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Plant structure and function

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Leaf Epidermis, stomata, mesophyll, veins.

Structure anil Junction Pyactical work

Stem

Stomata. Tension in stems.

Epidermis, vascular bundles and their function.

Root Outer layer and root hairs.

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A young sycamore plant is shown in Figure 6.1. It is The region of stem from which leaves and buds typical of many flowering plants in having a root arise is called a node. The region of stem between fwo systern below the ground and a shoot above ground. nodes is the internode. The shoot consists of an upright stem, with leaves and The leaves make food by photosynthesis (p.35) and buds. The buds on the side of the stem are called pass it back to the stem. lateral buds. 'When they grow, they will produce The stem carries this food to all parts of the plant branches. The bud at the tip of the shoot is the terrni- which need it and also carries water and dissolved salts nal bud and when it grows, it will continue the from the roots to the leaves and flowers. upward growth of the stem. The laterai buds and the In addition, the stem supports and spaces out the terminal buds may also produce flowers. leaves so that they can receive sunlight and absorb carbon dioxide, which they need for photosynthesis. An upright stem also holds the flowers above the

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ground, helping the pollination by insects or the wind (p.69). A tall stem may help in seed dispersal later on (p.72). The roots anchor the plant in the soil and prevent it falling over or being blown over by the wind. They also absorb the water and salts which the plant needs for making food in the leaves. The structure and functions of the plant organs will be considered in more detail in this chapter.

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6.'l Structure of a typical flowering plant

typical leaf of a broad-leaved plant is shown in Figure 6.2a (Figure 6.2b shows a transverse section through the leafl. It is attached to the stem by a leaf stalk, which continues into the leaf as a rnidrib. Branching from the midrib is a network of veins which deliver water and salts to the leaf cells and carrJ. away the food made by them. As well as carrying food and water, the nerwork of veins for:rns a kind of skeleton which supports the softer tissues of the leaf biade. The leaf blade (or larnina) is broad and a vertical section through a small part of a leaf blade is shown in Figure 6.2c, and Figure 6.3 is a photograph of a leaf section under the microscope.

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(b) transverse section

.'ihole leaf

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Epidermis The epiderrnis is a single layer of cells on the upper and lower surfaces of the leafl The epidermis helps to keep the leafs shape. The closely fitting cells (Figure 6.2c) redtsce evaporation from the leaf and prevent bacteria and fungi from getting in. There is a thin waxy layer called the cuticle over the epidermis which helps to reduce water 1oss.

Stomata In the leaf epidermis there are structures called stornata (singular: stoma). A stoma consists o{ a parr of guard cells (Figure 6.4) surrounding an opening, or stomatal pore. Changes in the turgor (p.30) and

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or close the stomatal most dicoryledons (i.e. the broad-leaved plants; see p.278), the stomata occur only in the lower shape of the guard cells can open

pore.

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monocofyledons (i... narrow-leaved plants such as grasses) the stomata are equally distributed on both sides of the leaf. epidenrris.

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Figure 6.5 Stoma

The dctailecl mechani.sm b1- u,hich stomata opell and close is not fullr, understoocl, but it is knou.n that.

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ce11 r,'acr.roles increascs. This lorvers the rvater potentra(p. 29) of the cel1 s:rp and rvater ellters the suar:d ce1i, b,v osrnosis (p 29) fl-orrr tl-reir neiqhbouring epidernu, cells. This inflou,' of urater raises the tlrrsor pressur:

irr'iclc tlrc Slr.rltl rclli. Figure 6.4 Stomata in the lower epidermis of a leaf (x 350)

In very general terms, stomata are open during the hours of daylight but closed during the evening and

52

most of the night (Figure 6.5). This pattern, however, varies greatly with the plant species. A satisfactory explanation of stomatal rhythm has not been worked out, but when the stomata are open (i.e. mostly during daylight), they allow carbon dioxide to diffuse into the leaf where it is used for photosynthesis. If the stomata close, the carbon dioxide supply to the leaf cells is virtually cut off and photosynthesis stops. However, in many species, the stomata are closed during the hours of darkness, when photosynthesis is not taking place anyway. It seems, therefore, that stomata allow carbon dioxide into the leaf when photosynthesis is taking place and prevent excessive loss of water vapour (see pp.60 and 61) when photosynthesis stops, but the story is likely to be more complicated than this.

