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Lab 4 Flowers, Pollination and Fruit

Flowers are designed on plants for sexual reproduction. They contain organs that produce gametes (sex cells), which, after fertilization, lead to the formation of seed and fruit. Anthers found in flowers produce pollen grains, which ultimately make sperm nuclei and egg cells are produced in ovules found in the ovaries. The transfer of pollen between flowers can occur by a variety of mechanisms including wind, insects, birds, bats and even water. Consequently, flowers are constructed to attract the appropriate pollinators or, as in the case of wind pollination, to maximize the collection of pollen. After successful pollination, seed develop from ovules within the ovaries, which then mature into fruit (fruit contain seed). In today’s lab we will study the structure of a basic flower as well the diversity of floral shape, color and size in flowering plants. We will investigate the process by which reproduction occurs and study the overall shape of pollen and pollination strategies. Finally, today’s lab will focus on the structure fruit and the fruit types produced by flowering plants. 1. Floral Structure Flowers are made up of four whorls of reproductive structures borne on a fleshy platform called the receptacle. The outermost whorl is called the calyx, which is constructed of one to several leaf-like structures called sepals. The calyx is usually green, but may also be colored and difficult to distinguish from the corolla. The corolla occurs just inside the calyx and is made up of colored petals. The shape and color pattern of petals is often used to attract animal pollinators. In wind and water pollinated flowers, flowers may not have petals or they may be significantly reduced in size. Just inside the corolla are the male reproductive structures, the stamens. Stamens are made up of a long stalk-like structure called the filament and a sac-like anther which produces plooen. The center of a flower is occupied by the pistil (composed of one or more carpels). The pistil has a bulbous ovary at its base that contains the ovules, a sticky stigma, which is a receptive platform for pollen and a slender style that connects the ovary with the stigma. Pollen, once deposited on the stigma, will germinate and produce a pollen tube that grows down through the style towards the ovules within the ovary. When fertilized by pollen, ovules develop into seed and stimulate the ovary into developing into fruit. Necataries may be found at the base of the flower often producing a small pool of sugary nectar, which serves as a reward for animal pollinators.

Actinomorphic flower Zygomorphic flower

Closely related plant species often share similar floral characteristics. However, all plant species have unique flower structures that do not vary significantly between individuals within the species. For this reason, flowers and floral structures are commonly used in plant identification keys.

Although flower morphology within a species is a very conservative trait, the diversity of flower size and shape can vary tremendously between plant species. For example floral symmetry can be quite different between species. Flowers with regular symmetry that have morphologically similar petals that radiate out from the flower’s center are said to be actinomorphic. Flowers that exhibit bi-lateral symmetry are said to by zygomorphic and in contrast to actinomorphic flowers, have individual petals that may have very different shapes. The point of insertion of the floral parts on the receptacle relative to the ovary is another floral characteristic that varies between species. If the sepals, petals and stamens are all inserted below the ovary the flower is said to be epigynous and the ovary is superior. In many flowers, however, the sepals, petals and stamens are inserted above the

ovary. In this situation the flower is hypogynous and the ovary is said to be inferior. Perigynous flowers have superior ovaries but the sepals, petals and stamens fuse along their lower portions to form a cup that surrounds (but is not attached to) the ovary. This cup structure is called the hypanthium.

Dissect 3 flowers that differ in symmetry and ovary position. In the space below, sketch and label all the floral parts observed on each dissected flower.

Inflorescence Types

Solitary flowers can be produced at the terminal end of a branch or in a leaf axil. This, condition is relatively rare, however, and most plants produce flowers in groups on flowering branches called inflorescences. Inflorescence types the first flower to open and mature is at the top of the inflorescence it is said to be a determinant inflorescence. If the first flower to open is at the bottom of the inflorescence it is said to be indeterminate. The diagram below illustrates different determinant and indeterminate inflorescence types. On display in lab today are many different inflorescence types. Use this figure to help identify the inflorescence type for each flowering plant.

