11 Chapter 11
LAB 11
Reproduction in Flowering Plants
Prepared by Jason R. Jones, Modified by J. Ray
University of North Alabama
OBJECTIVES
After completing these laboratory activities, you should understand/be able to:
- Define angiosperm, flower, pollen, fruit, ovule, seed, perfect flower, imperfect flower.
- The following flower parts and their functions: anther, carpel, filament, ovary, ovule, petals, receptacle, sepals, stamen, stigma, style.
- The botanical distinction between fruits and vegetables.
- The difference between simple fruits, aggregate fruits, and multiple fruits, and be able to give examples of each.
- The main mechanisms of how seeds are dispersed (wind, animals, and water), and some examples of each.
- The importance of seed dispersal.
- Why pollination is advantageous for angiosperms.
INTRODUCTION
The angiosperms (flowering plants) are the most diverse plant group, with 300,000+ species comprising 95% of all vascular plants, and 90% of all plants as a whole. The success of angiosperms can be directly attributed to their unique reproductive strategy that utilizes structures not seen in any other plants: flowers, pollen, and fruits. In these lab activities, you will learn about reproduction in angiosperms, and familiarize yourself with the anatomy of flowers, different types of fruits, examine the differences in anatomy and development of monocot and eudicot embryos, methods of seed dispersal, and the process of pollination, as well as the anatomy of a pollen grain.
STATION I: Flower Anatomy and Dissection
Examine the basic anatomy of a flower using a model of a “typical” flower, but also through dissection of a flower. Flowers represent the reproductive parts of flowering plants, and are essentially modified leaves. Depending on the species of angiosperm in question, (1) perfect flowers contain both male and female reproductive organs, (2) imperfect (unisexual) flowers have male and female reproductive organs on separate flowers or (3) monoecious species are either male or female (on separate individual plants). The flowers you observe and dissect today, are perfect flowers. Compare Figure 1 below to the flower model in lab. Identify all named structures on the plant model as you can. Know the functions of the flower parts on the following pages, as this material will likely be on your quiz next week.
Figure 1. Anatomy of a generalized flower.
Flowers are highly modified leaves. Non-reproductive regions called the sepals and petals emerge from the receptacle. One whorl of leaves is modified into sepals, which protect and support the flower. Sepals are often green, but may be other colors and look very similar to the petals. Another whorl of leaves form the petals. In animal-pollinated plants, petals are brightly-colored to attract pollinators. Some flowers may also have markings that reflect light waves in the ultraviolet (UV) spectrum, which we cannot see but are visible to some pollinators, even at night. Animal-pollinated flowers often have a nectary, a region containing a sugar-rich liquid, which serves as a reward for pollinators.
The reproductive structures of flowers includes the male portion called the stamen, which consist of several distinct parts: a stalk-like filament, which is topped by an expanded region called the anther. The anther contains several pollen sacs, which themselves contain cells that divide by meiosis and eventually develop into the pollen grains.
The female reproductive portion is the carpel, which includes several distinct structures. The ovary is the expanded base of the carpel, and contains ovules, which, after fertilization, become seeds. From the top of the ovary extends a stalk-like structure called the style, which supports at its top the stigma, which receives pollen grains during pollination.
When pollen (~sperm) is carried by animals from one plant to another, pollination occurs. If the pollen reaches the ovule, double fertilization occurs, in which one sperm cells fuses with the egg cell to form an embryo, and the other sperm cell fuses with embryo sac to form the endosperm, which serves as a source of nutrients for the embryo. The embryo and endosperm (the seed) are contained within the ovary.
Flower Dissection
Dissect one of the provided flowers; identify as many structures as you can. Make a cross section through an anther, and observe it under the dissecting microscope. See if you can identify the pollen sacs and possibly pollen grains inside the anther. Make a longitudinal section through the carpel of your flower, and examine it under your dissecting microscope. See if you can see any ovules inside the ovary.
