To review -- last week, we learned that the 4 major groups of plants on the planet are: Nonvascular “bryophytes,” such as mosses and liverworts. Vascular plants, which include: Seedless plants, such as ferns and horsetails Seed plants, such as gymnosperms and angiosperms Today, we’re going to shift our focus to plant structure and function, and we’ll focus on the flowering plants (angiosperms) because they are, by far, the most diverse group with about 250,000 species. Gymnosperms (conifers) = 550 species Ferns = 11,000 species Mosses = 9500 species
Lecture outline: First, we will look at the structure -- morphology and anatomy -- of the three vegetative (non-reproductive) organs, the root, stem, and leaf. Then, we will look at how these organs function . We will look at the structure of the flower when we do reproduction.
When we speak of plant structure, we mean both the plant’s morphology (external structure) and its anatomy (its internal structure). The most important thing to remember is that the structure of a plant organ reflects its function. For example, think if you had to design an organ to capture sunlight: what features you would need to include?
Root system : Some plants have a tap (or main) root with smaller lateral roots (e.g. dandelion); in other plants, the taproot is replaced by numerous smaller fibrous adventitious roots (e.g. grasses) Function -- anchorage, storage, absorption & conduction Shoot system : more complex: Functions: photosynthesis, reproduction, conduction Stem : Nodes, where leaves and buds form, and internodes Buds : Terminal or axillary (arise at the nodes in the axil/angle of the stem and the leaf; Vegetative -- give rise to lateral branches and leaves Floral -- give rise to flowers; Mixed -- (giving rise to both branches and flowers). Leaves consist of a blade and petiole, and may have stipules at the base of the petiole.
All plants begin as herbaceous plants – they grow in height . Some plants grow in girth also – these are what we call woody plants.
This image shows a root tip -- where growth in height occurs in the root. Root cap -- protects the growing root tip as it penetrates the soil. Also the center of gravity perception. Followed by several zones : Zone of cell division -- growth initiates in APICAL MERISTEMS Apical refers to the apex, or tip, of the root. Meristem = an area of rapidly dividing cells. Zone of elongation -- where cells elongate, and 3 TISSUES begin to differentiate : dermal , vascular (central core), & ground (“stuff” of the cortex). Zone of maturation AKA the root hair zone -- where the cells are fully differentiated and become fully functional = absorption occurs.
The root hair zone occurs only on the tips of new roots, and it is very fragile – this is where absorption occurs! Seen here -- a radish seedling. Pruning the canopy reduces moisture demands on the root until new roots can grow.
Plant roots have a couple of important SYMBIOTIC relationships with other organisms. One of the most critical for life on Earth involves LEGUMES and bacteria in the genus Rhizobium that live in nodules on their roots. (other plants with other bacteria). Why is this relationship so important for life on earth ? – Neither animals, nor plants, can use atmospheric N -- a gas (N 2 ), which is 78% of Earth’s atmosphere! It must be converted to AMMONIUM IONS or NITRATES before plants can use take it up in their roots. Legumes-- and their rhizobial symbionts -- are one of a very few plant groups that can do this. They incorporate the N into organic compounds , such as AMINO ACIDS , the building blocks of PROTEINS . Why is this important information for farmers and gardeners ? -- This is why we rotate crops with legumes, such as clover, or include legumes in our garden plots. And, what do we know about the diets of many cultures -- they combine grains with legumes (e.g. wheat & peas, corn & beans, rice & soybeans). In short, WE get our nitrogen by eating N-rich plants or protein-rich animals that ate plants. The association is mutualistic -- both organisms benefit: Plant -- gets N. Bacterium -- gets C
A 2 nd important symbiotic (mutualistic) association plants have is with mycorrhizal fungi . = “fungus root” Mycorrhizal fungi help plants absorb water and nutrients, especially phosphates -- greatly extending the surface area for absorption beyond what the root hairs alone could do. Types: Ecto - 15% of species, esp. pine, oak, willow, birch, eucalyptus, walnuts; our PNW forests are “ectomycorrhizal” (Randy Molina’s talk 10-22-09) Endo - = old terminology not used by mycologists any more: the group comprises 85% of species, esp. corn, wheat, legumes; but, they are now divided into three classes: Arbuscular (shown above, the largest group) -- form balloon-like structures inside the cell, but do not penetrate the plasma membrane; do not produce mushrooms, but very large spores). Ericoid -- unique to the Family Ericaceae Orchioid -- unique to orchids (seeds require for germination and growth)
This slide shows experimentally what happens in nature: growth is greatly increased when plants are grown in soil with mycorrhizal fungi. Fossils of earliest land plants show they were associated with mycorrhizal fungi -- imagine the bare, mineral soils those first plants encountered! (Randy Molina says even non-vascular plants, that have no roots, have mycorrhizal associations (with the thallus, in general, or with their rhizoids?) Herbicides, used after timber harvest to suppress “unwanted” vegetation, kill the very understory plants that host mycorrhizal fungi -- thus, the capacity of the soil to support the next generation forest is diminished . As a consequence (and ironically), foresters commonly inoculate young seedling conifers with mycorrhizal fungi to promote vigorous growth.