I

Study the plants on display in lab today. Use this table to organize your observations.

Species Flower shape/color

Flower Symmetry

Ovary position

Inflorescence type

Fruit type

Pollination

Flowers are adapted, in part, to maximize the probability of successful pollination. Consequently, floral shape and pollen structure can take on very different forms in plants that use different pollination strategies. For example, wind pollinated flowers usually do not produce showy flowers, but produce large numbers of pollen grains whereas insect pollinated flowers will use both visual and olfactory cues to attract the appropriate insect pollinator. Over 65% of flowering plants are pollinated by animals (insects, birds, bats) and co-evolution between flower and pollinator can be so strong that neither flower nor pollinator can persist without the other. Several pollination strategies that relate suites of floral characteristics to complimentary characteristics in their pollinators have been described, all of which underscore the co-evolutionary nature of this critical step in plant reproduction. The chart below describes a few of these pollination strategies (for wind, beetles, bees, butterflies, moths, hummingbirds and bats).

Some Pollination Strategies

Petals usually lacking; large amounts of pollen, often winged; large,

extensively branched stigmatic surfaces

Often zygomorphic flowers with light scent, not strong; yellow or blue flowers with UV pigments; nectar guides often present, moderate amounts of nectar

Mostly white or dull colored, heavily perfumed; open during night, nectar in

tubes with copious nectar, nectar guides absent

Vividly colored flowers, often red, open in day; abundant nectar often in tubes, no odor; flower often nodding

with no landing platform

Color usually drab or white, open at night; large amount of nectar and pollen; strong stale odor (musky), usually large flowered or in strong

inflorescences

Random and affected by gravity and wind direction. Wind

Clumsy fliers with poor vision; good olfactory development; feed on pollen. Beetles

Good fliers; diurnal; vision shifted into UV, does not see red; olfactory sense well developed; feeds on pollen and nectar. Bees

Good fliers, nocturnal; poor vision but good olfactory sense, attracted to sweet odors; feed on nectar. Moths

Good fliers, good vision especially in red, no vision in UV; olfactory not well developed; feeds on nectar and insects. Humming birds

Good fliers but large bodies, nocturnal; good vision but color blind; good olfactory development, attracted to musky odors; eats nectar and pollen. Bats

Dull or light colored with no special shape, but often bowl shaped; no nectar guides; strong fruit odor; produce copious pollen or nectar

Flower types Pollen vectors

Pollen type and Germination Pollen also can be very different between species. Shape, size and texture can all take on very important functional roles. Wind dispersed pollen is usually small and may have wing-like outgrowths to aid in their dispersal. Animal dispersed pollen may be large and nutritious to act as a reward for the pollinator. Some pollen is sticky to better attach to a visiting pollinator. Once deposited on the stigma, pollen must also contain enough energy to grow a pollen tube towards the ovules to complete fertilization.

In today’s lab we will study different kinds of pollen grains, germinate pollen grains to observe pollen tubes, and identify different pollination strategies in plants growing on campus. Germinating pollen. To observe pollen germinating you will need to collect ripe pollen grains and spread them on a fresh piece of onion epidermis. The onion epidermis acts like a receptive stigmatic surface and “tricks” the pollen into making a pollen tube.

Procedure:

• From a fresh yellow onion, cut a 15 mm wide section about 5 mm deep out of the onion

• Remove the section and peel the epidermis from the inner surface

• Place the epidermal peel on the slide, mesophyll side up (i.e. the side towards the outside of the onion) and trim to fit slide

• Brush ripe pollen on epidermis to deposit pollen

• Place slide in a Petri dish lined with a damp paper towel and cover to keep moist • Observe slide every 20 minutes to check for pollen tube formation

Make a sketch of some of the tubes in different developmental stages in the space below.