*Answer the questions on the worksheet at the end of this lab exercise relating to the above material*
STATION II: Simple Fleshy Fruits
After fertilization, the ovary, now containing one or more seeds, develops into a fruit. A fruit, defined botanically, is the seed-bearing structure formed from the ovary. Fruits function to ensure seed dispersal from the parent plant.
Fruits can be classified as simple fruits, (formed from a single ovary of a single flower), aggregate fruits, (from multiple ovaries of a single flower), or multiple fruits (from the fusion of multiple ovaries on multiple flowers). In the case where other structures fuse with the ovary and become part of it, they are called accessory fruits.
Simple fruits can be further subdivided into two main classes: fleshy simple fruits and dry simple fruits. Fleshy simple fruits are fruits in which the ripened wall of a flower’s ovary, and any accessory parts, when applicable) develop into soft, succulent tissue. At this station, you will learn about different types of fleshy simple fruits, and observe examples of them.
To help you learn and understand more about how fleshy simple fruits are classified, it is important to learn about the different layers that make up the pericarp (outer portion) in a fruit.
Outer: exocarp, which forms the outer skin of the mature fruit.
Middle: mesocarp, typically the thickest layer of the pericarp in fleshy fruits, and usually the part that is eaten. Inner: endocarp, which directly surrounds the seeds.
Figure 4. A fleshy fruit illustrating the layers of the pericarp (derived from the ovary), and parts of a seed (derived from an ovule).
Fleshy simple fruits are classified based on the characteristics of the layers of the pericarp (exocarp, mesocarp, and endocarp), as well as the number and types of seeds found within them.
Types of fleshy fruits:
Pome, an accessory fruit with an ovary whose pericarp (at least partially) forms a tough core (usually discarded) which contains the seeds, and fleshy tissue derived from the receptacle of the flower.
Example: Apple.
Figure 5. Note the core of the apple is derived from the pericarp (ripened ovary wall). Though not labeled, the darker line forming the outer boundary of the core is the exocarp. The fleshy portion of the apple that is eaten, including the skin (labeled “Floral Parts” in this figure) is derived from receptacle tissue.
Drupe, a fleshy simple fruit with a succulent exocarp (skin) and mesocarp (flesh), with a single seed surrounded by an extremely hardened endocarp. Olives and the peach shown in Figure 6 are drupes.
Figure 6. A peach (a drupe) showing the fleshy exocarp and mesocarp, with the seed being surrounded by the stony endocarp. Collectively, the endocarp and enclosed seed of a drupe are referred to as the “stone” or “pit” of a drupe.
Other drupes include almonds and coconuts. An almond is actually only the seed of the fruit, surrounded by parts which are not are eaten. In the unripe almond fruit in Figure 7, note the pale seed (which is eaten) in the middle of the split almond, and the layers surrounding the seed, which are not eaten.
Figure 7. An almond fruit (a drupe) showing the edible seed, and endocarp, mesocarp, and exocarp, which are not eaten.
Coconut. In coconuts, the exocarp and mesocarp of are fibrous, and forms the “husk”. The edible white “flesh” of the coconut is actually the endosperm from inside the seed, which is surrounded by the hardened “shell”, consisting of the seed coat of the seed itself, bounded to the outside by the stony endocarp. See Figure 8 below, and see if you can identify the exocarp, mesocarp, endocarp, seed coat, and endosperm of the coconut.
Figure 8. A coconut palm and its fruit (a drupe).
Berry. A fleshy fruit that develops from a single flower with a single ovary, containing one or more seeds, in which the entire pericarp develops into potentially edible tissue. Many fruits that have “berry” in their names (such as strawberries, raspberries, and blackberries) are not actually berries. Grapes are a classic example of berries (figure 9). These grapes differ from the grape in the diagram in that they are seedless. Seedless fruits cannot, by definition, reproduce sexually, and thus seedless varieties of fruits are produced by vegetative propagation or through genetic engineering.
