The earth is a complex place.

Environments vary by location




•soil type

•aquatic vs. terrestrial

•winds or currents





Environments also vary in time

•shifts between ice ages and conditions like the present (glacial and interglacial periods) over past 2 million years


Environments also vary in time

•warmer than present conditions existed during the reign of the dinosaurs (135 - 65 million years ago), as well as 300 million years ago when vast swamps existed, which ultimately became rich coal beds



•Organisms of different kinds are found in nearly every corner of the earth, and fossil remains show that organisms unlike any living today have existed in the past.


•Charles Darwin, concurrent with Alfred Russel Wallace, proposed an explanation for this diversity of organisms which fits with the diversity of environments.



•In 1859, Darwin published "On the Origin of the Species by Means of Natural Selection" to describe his theory.


Natural Selection


–exists within a species

–at least some of that variation is heritable


–organisms produce large number of offspring, more than just enough to replace the parents

•Struggle for Existence

–Competition for available resources

–Since resources are limited, many die

Natural Selection

•Inheritance and Accumulation of Favorable Variations

–Variations with survival value are inherited from generation to generation

–Other variations are eliminated

•Survival and Reproduction of the Fittest

–organisms with the most useful variations (the fittest) will leave more offspring than those less fit

–favorable variations will be present at a higher frequency in successive generations, as long as they are favorable




•Darwin and Wallace did not know how traits were passed from one generation to another. The field of genetics originated with Gregor Mendel a few years later, but wasn't widely known for a while.

•We now know that variations are coded for in genes carried by DNA on chromosomes.

What is the source of natural variation, or new genes?


Gene mutations

•Defined: Change in the sequence of DNA molecules that make up genes

–a mistake may occur as DNA in chromosomes is copied in preparation for mitosis

–occurs about once in every 200,000 gene replications

–natural cause of many mutations is ultraviolet light from sun

–if mutation occurs during mitosis, that new gene may be passed only to other cells arising from that cell

–mutations may be passed on to offspring only if it occurs during meiosis (cell division to produce gametes, or egg and sperm


Fate of mutated genes

•Most mutations harmful

–cell or organism dies before it can be copied and passed on

•Some genes are neutral or "silent“

–have no effect on the cell or organism

–mutations in these will passed on with no effect

•Very few (<1%) beneficial

–These may be passed on to successive generations (in the case of reproductive mutations) at rates higher than for the non-mutated gene if they lead to enhanced survival, competition and reproduction.


•Because environments vary across the earth, a trait that is favorable in one place may not be favorable in another.

–Large leaves with a thin cuticle work well in tropical rainforest

–but would wilt and die in a desert

–and freeze on the tundra

•This has given us giant philodendrons from the tropics, spiny cacti in the desert, and the small-leaved heather in the meadows of Mount Rainier.

•Each plant is the result of natural selection over many, many generations, where traits favorable to its own environment have been passed on to successive generations. We can call these favorable traits "adaptations".

Adaptations for Protection

Chapter 5

The garden habitat

•People, not nature, dictate which plants may grow and reproduce for our gardens, within climatic limitations.

•Plant breeding is the result of artificial selection, not natural selection.

Environmental modification

•Agriculture began when people first cultivated native plants (as well as animals) for their own use.

•Seeds chosen for planting came from parents with desired traits, such as large seeds or fruits. These may or may not be traits favorable for survival in the wild, but are propagated through artificial selection.


Environmental modification

•Also, the environment (farm or garden) is modified to ensure survival and growth.



–pest and weed control

–seed dispersal (planting)

–sometimes transplanted from greenhouse to nurture most sensitive stage, the seedling

Adaptations: Modified Roots, Stems and Leaves

Chapters 5 and 6

Plant organs

•Roots, stems and leaves are the basic organs of a plant.