Now, let’s turn our attention to the shoot – this is a longitudinal section through the growing tip of a plant. Shoots differ from roots: Nodes and internodes Lateral appendages: leaves and buds Vascular tissue on the periphery Purpose of stem vs. root = support leaves, grow lateral branches, reproduction vs. anchorage and absorption Primordium - the first, recognizable, histologically differentiated stage in the development of an organ.
This is a cross section of two herbaceous (non-woody) stems -- a eudicot and a monocot. Monocots and eudicot stems are distinguished by an important feature. Right -- monocot stem: epidermis; VBs arranged randomly in the stem, with no distinction between the pith and cortex. Left -- a eudicot stem: epidermis; VBs arranged in a cylinder around the outside, separating the ground tissue into pith and cortex. Phloem (sugar transport) Xylem (water/mineral transport) Bundle cap of sclerenchyma fibers (bast fibers made into cordage, such as jute, hemp) Stems that will eventually get woody must grow not just in height, but also in girth . They develop another meristematic region -- the VASCULAR CAMBIUM -- a ring of vascular tissue encircling the plant that forms between the phloem and xylem. The VC gives rise to growth in GIRTH = SECONDARY GROWTH , producing 2° xylem (e.g. WOOD) and 2ª phloem. This can only happen in eudicots; only they can get woody. Monocots cannot, and do not, ever form a vascular cambium; therefore, they are never woody. (palms = overlapping leaf bases)
Secondary growth is typical of woody plants, and it occurs only in eudicots. Secondary growth is produced by the VASCULAR CAMBIUM -- a lateral meristem. As the vascular cambium divides, it produces 2° xylem to the interior and 2° phloem to the exterior. Layers of xylem build up in the interior Each year is characterized by a GROWTH RING (alternating springwood and summerwood ) The buildup of xylem to the interior forces the outer layers to grow rapidly and expand: Eventually, the epidermis is replaced by mostly dead CORK tissue (protects the tree or shrub from the environment; produced by the cork cambium) Functional secondary phloem consists of just a layer or two of living cells. Together, these two layers -- cork + secondary phloem -- make up the BARK . Girdling a tree severs the bark, thus destroying the phloem and VC.
Here it is diagrammatically. Outer bark = cork, technical term = periderm. Distinguish heart- and sapwood from spring and summerwood. Ray -- vascular tissue extending horizontally throughout the wood.
Use cottonwood stems along Wallace Rd. and forsythia from backyard.
Monocots cannot develop 2° xylem because their VBs are scattered, so there’s no place for a VC to form. But, their lack of 2° xylem gives them flexibility, so they can attain impressive heights especially in tropical areas where they are buffeted by hurricane force winds. Bamboos are also monocots; basically, they are just tall grasses. They are even more flexible because of their hollow stems. Photo: Ma’alaea (Maui south shore)
Again, we see familiar tissues: dermal, ground, and vascular. What are the functions of the leaf ? photosynthesis; conduction of sugars; gas exchange (CO 2 in, H 2 O and O 2 out) What are the functions of the following structures? The cuticle (upper and lower) -- waxy layer outside the epidermis; prevents desiccation (water loss). The mesophyll (“middle leaf”) -- photosynthesis (what are the green dots? -- chloroplasts) The epidermis + stomates = pores surrounded by guard cells ; thus, the plant can control the opening and closing of the stomates; found mainly on the undersurface of the leaf; thus, note how the bottom layer of mesophyll is “spongy” allowing for gas exchange between the atmosphere and the veins.