Figure 9. Anatomy of a grape (a berry).
Pepo. A modified berry which develops from a single ovary on a single carpel (not formed from fused carpels) with multiple ovules, in which the exocarp forms a relatively hard outer rind. Examine the provided examples of pepos.
Hesperidium. A modified berry which develops from multiple carpels that fuse. The endocarp is thick and fleshy, and is divided into segments, indicating the boundaries between fused carpels. The mesocarp typically develops into a white pith, with the exocarp developing into a leathery rind.
Figure 10. An orange (a hesperidium), showing the thick outer leathery rind (exocarp), white pithy mesocarp, and a fleshy endocarp divided into segments separated by septa between fused ovaries. Credit: Ekko on Wikimedia Commons (http://en.wikipedia.org/wiki/Fruit_anatomy#/media/File:Orange_cross_section_description.png)
STATION III: Dry Simple Fruits
A dry simple fruit is formed from 1 pistil (which may consist of multiple fused carpels), and whose coat is dry at maturity. Dry simple fruits can be split into two main groups based on whether they naturally split open at maturity to release the seeds (dehiscent dry simple fruits), or do not naturally split open at maturity to release the seeds (indehiscent dry simple fruits).
First, we’ll start with dehiscent dry simple fruits. Again, these are dry simple fruits that naturally split open at maturity to release these seeds. Some examples of dehiscent dry simple fruits include legumes, capsules, and follicles, which, with some other types of dehiscent dry simple fruits can be seen below in Figure 11.
Figure 11. Examples of several types of dehiscent dry simple fruits (dry fruits that split open at maturity to release seeds).
Legume. A dry simple fruit that develops from a single carpel, and which splits along two seams at maturity. Peas and beans are classic examples of legumes. Peas are the actual seeds, containing the embryos of developing pea plants.
Capsule. A dry simple fruit that develops from a single made up of several carpels, which may split along multiple seams representing the boundaries between fused ovaries in the pistil.
Follicles. Dry simple fruits that develop from one carpel, and which split along a single seam at maturity to release the seeds. We do not have a physical example of a follicle, but the fruits of the milkweed plant are good examples of follicles. Examine the laminated picture of a milkweed fruit, and see if you can identify the single seam along which the fruit splits.
Indehiscent fruits are simple dry fruits that do not naturally split open to release the seeds at maturity. Examples include achenes, nuts, samaras, caryopsis, and others shown in Figure 11.
Figure 11. Examples of indehiscent dry simple fruits (dry simple fruits that do not split open to release seeds at maturity).
Achene is a dry fruit with a single seed which is attached to the fruit wall at a single point. Examine the provided example of an achene.
Nuts have a single seed, but with a fruit wall that is much thicker and harder than that seen in the achene. Some fruits with “nut” in the name, however, are not true nuts, but are technically drupes or other fruit types.
Samara is an indehiscent dry simple fruit that contains one or two seeds, with part of the pericarp (fruit wall) expanded into a wing-like structure. These seeds of these types of fruits are typically dispersed by wind, with wind currents catching the wing-like portion of the fruit, often carrying it considerable distances. Several trees, including elms, ashes, and maples produce this type of fruit. Examine the provided maple samaras. Take one and, holding it at shoulder height, drop it, noting its movement as it falls. Answer the question on the worksheet at the end of this lab exercise. (HINT: The questions on achenes, nuts, and samaras may be easier to answer after you have visited Station VI!)
Caryopsis. A one-seeded fruit in which the wall of the fruit and the seed coat of the seed inside are completely fused. This type is common in many monocots, such as grasses, corn, wheat, etc. Examine the provide examples of grains.
STATION IV: Aggregate Fruits, Multiple Fruits, and Fruit Identification
At this station, you will examine some examples of aggregate fruits and multiple fruits, as well as use what you have learned about simple fleshy fruits, simple dry fruits, and aggregate and multiple fruits to identify several fruits as to their types.
