chapter 24 parte 1

8/12/2019 Chapter 24 Parte 1

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CHAPTER OUTLINERoot Systems

Origin and Growth of Primary Tissues

Primary Structure

Effect of Secondary Growth on the Primary Body

of the Root

Origin of Lateral Roots

Aerial Roots and Air Roots

Adaptations for Food Storage: Fleshy Roots

-I-h" first structure to emerge from üe germinating sedI is the root, enabling the developing seedling to irecome

anchored in the soil and to absorb water. This reflects the twoprimary functions of roots: anchorage and absorption (Figure2+l). TWo other functions associated with roots are süorage

and conduction. Most roots are important storage organs, and

some, such as those of the carrot, sugar b€et, and sweet potato,

are specifically adapted for the storage of food. Foods manufac-tured above ground, in photosynthesiúng portions of the plant,move down through the phloem to the storage tissues of theroot. This food may evenrually be used by the root itself, butmore often the stored food is digested and the products trans-porled back through th phloem to the aboveground palts. Inbiennial plants, such as the sugar beet and carrot" which com-plete their life cycle over a frweyear perid, laqge fot¡d reserves

accumulate in the storage regions of the root during the fusty:.ar. These food reserves are then used during ttrc second year

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CHAPTER

to produce flowers, fruits, and seeds. Water and minerals, orinoqganic ions, absorbed by the roots move through the xylemto the aerial parts of the plant.

Hormones (particularly cytokinins and gibberellins) syn-thesized in meristematic regions of the roots are transported

upward in the xylem to the aerial parts, where they stimulateg¡owth and development (see Chapter 27). Roots also synthe-

size a wide variety of secondary metabolites, such as nicotine,which in tobacco is transported to the leaves (see Chapter 2).

In addition, roots function in clonal regeneration (the rootsof certain eudicots produce buds that can develop into newshoots), rcdistribution of water witliin the soil (see Chapter 30),and secretion of a rr¿ast arrzy of substances (root exudates) into

CHECKPOINTsAfter reading this chapter, you should be able to answer the following:

l. Name the tvro principaltypes of root systems, and describe ho¡rlthey differ ¡n both origin and tructure-

2. Wrat changes occur to the rootcap durfng elongatfon of theroot, and what are some functions of the rootcap?

3. What tissues are found in a root at the end of primary growth,and how are they arranged?

4. Describe th€ €ffect of secondary growttr on the primary bodyof the root

5. Ii/frele do hteral roots originare and wtry are üey said to beendogenous?

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,, {i:StÉngler.fig Be$mF g t}bas a ¡{ depqf@hlglr ln a r', l

," .uee bry a t*td or monk€y, ü€ srangler fig Ffüs, Eayvafint as

.,:. an' fte and $r9n e\tnG tts lory- ryI to üre ground as [t , :

i, . srnurndiGt.ii.il Here, *re hoft lsTa hgttrn, a temfle at Angkor

,., tlfat in Carnbodia lTre growür habü sf üe qangfu fqg is an:;f:,.*h{bihntor.*n¡rqginden+bresli,rt r+ütffetsinterrse,,,, ;

csnpeti$on fu sunlfght ald nutrbntE,:,.. l... .,,-. ,-, '..,' ,'' . 1 .:r'- :'..' : .. :r

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CH A PT E R 2 4 The Root: Strusn¡re and Development'

Thd'll¡nt'lUaintain¡ a Balancg betweln lts Shogt '

andRootSystqms:...j.¡.,.'.: : ..,:.::, j. ..,,;;.r :.

In a plarit, a balance is. maintained benxeen the totdsurface anca ar¡ailable for the l rnanufacture of food: (the pho-tosynthesinng surface) and ¡he surface Er€r.:available for the

absorption of wdcr and minerals. This functionat balance,which is between the fine mots and leaf arrlq,csn.bc expressed

as a ratio, the root:shoot'ratio. In seedlings, the total water- and

mineral-absorbing surface usually far exceeds the photosynthe-

sizing surface. Hc¡q¡ever, the root:sht, ¡atio.decreases gndu-ally as a plant ?gGS.,

ff damagg to tbc rod systfm saimsly reü¡ces its absorbing sufu, shot grqnrth is reü¡ced by lack of wm, mitsals.

