Chapter 33: Stems and Plant Transport

Chapter 34: Roots

Stem functions

• Support – leaves and reproductive parts– Photosynthesis/access to pollination

• Internal transport• Produce new living tissue• Other jobs– Asexual reproduction– Photosynthesis– Store starch

Herbaceous dicot stems

• Epidermis – protection, cuticle (cutin)• Cortex – inside epidermis, ground tissue– Parenchyma – photosynthesis / storage– Collenchyma + sclerenchyma - support

• Vascular bundles – conduction, support– Circle arrangement– Xylem (inner), phloem (outer)– Vascular cambium btw. Xylem and phloem

• Pith – center of stem– Large parenchyma – storage

• Because of arrangement of vascular tissues in bundles, there is no distinct separation of cortex + pith between vascular bundles

• Arrangement of parenchyma btw. Bundles = pith rays

Fig. 35-17a

Sclerenchyma(fiber cells)

Phloem Xylem

Ground tissueconnectingpith to cortex

Pith

CortexEpidermisVascularbundle

1 mm

Cross section of stem with vascular bundles forminga ring (typical of eudicots)

(a)

Dermal

Ground

Vascular

Keyto labels

Monocot stems

• Epidermis – cuticle• Vascular bundles – scattered– Xylem (inside), phloem (outside)– Bundle sheath - sclerenchyma

• No distinct cortex or pith– Ground tissue – same functions as dicot stem

• No lateral meristems– Primary growth only, no wood/bark

Fig. 35-17b

Groundtissue

Epidermis

Keyto labels

Cross section of stem with scattered vascular bundles(typical of monocots)

Dermal

Ground

Vascular

(b)

Vascularbundles

1 mm

Woody dicot + conifer stems

• Secondary growth – 2 lateral meristems (replace primary tissues)– Pith in center– Vascular cambium• Secondary xylem – wood• Secondary phloem – inner bark

– Cork cambium• Cork cells + cork parenchyma• Periderm (outer bark) = cork cambium, cork cells and cork

parenchyma– Replaces epidermis

• Woody stems starts with same primary tissues in vascular bundles

• Secondary growth = many changes – Vascular cambium becomes continuous ring• Produce cells inside (sec. xylem), outside (sec. phloem)• Primary tissues become separated from each other• Sec. tissues take over functions of primary tissues

(vertical movement of substances)

Fig. 35-11

Shoot tip (shootapical meristemand young leaves)

Lateral meristems:

Axillary budmeristem

Vascular cambiumCork cambium

Root apicalmeristems

Primary growth in stems

Epidermis

Cortex

Primary phloem

Primary xylem

Pith

Secondary growth in stems

Periderm

Corkcambium

Cortex

Primaryphloem

Secondaryphloem

PithPrimaryxylem

Secondaryxylem

Vascular cambium

Fig. 35-19a3

Epidermis

Cortex

Primary phloem

Vascular cambium

Primary xylem

Pith

Primary and secondary growthin a two-year-old stem

(a)

Periderm (mainlycork cambiaand cork)

Secondary phloem

Secondaryxylem

EpidermisCortex

Primary phloemVascular cambiumPrimary xylem

Pith

Vascular ray

Secondary xylem

Secondary phloem

First cork cambium

Cork

Growth

Cork

Bark

Most recent corkcambium

Layers ofperiderm

– Lateral movement through rays = chains of parenchyma that radiate out from center of woody stem or root• Formed by vascular cambium

– Cork cambium – makes periderm (replaces epidermis)• Continuous ring or series of overlapping arcs• Different cork cambia + rates of division different

bark types• Lenticels for gas exchange• New tissues in 2 directions

– Outside – cork cells; dead at maturity; protection– Inside – cork parenchyma; store water/starch

Wood terms• Sapwood – functional sec. xylem• Heartwood – older wood, center• Hardwood – flowering plants• Softwood – conifers, lack fibers, vessel elements• Knot – embedded dead branch• Annuals rings – concentric circles in cross section– Temperate zone only – age (spring and summer

growth)– Spring wood – more water, larger cells– Summer wood – less water, narrow cells

Fig. 35-19b

Secondary phloemVascular cambium

Secondary xylem

Bark

Early woodLate wood Cork

cambium

Cork

Periderm

0.5

mm

Vascular ray Growth ring

Cross section of a three-year-old Tilia (linden) stem (LM)

(b)

0.5 mm

Fig. 35-22

Growthring

Vascularray

Secondaryxylem

Heartwood

Sapwood

Bark

Vascular cambium

Secondary phloem

Layers of periderm

Internal Transport

• Xylem– Roots only upward to stems, leaves, flower, fruits,

seeds– Water and dissolved nutrient minerals

• Phloem– Leaves to parts throughout plant– Downward or upward– Sugar in solution

Xylem transport – xylem sap

• No energy• Path of water– Soil root tissues root xylem stem xylem

leaf xylem leaf mesophyll atmosphere

• 2 mechanisms for upward flow:– Root pressure– Tension (transpiration) – cohesion model

Root pressure

• Water – osmosis – soil to root• More water = more pressure at root• Water is pushed upward

Tension (transpiration)) – cohesion Model

• Water is pulled up plant by tension (caused by transpiration pull)

• Water column can’t be broken– Cohesion = water-water (H bonds)– Adhesion = water-xylem cells (H bonds)

Phloem Transport – phloem sap

• Glucose sucrose (glucose + fructose) in solution

• Pressure – Flow Mechanisms– Source (excess sugar) sink (area of storage-

roots, fruits, seeds, apical meristems)– Pressure gradient– Sugar in leaf mesophyllcompanion cell sieve

tube member by active transport (ATP)– Then water moves from xylem to sieve tubes

by osmosis increases pressure

– At sink – sugar unloaded from sieve tube– Water follows– Decrease in pressure

Ch. 34 Roots

• Functions– Anchor– Absorb water + dissolved nutrient minerals

(nitrates, phosphates, sulfates)– storage

Herbaceous Dicot roots

• Epidermis – no cuticle, has root hairs increase absorption

• Cortex – loose parenchyma– Storage– Spaces – water path, aeration– Endodermis – inner layer of cortex; regulates

movement of minerals into xylem

• Stele – center or dicot root; central cylinder of vascular tissues– Pericycle – outermost layer of stele, just inside

endodermis; makes lateral roots– Xylem – centermost of stele; “xylem arms”– Phloem – between xylem arms

Fig. 35-14a1

Root with xylem and phloem in the center(typical of eudicots)

(a)

100 µm

Epidermis

Cortex

Endodermis

Vascularcylinder

Pericycle

Xylem

Phloem

Dermal

Ground

Vascular

Keyto labels

Fig. 35-14a2

Vascular

Ground

Dermal

Keyto labels

Root with xylem and phloem in the center(typical of eudicots)

(a)

Endodermis

Pericycle

Xylem

Phloem

50 µm

Fig. 35-15-3

Cortex

Emerginglateralroot

Vascularcylinder

100 µm Epidermis

Lateral root

321