Raspberries. An aggregate fruit, which is a fruit made up of multiple separate ovaries on multiple separate pistils within a single flower. Figure 12 shows that each individual ball-like unit of a raspberry is actually a small single drupe, with a single raspberry itself consisting of numerous such small drupes, formed from multiple individual ovaries on the same original flower.
Figure 12. A raspberry flower (left), showing multiple carpels (each containing a single ovary), and a raspberry fruit (aggregate fruit) formed from the fusion of multiple individual drupes, each formed from a single ovary. Technically, a single raspberry consists of numerous tiny fruits held together by the receptacle of the original flower.
Pineapple. A multiple fruit, which is a fruit formed from carpels on multiple individual flowers, often fused together with other accessory structures on each flower. See Figure 13.
Figure 13. Pineapple (a multiple fruit). Each scale-like/plate-like structure on the surface represents a single fruit formed from a single carpel of a single flower. Thus, an entire pineapple is formed from multiple fruits formed from multiple flowers. The “core” of a pineapple is formed from the peduncle, or the stalk to which all of the flowers were attached. On the right is a single fruit from a single pineapple flower. Note the bract, which is a specialized leaf, often associated with a reproductive structure. The flesh of each pineapple fruit is composed of ovary tissue. The individual fruits of a pineapple are technically berries.
Identifying “Mystery” Fruits
After the raspberries and pineapple at this station, there are several other fruits in trays at this station. In the worksheet at the end of this lab exercise, make a list of all of these “mystery” fruits in the provided table, and using what you’ve learned about fleshy simple fruits, dry simple fruits, aggregate fruits, and multiple fruits, and try to see if you can identify the specific type of each of the provided fruits.
STATION V: Embryonic Development in Monocot & Eudicot Seeds
Angiosperms differ in the anatomy of their embryos, as well as in the development of those embryos. Monocots and eudicots differ in terms of the number of their embryonic “seed leaves”, which are called the cotyledons. Monocots have a single cotyledon (‘mono-‘, one). Eudicots have two cotyledons. Cotyledons are not true leaves, because in most angiosperms, the cotyledons are not capable of photosynthesis. Instead, the cotyledons serve to access the stored nutrients (endosperm) inside of the seed, providing those nutrients to the embryo until its first true leaves (two in eudicots, and one in monocots) emerge from the ground and begin photosynthesizing.
The seed is surrounded by a seed coat, and the embryo is housed inside the seeds of each. The embryos of each group also have several features in common.
Figure 14. Embryonic development in eudicot (bean) and monocot (corn) seeds.
At this station, you will dissect both a germinating monocot (corn) and eudicot (bean) seed, each containing an embryo. The best way to do this is to simply use a scalpel or razor blade to slice through the corn seed in a longitudinal section right through the middle of the seed. For the bean seed, you can probably actually slide the seed coat off of the seed with your hands, and then split the bean manually along its midline, separating the two cotyledons.
After dissecting each of these seeds, compare them to Figure 14, as well as the provided models, and see if you can locate and identify all of the structures mentioned in bold above, as well as shown in Figure 14. You will also note that at both the monocot and eudicot dissection stations, you have been provided with a dropper bottle of Lugol’s iodine solution, which can be used to test for the presence of starch. Normally, this solution is amber/dark orange in color. However, in the presence of starch, this indicator turns darker, often becoming bluish-black in color. After you have dissected each seed, use the provided Lugol’s iodine, and place a single drop of it on the interior surface of each seed, and note your observations on the worksheet at the end of this lab exercise. (NOTE: Careful with the Lugol’s iodine– it will stain your skin and clothes!)