OPEN – for gas exchange during psn. CLOSED – during heat of day, esp. in arid climates; need to conserve water, but then cannot psn. How plants cope: Morphological differences : broadleaves where water plentiful; needle-shaped, or no, leaves where arid Biochemical differences ; e.g. psn consists of two separate sets of biochemical reactions: One set needs stomates to be open to take up CO 2 . The other set uses sunlight energy to convert the CO 2 into sugar, which does not need the stomates to be open. Thus, these plants have figured out a way to keep the two sets of reactions separate – in space or in time – and only keep their stomates open when they are needed to take up CO 2 . Adjust their growing season to times when water is available: in PNW coniferous forests, growing season mainly in spring, fall; even in winter, low level of psn. occurs; basically, dormant during hot, dry summers of this Mediterranean climate.
Temperate, deciduous forests are, typically, found in areas with warm, moist summers -- the leaves of these trees can afford to have their stomates open for photosynthesis even if they lose a lot of water. In the PNW, our forests are dominated by conifers = unique among temperate forests of the world. Their needle-shaped leaves are well adapted to conserve water (small surface:volume ratio, thick cuticles, sunken stomata) during our dry summers. The trees go mostly dormant in summers, restricting their growing to fall and spring In deserts , plants either get rid of leaves altogether -- succulent stems photosynthesize and hold water; OR, the leaves are very small, covered with resins or thick, waxy cuticles. In the Tropics , leaves are large and evergreen. Thus, leaf structure reflects its function as an organ of photosynthesis, and the enormous variation we see in leaf structure reflects the kind of environment in which the plant grows.
The reproductive structure of flowering plants is -- the flower! Pedicel, receptacle, 4 whorls When we do the plant ID section in March, we’ll see the seemingly infinite variations on this general theme that characterize different plant families -- all related to fit to pollinator.
This shows a flower with one ovule in its ovary. Pollen is released from anthers; travels by wind or animal to the stigma of another flower = POLLINATION : unique to the seed plants. The pollen grain germinates on the stigma of the plant, sends a pollen tube through the style to an ovule inside the ovary . FERTILIZATION occurs when the egg unites with the sperm. The SEED is a fertilized ovule. Once the seeds mature, outer 3 whorls drop off; ovary enlarges into the fruit. Therefore, a FRUIT is an enlarged ovary containing seeds. Fruits are dispersed to new location, where seeds germinate into a new plant.
Advantages: Generates genetic diversity in the offspring – increases the likelihood that at least some of the offspring will be adapted to, and be able to colonize, new environments. Offspring are heterozygous (Aa), so the harmful recessive genes are not expressed. You’ve heard of “hybrid vigor” or “heterozygote superiority.”
Flowering plants have several mechanisms for avoiding SELF-FERTILIZATION . This slide shows a structural mechanism that promotes cross-pollination (and, therefore, avoids self-fertilization). Two different MORPHS/FORMS (PIN AND THRUM) : differences in the length of the style vs. filament. The structural differences are backed up by physiological and genetic mechanisms: Physiologically, the pollen from a pin flower cannot germinate on the stigma of a pin flower; that is, each form is SELF-INCOMPATIBLE . Both the structural differences and the incompatibility mechanisms are controlled by genes that are, themselves, linked Self-fertilization is the rule in some plants: colonizing species, such as weeds; e.g. dandelion. Show primroses!
Sometimes floral parts mature at different times = DICHOGAMY . Shown here, in Magnolia grandiflora , the stigmas mature first = protogyny If the anthers matured first = protandry .
Monoecious and dioecious plants have unisexual flowers . Dioecious = most effective mechanism to prevent selfing! Many dioecious species will not set fruit unless you plant both sexes; e.g. kiwi (but new varieties? avocado Can you think of other monoecious and dioecious plants?