The cc1l u.al1 ncrt to thc stornatal pore is thicker th:r: elsel.l'rere in the cell :rnd is less able to stretch (Fi8;rrr. (r.(r). So. althor,rgh the increased tursor tencls to expalr. the rl,hole gr.rard ce11. the thick inner u,a1l cannot c\palrtjThis causes the euard cells to cluve in such a wrw that th:

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Figure 6.6 Structure of guard cells

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'. ::r: ;.qs,15\innt ions leave the gu:rrd ce1l, the nrater : '..,.11 ri:c-s. rvater passes orlt of the cells b.v osnlosis, ..r'SL)r pressllre falls arrd the guard cells straishten .-: close thc stolra. ::r'e tlte potassilur ions come- frotn and rvhat trig_ ..r:il rtrovernent into or oltt of the guar.1 cells is

The function of the palisade cells and _ ro a lesser extent - of the spongy mesophyll cells is to make food

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by photosynthesis. Their chloroplasts absorb sunlight and use its energy to join carbon dioxide and watei mol_ eculesto make sugar molecules as described on p. 35.

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In daylight, when photosynthesis is rapid, tle meso_ ..1..r' ,r.'tir c irrr esrig.rtiorr. phyll cells are using up carbon dioxide. is a result, the r -1 \\'ill notice from Figr,rres 6.5 aucl 6.6 that the concentration of carbon dioxide in the air spaces falls - ,r :el1s are thc onlv epiclemtal cells containing clrloro_ to a low level and more carbon dioxide Jiffuses in , ,\r one tintc it il,as thonglrt that the cldor:oplasts (p.26) from the outside air, through the stomata -.n slls:rr by photosl.nthesis dudng d:rylight, that the (Figure 6.7). This diffusion continueslhrough - : nlade the cell sap ntore concentrated and so causecl spaces, up to the cells which are using carbon the air dioxide. .---re:rse in turgor. In fact, little or no pl-rotosynthesis T]r1e cells are also producing oxygen as a by_product r, '-.l.lce in these chloropl:rsts and their firnction has not of photosynthesis. 'When the concentration of oxygen : r\plai11ed, though it is knou,n that starch accllntll_ in the air spaces rises, it diffuses out through Ihe r11 thellt during the hours of clarkness. In sorne stomata. .) of plauts, the gurard cells h:rve no chloroplasts. betureen the upper and lorver epidenris

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mesophyll (Figtrre 6.2c).It consists of tu.o zones: :rer, palisade rnesophyll and the lorver, spongy - -!ophyll (Fig;Lrre (r.7). The palisade cells :rre usuallv :.-.

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The water needed for making sugar by photosynthesis is brought ro the mesophyll celli bv tlie veins. The mesophyll ce1ls take in the water by osmosis (p.29) because the concentration of free water molecules in a leaf ce1l, which contains sugars, will be less than the concentration of water in the water vessels of a vein. The branching nerwork of leaf veins means that no cell is very far from a water supply.

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The sugars made in the mesophyll cel1s ar:e p:i-,sed ro the phloem ce1ls (see below) of rhe r-eins. :rnd rhese cel1s carry the sugars away from the leaf into the stem. The ways in which a leaf is thought to be well adapted to its function of photosynthesis are listed on p. 41.

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What are the functions of a the epidermis, b the mesophyll of a leaf?

2 Look at Figure 6.7. Why do you think that photosynthesis does not take place in the cells of the epidermis?

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Vascular bundles These are made up of groups of specialized cells which conduct water, dissolved salts and food up or down the stem. The vascular bundles in the roots, stem, leaf stalks and leaf veins all connect up to form a transport system

Questions

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the epiderrnis which allow the cissues inside ro take up oxygen and get rid of carbon dioide. In woody srems, the epidermis is replaced by bark, which consists of many layers of dead cells.

During bright sunlight, what gases are

a passing out of the leaf through the stomata, b entering the leaf through the stomata?

4

What types of leaves do you know which do not have any midrib?

5

ln some plants, the stomata close for a period at about midday. Suggest some possible advantages and disadvantages of this to the plant.

throughout the entire plant (Figure 6.9). The rwo main tissues in the vascular bundles are called rylern and phloern (Figure 6.10). Food substances travel in the phloem; water and salts travel mainly in the xylem. The cells in each tissue form elongated tubes called vessels (in the *yl.-) or sieve tubes (in the phloem) and they are surrounded and supported by other cells.

