Chapter 21: Plant Organs and Organ Systems1. Root – develops from a portion of a seed embryo called hypocotyl. They:

Anchor plants into soil Absorb water and minerals from soil Conduct water and dissolved minerals to and from the shoot

Root system – entire mass of underground roots. There are 2 types: Diffuse/Fibrous root system – consists of several long and slender roots

with numerous, smaller and thread-like roots that branch from the main roots. Monocots have this kind of root system.

Tap Root System – consists of a single main root, producing smaller, lateral roots. Dicots have this kind of root system.

2. Longitudinal Sections of the Roots Root cap – thimble-shaped mass of cells that covers and protects the root

tip. It secretes mucigel for: Protection Lubrication Water absorption Nutrient absorption

Meristematic Region – zone of cellular division Elongation Region – cells undergo enlargement in length, cells do not

enlarge above this region Maturation Region – region of differentiation

3. Cross-Sectional View of the Roots Epidermis – outer surface layer which protects and covers inner tissues,

except root cap, contains cuticle. Cortex – irregularly shaped parenchyma cells with many intercellular

spaces, for food and water storage. It has 3 layers: Hypodermis – outermost layer of cortex, lined with suberin, a

waxy substance that slows the outward movement of water and nutrients from roots.

Storage parenchyma – contains starch and separated by many intercellular spaces.

Endodermis – innermost layer of the cortex, lined with a waxy substance made of lignin and suberin called Casparian Strip. This layer prevents outward movement of water from inner tissues of the endodermis.

Pericycle – inner layer next to the endodermis gives rise to secondary roots. (The central cylinder in the root that includes the pericycle, vascular tissues and pith is called the stele)

Vascular Tissues – the xylem and the phloem. In dicots, the xylem radiates from the center and the phloem develops in the wedges between the spokes of the xylem. In monocots, the phloem and the xylem are arranged alternately within the pericycle, but surround a core of parenchyma cells called the pith.

In some plants the vascular cambium develops. It accounts for secondary growth.

4. Modified Roots Storage Roots – large and fleshy underground roots that contain a large

amount of water and starch. Adventitious Roots – develop from leaves or stems Prop Roots – outgrowths that provide support Aerial Roots – allow plants to “breath” in places with low oxygen content Contractile Roots – contact by shrinking deep into the soil, done during

droughts Photosynthetic Roots – contain chloroplasts and therefore perform

photosynthesis5. The Stem

Shoot system – includes plant parts above the root systemEpicotyl – structure that extends from the hypocotyl, where primary stem originatesNodes – areas where leaves and branches originateInternodes – areas between 2 nodes, for support, water storage and fluid transfer from roots to leaves, and vice versa.Soft/Herbaceous stems – soft and green, have small diameters and covered with epidermis, die in a yearWoody stems – tough, not green, can grow considerably in height and diameter and can live for many years

Tree – woody stemmed plant with a single trunk Shrub – woody stemmed plant with several main stems of the same size,

smaller and shorter than trees.6. External Structure of a Woody Stem

Lenticels – tiny pores for gas exchange Leaf scars – marks left by fallen leaves Buds – masses of meristematic tissues that may result in growth in length

or production of leaves/flowers. They can be: Terminal – located at tip of stem Lateral/axillary – located along side of a stem Flower bud – if it produces a flower Leaf/branch bud – if a leaf/branch id produced Naked bud – if it is not covered by leafy scales Covered bud - if it has leafy scales Active/dormant – resting

Bud scales – leafy structures that cover and protect the buds from drying out

Bud scars – marks left by fallen buds scales, form a ring around a twig.7. Internal Structure of a Woody Stem

Bark – outer layer Cork Cork Cambium – produces cork cells for protection Cortex – stores nutrients Phloem

Wood/xylem – inner part, contains the annual rings

Annual ring – ring of xylem tissues added during 1 growing season Spring Wood – made of large xylem vessels and light in color Summer Wood – made of smaller xylem vessels and dark in

colorIn dicot woody stems, such as in roots, the pith in present. Laterally extending out the pith, are vascular rays. The vascular cambium is found between the bark and the wood.

8. Structure of a Herbaceous Dicot Stem Epidermis – similar to woody stems Cork – absent Cortex – thinner but similar to woody stems Chlorenchyma cells – type of parenchyma cells, have chloroplasts and

make up the cortex Stele – same as woody stems Xylem and phloem – larger in herbaceous dicot stems than in woody

stems, separated by vascular bundles Cambium layer - present, but little growth/activity

9. Structure of a Herbaceous Monocot System Vascular bundles – scattered throughout the stem Cambium layer – absent Closed bundles – vascular bundles of a herbaceous monocot stem Open Bundles – vascular bundles in dicot stems

10. Modified Stems Aboveground Modified Stems

Runners/Stolons – grow horizontally or along the soil surface for reproduction

Tendrils/twining shoots – coil around objects to help support plant body

Cladodes/cladophylls – perform photosynthesis Thorns – protection from grazing animals Succulent stems – store large amounts of water

Underground Modified Stems Rhizomes – grow horizontally near the surface, for reproduction

and food storage Bulbs – fleshy, overlapping leaves for storage, reproduction and

adaptation during unfavorable conditions Corms – grow vertically with thin leaves on the surface for storage,

adaptation and reproduction Tubers – rounded and fleshy for storage and reproduction

11. Leaves – major sites of photosynthesis12. External Structure of a Leaf

Blade – for absorption Leaf apex – tip of leaf Petiole/stalk – attached to node of stem (leaves without petioles are calles

sessile leaves) Midrib – central vein

Leaf margin – edges of a leaf Stipules – tiny flaps of tissues, perform photosynthesis and ensure

pollination13. Variation in Leaves

Arrangement of leaves Spiral – one leaf attached to each node Opposite – two leaves attached to each node Whorled – three/more leaves attached to each node

Number of leaves attached to the petiole Simple – single leaf blade attached to a petiole Compound leaves – several leaflets attached to petiole

Pinnately compound – leaflets are attached along a central stalk

Palmately compound – leaflets radiate from a common point/petiole

Bipinnately compound – leaflets are further subdivided into smaller leaflets

Leaf venation/arrangement of veins Parallel – veins are equidistant and perpendicular to midrib Netted – main veins that rebranch to veinlets, forming a network

pattern Pinnately netted – netted venation with a single midrib Palmately netted – several main veins extending from end of

the petiole to the leaf margin Shape of Leaf Margin

Entire – smooth edge Serrate – saw toothed edge Lobed – has indentions

14. Internal Structure of a Leaf Epidermis Mesophyll – contains chlorophyll rich cells

Palisade layer – below upper epidermis, consists of vertically elongated cells

Spongy Tissues – consists of horizontally elongated cells15. Modified leaves

Bud scales – overlapping leaves that protect internal bud Bulb scales – for storage Tendrils – climbing and clinging to other object for support Spines – reduce rate of transpiration, protection from grazing animals Bracts – colorful, attracts pollinators Insect-trapping leaves – attract, capture and digest insects Reproductive leaves – produce new plants from petioles or marginal

notches