Amorphophallus -- about 170 species in the Family Araceae This is not a flower, but an INFLORESCENCE : a spadix surrounded by spathe (like skunk cabbage); note leaf at bottom. Distribution -- Old World tropics and subtopics Monoecious -- very top = sterile flowers; male flowers next; female at bottom Individual flowers open and available for pollination for one day only. Most have odor of decaying flesh that attract insects (some have pleasant odors) ingenious methods trap insects inside each flower until flower is pollinated. Dichogamous -- male flowers shower insect with pollen when female flowers are closed and, therefore, not receptive; then, insect escapes and flies to another plant whose female flowers are open and males are not, enters an open female flower, and pollinates it.
Just as we saw with that angiosperm flowers are adapted for pollination by animals, so too are angiosperm fruits and seeds adapted for dispersal by animals, especially birds and mammals. It is advantageous for seeds to be dispersed at some distance from the parent so that the offspring do not compete with the parent.
One of the most remarkable features of flowering plants is their capacity to suspend growth of the embryo and go dormant until environmental conditions are suitable for germination. DORMANCY -- the suspension of growth and development -- ensures that germination will only occur when the time and place are most advantageous. GERMINATION -- the resumption of embryo growth SOIL SEED POOL/BANK – the seeds produced in any given year do not all germinate the following spring; some may remain dormant in the soil for many years = insurance against a late spring freeze, or drought, that kills off seeds that have germinated already. Allows for rapid re-vegetation after a fire, catastrophic weather event, agricultural or timber operation. Think of effects of burning at Deer Creek Park – rapid return of plants long thought extinct from the site after years of agricultural use. Oldest seed ever germinated was 2000 years old, obtained from an archaeological site in Israel and reported on Science Friday by Ira Flatow in 2005. The date palm seeds had been excavated in 1970s and stored in a drawer since then. Apparently, in ancient times this area had been covered with a forest of date palms!
In addition to the factors for germination required for all plants, some plants require additional factors, such as fire. Fireweed seed require the light that occurs when an area is burned over (or clearcut!). Chaparral plants (e.g. chamise) require fire to burn off toxins released by parents that inhibit growth of other plants, including their own seedlings.
Plants also reproduce asexually. They may be some of our best loved plants , such as tulips, lilies, strawberries. … Or our worst weeds , like bermuda grass, yellow flag iris or creeping buttercup. In native environments, asexual reproduction is typical of plants that: Are colonizers of disturbed environments (these plants also tend to be selfers) , where it’s unlikely there are other plants with which they can reproduce sexually; e.g. aspen colonizes after fire; in the RM, essentially no reproduction by seed (although germinate readily in a greenhouse); root systems may have been around since the end of the Ice Ages. Live in environments, such as the alpine tundra, where pollinators are limited .
It’s pulled by the evaporation of water off the leaves! Fancy word = EVAPOTRANSPIRATION is vital to moving water in a plant -- stomates must be open for this to happen.
Cohesion and adhesion = a result of hydrogen-bonding. If you break the tension, and thus the bonding between the water molecules, no matter how much water you give a plant, it cannot take up any more water. So, you can forget to water your garden just so long before plants are irreparably damaged and cannot recover.
We know that plants require sunlight energy to make sugars; that is, photosynthesis = an active process The transport of those sugars produced by psn. within a plant is also an active process = the plant must expend energy to move sugars from the leaves where they are made to the stem and roots. Sugar transport occurs in the phloem and movement is bi-directional : When growth begins in spring , before leaves bud out, plants move sugars stored in the roots to growing parts. In the fall , sugars produced during the summer are moved to the roots where they are stored over the winter.
To make sugars, plants need C, O, H -- obtained from the atmosphere.. Plants also need other nutrients to carry out their metabolic functions. Plants have the same macronutrients that we do + 2 more: Ca (middle lamella/calcium pectate) and Mg (center of the chlorophyll molecule). Minerals transport is partly a passive process (from the root hairs to the vascular cylinder) and partly an active process (transport into the vascular cylinder, across the endodermis).