Compare pollen from different plant species. Sketch pollen grains making notes on the relative size, texture and color of the pollen grains from different species. For each type of pollen, list the properties that are related to their pollination vector (i.e. How is size related to wind pollination or to their value as a reward to pollinator? Is sticky pollen related to its pollinator type? etc.) While your pollen is germinating, identify different pollination strategies from flowers growing on campus. Use the table below to help you organize your observations. Species Color or

pattern Shape Odor nectar Nectar

guides Pollinator

Fruit

Fruits develop from ripened ovaries, but may also contain other structures such as the hypanthium and the receptacle. The ripened ovary wall is called the pericarp and can be divided into three more or less distinct layers. The endocarp is the inner most pericarp layer. The mesocarp, which is the middle pericarp layer and it can be fleshy as in the tomato or indistinguishable from other layers as in the bean pod. The exocarp is the outermost layer and forms the protective skin on a fruit. Fruit

types are highly variable and the classification of fruit is largely artificial (i.e. not reflecting taxonomic relatedness). The simplest classification schemes place fruit into three categories: simple – fruit that develop from a single ovary; aggregate – fruit that develop from multiple ovaries within a single flower; and multiple – fruit that develop from several flowers coalescing into a single fruit. Using the fruit available today for dissection, identify the different regions of the pericarp. Make representative sketches of the fruit dissections below. Make sure you label the sketches and indicate the fruit type that you are working on. Use the key to fruit types to help fill in the table below. Species Fruit type

Key to common Fruit Types 1. Fruit derived primarily from ovary of the flower………………………………2 1. Fruit derived mostly from floral parts other than ovary. Accessory fruit: calyx or receptacle fused with ovary wall; can be simple, aggregate or multiple…………..15

2. Fruit derived from a single flower…………………………………………..3 2. Fruit derived from multiple flowers whose ovaries have coalesced into a single fruit. Multiple fruit

3. Fruit derived from a single ovary…………………………………………4 3. Fruit derived from multiple ovaries………………..Aggregate Fruit

4. Pericarps flesy at maturity…………………………………………….5 4. Pericarps dry at maturity………………………………………………8

5. Pericarps fleshy throughout………………………………………..6 5. Pericarps divided into thin, skin-like exocarp; thick mesocarp; and hard, stony endocarp……………………………..Drupe

6. Fruit with leathery exocarp………………………………..7 6. Fruit without a leathery exocarp; fleshy throughout……………………Berry

7. Exocarp leathery and glandular; exocarp easily separated from inner layers………………….Hesperidium

7. Exocarp leathery to woody, not glandular; inseparable from inner layers……………………..Pepo

8. Fruit splits open when ripe (dehiscent); contains numerous seeds….9 8. Fruit doesn’t split when ripe (indehiscent); usually with only 1 or 2 seeds…………………………………………………………….……..12

9. Fruit one carpel………………………………………………..10 9. Fruit two or more carpels………………………………………11

10. Fruit splits along 1 seam….Follicle

10. Fruit splits along 2 seams…Legume

11. Fruit 2 or more fused carpels that separate, leaving a persistent partition between…………………..Silique

11. Fruit 2 or more fused carpels that split in a variety of ways………..Capsule

12. Fruits with wing-like outgrowths….Samara

12. Fruit without wing-like outgrowth…………………………..13

13. Fruit with hard exocarp; usually attached to extra material (e.g. acorn cap)…………………………Nut

13. Fruit without hard exocarp and without extra material attached …………………………………………………14

14. Fruit with one seed that is attached to pericarp at a single point …………….Achene

14. Fruit with a single seed that is completely fused with pericarp………..Caryposis (grain)

15. Simple fleshy fruit derived from inferior ovary surrounded by a fleshy hypanthium …………………………………………..Pome

15. Most of fruit derived from receptacle; individual ovaries form berries, drupes, or achenes (eg strawberry or blackberry)…..Accessory aggregate