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Stem In Figure 6.8 a stem is shown cut across (transversely) and down its length (longitudinally) to show its internal structure.

transverse section

xylem phloem

Figure 6.9 Distribution of veins from root to leaf

Vessels longitudinaltangential Iongitudinal

section

radial section

Figure 6.8 Structure of plant stem

Epidermis 54

Like the leaf epidermis, this is a single layer of ce1ls which helps to keep the shape of the stem and cuts down the loss of water vapour. There are stomata in

The cells in the rrylem rvhich carry warer becon:. A vessel is made up of a series of long cel joined end to end (Figr:re 6.1la, p. 56) Once a regic vessels.

of the plant has ceased growing, the end rvalis of the,. ceils are digested away to form a continuous, fine tul-. (Figure 6.10c). At the same tirne, the ce1l rvalls a:. thickened and impregnated with a substance ca11e-. lignin, rvhich makes the cell wall very strong arinrpermeable. Since these lignified cell walls preve: '

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6.10 Structure of plant stem

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of water and nutrients, the cytoplasm does not atfect the passage of water in the Xvlem also contains many elongated, lignified passage

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is made, to any part of the plant which is using or storing it. Vascular bundles have a supporting function as well

a transport function, because they contain vessels, fibres and other thick-walled, lignified, elongated cells. . rubes -:,nducting cells in the phloem remain alive and In many stems, the vascular bundles are arranged in a -:r-e tubes. Like vessels, they are formed by ver_ cylinder, a little way in from the epiderrnis. This ',lunns of cells (Figure 6.Ilb, overleat). perfora_ pattem of distribution helps the stem to resisr the side.:pear in the end walls, allowing substances to ways bending forces caused by the wind. In a root, the - n-r ce1l to cell, bur the cellwalls are nor lignified vascular bundles are in the centre (Figure 6.1.2, overleaf) where they resist the pulling forces which the '.. ce11 contents do not die, although they do lose ,rclei. The perforated end wails are called sieve root is likely to experience when the shoot is being , ..:! blown about by the wind. The network of veins in many leaves supports the :m contains supporting cells as well as sieve tubes. soft mesophyll tissues and resists stresses which could :;": :rions of vascular bundles lead to tearing. .:ra1, water travels up the stem in the xrylem from The methods by which water, salts and food are ' 'rs to the leaves. Food may travel either up or moved through the vessels and sieve tubes are dis,he stem in the phloern, from the leaves where cussed in Chapter 7 (p.59). as

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Figure 5.12 Transverse section through a root (x 40). Notice that the vascular tissue is in the centre. Some root hairs can be seen in the outer layer of cells

Figure 6.'l 1 Conducting structures in a plant

Cortex and pith The tissue betu,een the rrascular bunclles ancl the epidernris is ca1lec1 thc cortex. Its ce11s often storc starch. In green ste111s, the outer cortex cel1s contain chloroplasts ancl nrake food by photosylrthesis. The centr:r1 tissue of the stenr is c:rl1ed pith. Thc cclls of the pith and corter act as packing tissues and help to sLlppol-t the stem in the sa11re \vay that a lot of blorvn-up ba1loons p:rcked tightl.v into a plastic bag u,ould form quite a rigicl

strr.rctr.rre.

Questions 'l Make a list of the types of cells or tissues you would

region of elongation

expect to find in a vascular bundle.

2

What structures help to keep the stem's shape and

upright position?

3

region of cell divisic-

What are the differences between xylem and phloem

a b

in structure, in function?

root cap

Figure 6.13 Root structure

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56

Outer layer and root hairs

The internal structure of a tvpical root is shou,n in There is no distinct epidennis in a loot. At the root tr: Figure 6.13. The rrascLrlar bunciie js in the crentre of the :rre several la,vers of ce1ls forn-ring the root cap. The.. root (Figure 6.12), Lrnlike the stem u,hcre the bunclles celis are continually replacecl as fast as they are \vol: :rrva,v u.hen tl.re root tip is pr-rshed through the soil. fornr a c,vlinder in the corlex. In a region above the root tip, rvhere the root h.' The 5.1c111 carries r,vater :urcl salts fi-om the root to just fi-otn steur stoppecl gror.r.ing, the cel1s of the outer 1ar:: bring {bocl the The phloenr rvill the stem. to the root, to provicle the root cells rvitl'r substatrces prodr-rce tirrrr, tr-rbe-like outgrorvths called root hair' (Figtrre 7.6, p.63). These can just be seen as a dou-r. lbr their elrer!l' atrd qrou,th.