat uniform all year and annual rings do not appear.
Additional changes take place as the tree ages. No new ground tissue isadded to the tree, and the primary tissue may be displaced by secondary tissue.As in the root, the cork cambium is formed. The cork cells produced by thislayer will accumulate in layers. The actual cells die but the cell walls remain toform cork, which offers protection against mechanical damage and damage
annual ring: the increase in the amount ofsecondary xylem during one year. The numberof annual rings indicates the age of thewoody plant.
Figure 4
Drawing of part of a cross section of (a) athree-year-old woody plant, Tilia (basswood),and (b) photomicrograph of a Tilia
Figure 5
The major tissues within a woody stem.Secondary growth causes an increase indiameter each year.
(b)
(a)
522 Chapter 13
caused by bacteria, fungi, and insects. As the stem expands, the cork cracks andis renewed from the inside.
Bark is composed of everything from the vascular cambium outwards:phloem, any remaining cortex, cork cambium, and cork. If the bark is removed,the phloem will be destroyed and no further carbohydrates will be transportedto the root for storage. The tree will die, although it may be a slow death takingplace over three or more years.
As trees age, old xylem cells become plugged with various substances such asoils and resins, which prevent water and dissolved materials from moving up ordown. The oils and resins often cause the xylem tissue to darken. This darkenedregion is called the heartwood, because it lies at the heart or centre of the stem. Theliving xylem tissue surrounding the heartwood is called the sapwood and carrieswater and dissolved minerals. In sugar maples, the xylem also stores and transportscarbohydrates.This area is often somewhat light in colour. Each year some sap-wood is changed to heartwood. The fluid within any part of the plant is called sap.The trunks of trees are simply stems on a large scale (Figure 6).
bark: the outer layers on older stems,branches, and trunks. Bark consists of everylayer from the vascular cambium outwards:phloem, any remaining cortex, cork cambium,and cork.
heartwood: the older, harder, nonlivingcentral wood in tree trunks. It is often darkerdue to the accumulation of oils and resinsand its basic function is to provide support.
sapwood: the younger, softer, outer woodin tree trunks that is important for transportingwater and dissolved materials as well as forsupport
sap: the fluid within any part of a plant,found mostly within the xylem and phloemtissues
(a) (b) (c)
Figure 6
Stems of trees are the largest structuresproduced by plants.(a) Trunks of giant sequoia trees(b) Evidence that very large trees once grew
in the heart of Toronto(c) Palm tree trunks
Stem Adaptations
Just as leaves and roots are often modified to meet special needs, stems exhibit avariety of specializations (Figure 7). Cacti have thick, fleshy stems that performphotosynthesis and contain large amounts of parenchyma tissue for waterstorage. Sometimes stems are modified for asexual reproduction. For example,regular white potatoes are tubers, a form of underground stem, and the thin,reddish strawberry runners are stems, not roots. Some plants have stems whichtwine completely around other plants or structures. Members of the same speciesalways twine in the same direction. The Cecropia tree of Central and SouthAmerica has hollow stems which are home to symbiotic ants. The ants benefitfrom the protective habitat and, in turn, offer protection for the Cecropia byattacking leaf-eating insects. Have you ever wondered why birch trees havepeeling bark? Many tropical rain forest trees such as the Gumbo limbo have shed-ding bark as well. Shedding bark helps get rid of epiphytes which grow on thetree trunk and branches.
tubers: thick underground stems special-ized for carbohydrate storage and asexualreproduction
runners: thin stems which grow along theground producing roots and shoots at theirnodes
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13.5
Stems
1. Stems provide support, serve as a transport link between roots and the restof the plant, and store nutrients.
2. Both woody and herbaceous stems have primary growth.
3. Herbaceous stems are green, soft, short-lived, have little or no wood, andare usually short.
4. Primary growth occurs at apical meristems.
5. Stems of dicots, such as trees, undergo secondary as well as primary growth.
• Secondary growth occurs in the vascular cambium.