STATION VI: Seed Dispersal
One major advantage provided by the unique reproductive method of angiosperms (production of flowers and eventually fruits) is that fruits (containing seeds) provide an opportunity for seed dispersal, or movement of fruits (with the seeds and embryos within) away from the parent plants that produced them. You might be wondering why this might be such an advantage, but a little bit of thought about basic plant biology can provide us with several lines of insight. First, consider the fact that (for the most part), plants are pretty much confined to the environment in which they sprouted. If a parent plant’s seeds fell very close to the original parent plants (which would be larger than the developing seedlings), the developing offspring would be in direct competition with their parents for water, nutrients, and light. In such a case, the larger parents would be at an advantage with their larger size, and would likely outcompete their own offspring for all of the above resources. As a result, the fitness of the parental plants (likelihood of survival of their offspring to eventually produce their own offspring, perpetuating their parents’ genes into future generations) would be reduced.
Additionally, what if the parental plants originally germinated and grew in an environment that was less than ideal, but more suitable habitats could be found some distance away? In both scenarios, the ability to disperse their offspring to other locations is beneficial to both the parents (due to reduced competition) and the offspring (additional reduced competition, as well as the possibility of landing in habitats superior to those of their parents).
In examining the fruits (and enclosed seeds and embryos) of angiosperms, we see a wide variety of adaptations that allow the embryos within the seeds (within the fruits) to be dispersed to areas of varying distances from their parents. Given the diversity of angiosperms, the diverse range of modifications to fruits that allow for various methods of seed dispersal should come as no surprise. However, there are several main mechanisms of seed dispersal that can be observed using different morphological features of fruits themselves: dispersal by wind, dispersal by animals, and dispersal by water. Some less common (but still observed) methods of seed dispersal include dispersal by explosion, as well as dispersal by gravity. Figures 15-17 below and on the following pages provide some examples of seed dispersal mechanisms, as well as examples of plants that utilize those methods of dispersal.
Figure 15. Examples of seed dispersal by wind.
At this station, you should observe all of the provided fruits (again, containing seeds, which contain plant embryos), as well as read about the methods by which they are dispersed. After observing each of the example fruits, answer the questions at the end of this lab exercise.
Figure 16. Examples of seed dispersal by animals.
Figure 17. Examples of seed dispersal by explosion.
STATION VII: Pollination and Pollen Tube Growth
Pollination is another advantage to angiosperms in their reproduction. Pollination refers to the process of transfer of pollen from the anther of a flower to the stigma of a flower. Some plants have the ability to self-pollinate, when pollen from a flower on one plant is deposited on the stigma on a flower on the same plant. However, some plants exhibit what is called self-incompatibility, meaning that they are unable to pollinate themselves. At its heart, however, pollination can be advantageous when pollen from one plant is deposited onto the stigma of another plant of the same species, or cross-pollination. Cross pollination is advantageous, as it is a form of sexual reproduction, which, as we have learned, introduces genetic variation into populations. Remember, in sexual reproduction, essentially genetic information from each parent is being randomly shuffled together into each offspring produced, providing for the opportunity of numerous new combinations of genes, any of which may provide advantages in a given stable (or often changing) environment.
The pollination method(s) employed by angiosperms can often be deduced from the structure of their flowers. Wind-pollinated flowers tend not to be very showy, as they don’t need colors, flashy structures, or nectars to attract animal pollinators. However, wind pollination tends to be very hit-or-miss, so wind-pollinated plants tend to produce LOTS of pollen.
Plants that are animal-pollinated vary in terms of their flower characteristics, based on the identity and biology of their primary pollinators. For example, plants pollinated by diurnal (active during the day) insects tend to have brightly colored flowers, often with markings that reflect UV light (invisible to humans, but visible to many insect groups). Insect-pollinated plants also tend to have highly fragrant flowers, which attract insects, who have finely-tuned chemical senses. These fragrances are not always very pleasant, however. For example, fly-pollinated flowers often have fragrances similar to rotting meat, which attracts flies that typically feed on dead and decaying organisms. Insect-pollinated flowers also often have nectar, which provides pollinator insects with a sugary “reward” for transferring their pollen, and the structure of insect-pollinated flowers is often such that to obtain a nectar reward, the insect has to crawl into the flower, and thus get coated in lots of pollen which can be transferred to other plants of the same species when those pollinators visit those individuals’ flowers.