•All parts of a plant are some modification of one of these three parts, as a result of mutation and either

–(a) evolved through natural selection because they improved fitness, or

–(b) bred through artificial selection as humans encouraged reproduction in plants with traits they found desirable.

Modified roots

Food-Storage Roots

•sweet potatoes; yams; carrots; beets; turnips; radishes

Water-Storage Roots

•Marah, a wild cucumber vine with large root

Propagative Roots

•new shoots from same root system, appears as a new plant from above ground, but connected underground

•aspen trees all connected underground


•to get air for some submerged roots, especially important in mangroves

Contractile Roots

•pull bulb-like corm of gladiola below frost level

Aerial Roots (above ground)

•Velamen roots of orchids

•have extra tissue surrounding roots to prevent water loss

Prop roots

Adventitious roots

•Any adventitious plant organ is one growing from an unexpected position, like roots from a node on a stem

Buttress Roots

Parasitic Roots

•Mistletoe on a branch of ponderosa pine


•a symbiotic association between roots of certain species of plants (including most of our trees) and certain species of fungi

•beneficial to the fungus - gets food from the plant

•beneficial to the plant - gets extra water absorption from fungal hyphae in soil, acts to extend the roots surface area even more than root hairs


Root Nodules

•found mostly in members of pea family (soybeans, peas, beans, alfalfa, clover, peanuts, etc.), plus a few other plants such as red alder (one of our native trees)

•habitat for nitrogen-fixing bacteria, essentially provides plant with built-in fertilizer factory

Modified stems


•horizontal stems that grow below ground

•may or may not be swollen for food storage

•common in iris, ginger


Runners and Stolons

•more or less horizontal stems that grow above the soil surface

•may produce new plants at nodes or where tips touch the soil

•found in strawberries, spider plants


•stem tuber is the swollen tip of a rhizome for food storage

–example: common potato

•root tubers are found in other species (dahlia, sweet potato)


•Short, flattened stem with fleshy food-storage leaves

•Examples of bulb producing plants

–lilies, hyacinths, tulips



•Fleshy stem with papery scale-like leaves

•Examples of corm producing plants

–crocuses and gladioli



•Flattened stems with leaf-like appearances

–example: prickly pear cactus

Succulent stems

•water and food storage


•Modified short branches arising from axillary buds, stiff and sharp for protection are called thorns

•different from spines and prickles (other plant organs)

•note the origin of the part to tell the difference

Modified leaves

Sun vs Shade Leaves

•sun leaves form in sunny part of canopy (upper picture)

•thick palisade layer forms to maximize photosynthesis where light is best

•shade leaves thinner and have fewer hairs

•one cannot develop into  the other

Leaves of Arid Regions

•Dense hairy coverings

•Thick, leathery leaves

•Few stomata (or sunken stomata)

•Thick cuticle

–Conifers have all but first


•Modified leaves or leaflets

•Coiled structures

•Function in support

Spines, Thorns, and Prickles

•Spines (modified leaves) ex. cactus

–Reduces water loss

–Protection from herbivores

–Sometimes spines are dense enough to shade the plant


Spines, Thorns, and Prickles

•Thorns are modified stems (see above)

•Prickles (neither leaf nor stem)

–Outgrowths from epidermis or cortex

–Roses, blackberries and raspberries as examples


Storage Leaves

•Succulent leaves, retain water in thin-walled parenchyma

•Fleshy leaves (onion, lily bulbs); store food

"Flower Pot" Leaves

•Urn-like pouches that become home to ant colonies

•Ants carry in soil and add waste products

•Urn-like "flower pot" then produces adventitious roots

Window Leaves

•Plants of Kalahari Desert (South Africa)

•Leaves shaped like  cones buried in the sand

•Dime-sized top of the "cone" is exposed at the surface

•Transparent epidermis allows light to enter the "window"

•Photosynthesis occurs along inside edges of leaf

•Adaptation to the drying winds of the desert

Reproductive Leaves

•produce plantlets from tips (vegetative reproduction)