• Secondary xylem forms wood and each year a new layer is added.
• Cork cambium produces cork cells.
• Bark consists of all tissue from the vascular cambium outwards.
• Heartwood is darker, dead, older xylem that is plugged with resins and oils.
• Sapwood is lighter, live, younger xylem and functions in the transport ofwater and dissolved materials, and in some nutrient storage.
6. Stem adaptations include peeling bark, twining stems, tubers, runners, andhollow stems to house symbiotic insects.
Activity 13.5.1
Stem Anatomy
In this activity, you will identify and compare the plant tissues in stems from avariety of representative plants. You will examine whole specimens, microscopicsections, and models.
Questions
What types of tissues are found in monocot and dicot stems?How are these tissues arranged?What pattern of tissue growth results in the formation of a woody stem?
Materials
prepared slides of stem cross sections from Zea (corn), Medicago (alfalfa), andTilia (basswood)model of a mature tree cross section or an entire cross section of an actual treetrunk
(a) (b) (c) (d)
Figure 7
Modified stems(a) The fleshy bodies of cacti(b) Runners of silverweed(c) Twining stems of black swallowwort
wound around a branch of a large shrub(d) Peeling white birch trunk
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single-edged razor blade microscopemicroscope slides and cover slips fresh Coleus stemphloroglucinol stain in dropper bottles
Procedure
Part 1: Comparing Monocot and Dicot Stems1. Obtain a prepared slide of a Zea stem cross section. Observe this monocot
stem under low power. Notice that the vascular bundles are scatteredthroughout the stem. Observe a single vascular bundle under medium orhigh power. Try to identify the large xylem vessel cells—they often look likethe eyes and nose of a face. The thinner-walled phloem cells appear to theoutside of the xylem cells and are clustered together. You may be able todistinguish the sieve tubes and their companion cells. Forming a fibroussheath of ground tissue on the outside of the vascular bundle is a layer ofthick-walled sclerenchyma cells. Draw a single vascular bundle and labelthe xylem, phloem, and sclerenchyma cells.
2. Obtain a prepared slide of a Medicago stem cross section. Examine thisdicot under low power and observe the location and arrangement of tis-sues. Draw a cross-sectional “pie wedge” of the stem and label the epi-dermis, phloem, xylem, and vascular cambium tissue.
Part 2: Woody Stems3. Observe a cross section of a young Tilia stem under low power. Note the
wide inner region of xylem cells surrounding a smaller central pith. If thestem is two or three years old you will observe concentric layers of xylemtissue. Draw a simple “pie wedge” of this stem, labelling the cortex, phloem,vascular cambium, xylem, and pith regions.
4. Examine a model of a mature tree cross section or a cross section of anactual tree trunk. Attempt to count the growth rings in the xylem to esti-mate the age of the tree specimen. Record your estimate.
5. Each ring consists of a lighter band of larger cells produced during the springand a dark band of smaller cells produced during the summer of the sameyear. Compare the width of annual rings from different years.
6. Look for evidence of heartwood in your specimen or model. Where is itlocated? Try to estimate how many years of heartwood exist.
Part 3: Examining a Living Stem7. Using a single-edged razor blade, cut a very thin cross section of a fresh
Coleus stem. Prepare a wet mount of this section and observe under lowpower. Note the location of the vascular bundles. If available, use a singledrop of phloroglucinol stain for the wet mount medium. Phloroglucinolstains xylem a deep red.
Analysis
(a) Explain why there is a difference between spring and summer xylem. Howdoes this difference affect the appearance of these two regions within asingle annual ring?
(b) How old was the mature tree sample you examined? Why was the absoluteage difficult to establish? How did you obtain your estimate of the age ofthe heartwood?
(c) How can you determine if Coleus is a monocot or a dicot?
Phloroglucinol stain is toxicand can cause an itchy rash.Avoid skin and eye contact.Wash all splashes off yourskin and clothing thoroughly.If you get any chemical inyour eyes, rinse for at least15 min and inform yourteacher.