2+-2 Taproot and fibrous rootsystems Two tlpes of root systetns are

represented here by nro'pnirts prlmte. ' ' '

(a)Taproot s)4stem of the blazfnf star(Liatris punctato),a eudicot. fbl Fibrous

: 'foot system of wire grass (Arlsttdq. purpuru),a monocot. Each of the

vertlcat unlts deplcted here represents'

30 cent¡meters. Taproot s¡rstems. generally penetnte deeper into the soll

than fibrous root systerns.

...)

Fine roots are generally short-üveú persisting on average for just

a few months, alüough sorne rnay lir¡e much longer. The short-lived fine roots of ttees are in a state of constant flux, with death

and replacenrent ocgúrrfuig simultaneously; therefore, the flucn¡a-tion in the populatión and conoentration of rogts in the soil is as

dynamic as that of 'ttrc leaves and twigs in the air above. It has

been estimated üat,as much as 33 pereent of global annr¡al netproduaivity in terrestrial ecosystems is devoted to the productionof fine rcots. Even when plants are carefully transplante{ the bal-ance bctween shoot and root is inr¡ariably distuüed. Most of the

ñne roots are left behind when the'plant is removed from the soil.Cutting back the shoot helps to reestablish a balance betrreen the

root system and the shoot system, as does repotting a pl4qt that isroot bound into a larger container.

Origin qni,Groiilth 9{ Brimaryli¡:uesThe grourth of many rcots n Oppg*Rtly a cgntingoru proces$ ttn .stops onlf unde¡ sqcü adverse conditionq as

{¡ougtnmd

fowem-,

peratrf{E$. Durlng their grg\rü thr-oogh ¡rs so4, rgqts follm, thepaü of least resistance and fteqüently follow spaces left by w-lier rmts that have died and rotted.

The Tip of the Root ls (overed by a Rootcap,

WhichProducesMucilage,]',-,..,-,,i,,.,The motcap is a thimblelike nrass of living paredchyma cellsthat proteca the apical rneristem behind it and aids the root inits penetration of the soil (Figures 2+3 thoügh 2Á)- As ttleroot gros¡s longer and the roo{cry is pushed foflrarü alls ontbc peripbry of the rootry,rcctrEte largp arñxrnts sf nudryc



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Root hain

Muc¡Lagre/ sheath

Eudicot root A portion of a eudlcot rool showing therelatlonshlp between the rogtcap and the reg¡on of root

and (near the top) the sltes 9f emergence of the latenl roots,

arlse from deep wlürln the parent rtoot New root halrs arlse

behind the reglon of elongatlon at about the same rate as thehairs die off. The root tip ls,covered by a.rnucilage sheath,

lubrlcates the root durlng its passage through the soll.

highly hydrated polysaccharide), which lubricates the rootphssage through the soil (Figure 2+4). Eventually,

periphal cells are :released from the rootcap. Calledthese rootcap cells are programrned to separate

üe rmcap md from eacü other as t@ reacü úe root-

periphery- Oo their release- ttre bmder cells-which may

0rigin and Growth of Primary Tissues 56

,"-t.-.. .. : r.:i :.r.:(b)

24-4 Mucilage sheath on root tip fol Mucilage o.n thB rootcapof a rnaize (Zeo moys) root, contain¡ng border cells. fb) Dark-fieldimage of a living root showing a "cloud'of border cells suspendedinthemucilagesheath,whichlsnotapparentin.thlsvlew.

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remain' ¿llirc in the rhizosphere for several weeks--undergochanges,'in gene expression that enable them to iiioduce and

exude Specinc proteins thar are completely differenr'fioni thoseof thb rootcap. As the border cells are releaseü 'new'cells areadded to'the'rü)tcap. The number of border cells relerised eachday varies, in part" with the plant farnily. For example, as fs¡as l0 border cells are shed daily for tobacco (family Solanaceae), compared with as many as 10.000 for cotton (farnilyMalvaceae). Border cells and their products may contribute upto 98 percent of the weight of carbon-rich material released intothe soíl as root exudates. with rising levels of saóou dioxide inttp atmosphere and associated climue change, the role of soiin sequestering carbon is of grear interest.

Sewral fimctions harrc treen anributed to bsder cells md

ttgir exsddes. Among these functions are protwrion of üe arca

chapter 24 parte 1

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