•Kalanchoe plant, cactus relative

Floral Leaves (Bracts)

•Found at base of flowers or flower stalk

•Poinsettia, dogwood, examples

Insect-trapping Leaves – Pitcher plant

Insect-trapping Leaves - Sundews

Insect-trapping Leaves – Venus’s flytrap

Insect-trapping Leaves – Bladderworts

Hydrophyllic leaves

•aerenchyma in petioles for leaves to float

Adaptations for protection

Protection from freezing


–whole plants (or in annuals, seeds) go dormant

•Deciduous leaves

–remove parts where damage most likely to occur

•Protect meristems

–bud scales and cork tissue for insulation


–if leaves are persistent, cells take on high concentrations of sugars and other substances to depress the freezing point

•Cold hardening

–evacuate water from within cells through leaky cell membranes

Protection from drying out

•See above under modified leaves

Protection from herbivory


–spines, thorns, prickles


–Living stone, or pebble plants

–Can you find five livingstone plants in this picture?

Ants as protection

•Mexican Bullhorn Acacia

–ants live in swollen thorns of acacia

–feed on nectar and protein-rich Beltian bodies produced by acacia

–in return, ants protect acacia from attacking insects or competing plants

Wound healing

•Epidermis and cork are normal barriers to water loss and infection

–tannin present in cork acts as natural fungicide

•Injury to an herbaceous stem

–exposed cells collapse and die

–waxes seal the wound

•Injury to woody tissue

–callus tissue (another form of parenchyma) first grows over wound

–eventually the nearest cambium will produce new cork, phloem and xylem tissues to grow over wound

Wound healing – fire scars

Wound healing (con’t)

Contaminated tissues are isolated

•fungi cause the majority of plant diseases

•interconnected phloem and xylem cells provide ideal habitat for growth of fungal hyphae

•to prevent spread, a substance called callose is produced to plug holes in sieve tube plates of phloem

•barriers of other substances isolate infected from healthy tissues

–resin in conifers

–gums in Acacia and Eucalyptus

–latex from Fig and Euphorb families

Chemical Defenses

Two basic parts of a plant's biochemistry:

•Basic metabolism, same for all plant species



–synthesis of cellulose, starches, proteins, lipids

•Secondary plant products

–variable among species

–include many defensive chemicals

–two main categories: tannins and alkaloids


•found in leaves, bark, unripe fruit, heartwood, roots

•easily bind with proteins to disable them

•in living tissues, plants store tannins in isolation to prevent damage to its own tissues

•gives unripe fruit astringent taste

•desired in tea, wine, cocoa

•used to "tan" animal skins

•deter insects, inhibit growth of fungi and bacteria


•bitter or toxic to many animals, prevents herbivory (animals eating plants)

•some used for traditional medicines, and as origins to at least 25% of modern pharmaceuticals

•pain relievers, cardiac and respiratory stimulants, muscle relaxants, blood vessel constrictors, cures for malaria, pupil dilaters



•caffiene in coffee and tea

•nicotine from tobacco

•cocaine from coca plant

•morphine and heroine from opium poppy

•many hallucinogens


Alkaloids in Solanaceae

•Solanine in members of family Solanaceae

•all but fruit and seeds of tomato plant



•green peppers

•deadly nightshade, source of belladonna

Alkaloids in Brassicaceae

•substance that impairs thyroid function

•blocks uptake of iodine

Other plant defense mechanisms


–leaves and stems contain crystals of calcium oxalate

–if eaten cause swelling and pain in mouth and throat, loss of speech

•Odors or tastes to deter animals

–acrid, stinging, or hot and peppery

–doesn't prevent human herbivory

•Accumulation of minerals from soil

–copper, lead, cadmium, manganese, selenium, nitrates

–toxic to animals in sufficient quantities

•Lignin added to cell walls in woody plants for strength is indigestible to most animals