One can diagnose a nutrient deficiency by looking for various patterns: Where on a leaf does the symptom appear? -- along the margin? Along the veins? What color is the symptom? Yellowing (chlorosis); anthocyanin pigments? Does the symptom show up in young, or old, leaves? e.g. N, P, K deficiencies show up in older leaves because these nutrients are PHLOEM MOBILE -- they can be translocated out of older leaves and transported to young leaves. e.g. In contrast, CA and B cannot be moved from older leaves, so their symptoms show up in young, new leaves
Animal development = like a symphony (perfectly timed series of stages, ends as adult); plant development = like a jazz concert (improvised, ongoing) = a consequence of their being sessile . Indeterminate growth = always embryonic regions (growing tips). Totipotency = the capacity of plant cells to de-differentiate dependent upon the internal and external stimuli they are exposed to (like animal stem cells)
The study of plant hormones is complex because their effect depends upon which part of the plant body they’re found in, their concentration, and their interactions with other hormones. Today, I will just use a couple of examples to illustrate their effects.
First, I will use auxin to illustrate how one hormone may affect several processes . A tropism -- a growth response to an external stimulus. This is an example of phototropism . Others: gravitropism, thigmotropism, Auxin is produced in growing tips. Auxin travels to the side of the stem away from light where it causes cells to elongate and grow toward the light.
Auxin produced in shoot tips also inhibits lateral buds from expanding. Remove the shoot tip, and lateral buds will begin to grow. Pruning, or nipping off buds of, plants makes for bushier plants. Conifers do not have (many) latent buds, so this doesn’t work with them (such as mugos). Giant sequoia is best example of symmetrical display of apical dominance = perfect pyramid.
Auxin may also be used as an herbicide: 2, 4-D is a synthetic auxin used to control broadleaf weeds. Most commonly used herbicide in the world; third most commonly used in the US behind…? Causes uncontrolled growth. 2, 4, 5-T, another synthetic auxin, was combined with 2,4-D to produce Agent Orange A dioxin compound was a contaminant in Agent Orange, and was implicated in the killing and maiming of many people during the Vietnam War.
We saw that auxin affects several plant processes. Some plant processes require the interaction of several different hormones . Most plants go dormant – and drop their leaves -- whenever water is in short supply -- in winter in our area; other areas = in summer (e.g. California buckeye Aesculus californica ). Used to think – leaf abscission was controlled only by abscisic acid; now know that the process is influenced by three hormones: ABA, auxin, and ethylene.
Some hormones have commercial applications . For example, gibberellins affect stem and leaf elongation by stimulating both cell division and cell elongation . Parthenocarpic -- development of fruit without fertilization.
Ormone production is influenced by environmental cues. Often guess T -- but this can fool them (warm day in January) DAYLENGTH -- the relative proportion of light to dark in a 24 hour period -- is a much more reliable cue.
This is from a birding website. Plants can detect the relative proportion of light and dark in a 24 hour period = the photoperiod.
The terminology is confusing because, initially, it was thought that plants were measuring the length of the day. Turns out, they are really measuring the length of the night. SHORT DAY plants, such as many composites, measure the night: they need long nights to flower . LONG DAY plants, such as most spring herbs, measure the night: they need short nights to flower . Simply by controlling the light regime in a greenhouse, florists can force plants to flower out of season. Pigment involved = phytochrome ; But the “hypothesized” hormone (florigen) has not been characterized chemically. It is produced in the leaves and affects shoot apical meristems.
A stunning characteristic of the angiosperms is their production of an incredible array of chemicals, such as caffeine, morphine and quinine. Why do plants do this? Originally thought to be waste products. Now know they are for defense -- remember, plants are sessile; they cannot run and hide from predators; they must defend themselves in place!
Coevolutionary “arms race.” Insects eat plants. Plants evolve chemical defense, which deters most insects. Insects specialize on a plant family; use chemicals as feeding cues. Plants evolve more chemicals (usually variations on a theme: alkaloids in lupines, terpenes in conifers).
This is an example where a species of butterfly is adapted to feeding on one species of lupine. Lupine known to contain many alkaloids. Butterfly orients by smell -- I’ve watched them hover around KL when Lupinus polyphyllus is nearby. Fossil record shows that angiosperms evolution is inextricably linked to the evolution of insects and birds who feed on these plants = leads to diversity!
These are two of the butterfly’s adult nectar plants -- native species typical of WV prairies. So, my final words to you on this section of the course is that, if you can promote plant diversity in your gardens, you’ll promote diversity in all of the insects that pollinate the plants, birds and mammals that disperse their seeds, and so on up the food chain. Happy Gardening!
Plant Structure and Function Kareen Sturgeon Professor Emerita, Biology Department Linfield College [email_address]