Bird-pollinated flowers are also typically showy and bright in coloration, to attract their highly visual avian pollinators. Bird-pollinated flowers also often have lots of nectar rewards, but may not necessarily be very fragrant, as the sense of smell of birds is often comparatively poor.
Flowers that are pollinated by nocturnal animals such as moths and bats tend to be white and ghostly in color, to provide contrast during the dark environment at night, and also tend to be heavily scented, as well as contain abundant nectar, as both moths and bats have strong chemical senses. Bat-pollinated flowers also tend to have very distinct shapes, which form relatively unmistakable “audible pictures” of their anatomy when “viewed” by bat echolocation. Examine Figure 18 for just a small sample of pollination by animals.
Figure 18. Two examples of animal pollination in angiosperms.
At this station, you will make microscopic observations of pollen grains by making wet mount slides of pollen that has been incubated in a solution of sucrose and boric acid, both of which stimulate the growth of pollen tubes. Note, pollen tube growth in this lab has been pretty hit or miss over the years, depending on the species used, as well as varied experimental concentrations of both sucrose and boric acid. You may not be able to observe pollen tube growth, but you should definitely be able to make microscopic observations of pollen grains themselves.
After making and observing a wet mount of the provide pollen grains in solution, answer the questions on the worksheet at the end of this lab exercise. For a bit of help/context, you may wish to refer back to the information regarding angiosperm life cycles covered under Station I.
BI 102 Lab Worksheet: Plants II Name _________________________________ Section _______
STATION I: Flower Anatomy and Dissection
1. Carefully dissect a flower and identify the parts. Sketch the basic parts of a flower below. Have your
instructor approve your dissection by initialing here _____.
2. What is the main function of flowers in angiosperms?
3. Which part of the flower produces the pollen: Male or Female?
4. The 2 main parts of the male portion of the flower:
______________________, _______________________
5. The 4 main parts of the female portion of the flower:
___________________, _____________________, ___________________, _____________________
STATION II: Simple Fleshy Fruits
6. What part does the fruit develop from? _________________________________
7. What part does the seed develop from? _________________________________
8. Which fruit is normally thought of as a “bean”, but is actually the fruit of a berry?
9. Look at one of the cross sections of one of the provided hesperidia. Below, name the hesperidium you chose, and give the number of visible segments (representing the number of fused carpels) in that fruit.
STATION III: Dry Simple Fruits
10. Based on what you know about the provided achene and nuts, how would you guess that the seeds of these fruits are dispersed?
11. What is advantageous about the wing-like shape of the samara fruits of maples, ashes, and elms?
STATION IV: Aggregate Fruits, Multiple Fruits, and Fruit Identification
12. Fill in the table below with information about all of the provided “mystery” fruits.
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Fruit Name |
Simple Fruit (Y/N) |
If Simple, Fleshy or Dry? |
If Simple, Specific Type? |
Aggregate or Multiple? |
If Aggregate or Multiple, Individual Fruit Type? |
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STATION V: Embryonic development / Seed Germination
13. Is corn a Eudicot or Monocot? Is a bean and peanut a Eudicot or Monocot?
14. On the peanut, find and define the following:
seed coat:
cotyledons (2):
embryo:
endosperm:
STATION VI: Seed Dispersal
15. How is the fruit of a chestnut dispersed? _______________Sycamore dispersed? _______________
Coconut dispersed? ________________
16. Name at least one other species of plant that exhibits seed dispersal via the following methods:
By wind – By animals – By water –
STATION VII: Pollination and Pollen Tube Growth
17. Which angiosperm flowers tend to produce more pollen: Wind-pollinated or Animal-pollinated?
18. What was the most interesting thing you learned about angiosperms in lab?