Activity 3 -- Tracking Variation within the Normal Growth and Development of a Population of AstroPlants
With four students working as a team with one PGC, each student will be responsible for two plants in a subpopulation of eight. Students will sow four seeds in each film can wick pot and place them in an environment conducive to germination, growth and emergence. After plants emerge students will select two of the four plants and track their growth and development by measuring plant height at specified days after sowing (das).
Data collected by students on their plants will become part of a class data set, which will be organized, summarized, analyzed, plotted and displayed so that students may gain a better understanding of the normal variation within a population of AstroPlants as they grow and develop.
How much variation is exhibited within and among subpopulations of AstroPlants grown under standard environments in classrooms across the United States and Ukraine?
A normal amount of variation will exist.
- Subpopulations of AstroPlants are grown in classroom PGCs, specified growth parameters (height, etc.) are measured and summarized results compared with other subpopulations and with experimental subpopulations grown in microgravity on the Space Shuttle orbiter.
- Students will record data on their AstroPlants Growth Group Data Sheet (page 41).
- It is important for each class to have at least 2 sets of 4 film can wick pots in extra PGCs to serve later as unpollinated control plants. Plant and maintain the extra PGCs in the same manner as the experimental PGCs, being careful to avoid pollen transfer once the control plants are in flower.
A period of 16 days from the sowing of seed is required for the growth of the AstroPlants and the completion of the activity. Class time required daily will vary depending on the developmental stage of the plants and the activity.
In participating in this activity students will:
- learn about plant growth by observing the emergence of seedlings, observing and measuring increases in plant size and in number, size and complexity of plant parts;
- understand the role of environment in regulating plant growth;
- observe, measure and analyze variation in growth and development among individuals in a population of plants;
- consider the use of statistical and graphical representation of growth and development within a population; and
- understand that growth in plants represents an ordered sequence of developmental events which vary between individuals of a population within limits that are defined as "normal" (see page 26).
- black fine-tipped marking pen
- PGC with seeded film can wick pots, three days after sowing (Day 3)
1. On Day 0, students planted AstroPlants and placed the experimental and control PGCs under recommended lights (Activity 2, page 35).
2. By Day 3, seedlings will have emerged. At the time of sowing, film cans were numbered for students 1 to 4. Refer to the AstroPlants Growth Group Data Sheet and record the number of seedlings that have emerged from each film can wick pot.
3. On Day 7, thin the plants in each film can wick pot by carefully snipping off two of four plants at soil level with fine scissors. Number the remaining two plants on the white tape label of each pot. Each team of four students should have plants numbered from 1 to 8 according to the AstroPlants Growth Group Data Sheet.
4. On Day 7, each student should measure the height of his/her two plants (in millimeters) and record the measurements on the group's AstroPlants Growth Group Data Sheet.
Note: Measure height from the soil to the apex as indicated in the illustration, not to the highest part of the leaf.
At this time students will have generated a group data set. Teachers may wish to have students practice some simple organization and analysis activities on the group data and have students enter data into a class population data set.
5. Continue to observe the developing plants. As the plants grow, they will draw increasingly more liquid from the system; be sure to check reservoirs daily and fill as needed.
Around Day 9 students will notice the first appearance of small flower buds in the apex of the growing shoot. Within these buds the tissues that lead to the production of the male and female sex cells are differentiating.
Over the next six days the buds will enlarge as male gametes develop within the anthers and as female gametes develop within the ovules of the pistil (page 43).
6. On Day 11, each student should again measure and record the height of his/her two plants. Notice that the plants are beginning to elongate more rapidly. All of the leaves on the main stem have formed and flower buds are more prominent.
7. Sometime between Day 12 and Day 14, flowers on individual plants will begin to open. For each plant, record the number of days after sowing (das) when the first flower opens.
8. A final measurement of height should be made on Day 14, even if some plants are not in flower. After all of the students' data are in and recorded, it is a good time to observe and discuss the variation within plant population growth over time.
9. Following Day 14, many of the flowers will be opening on the plants, awaiting pollination. The pollination activity (Activity 4, page 46), should be carried out on Day 15 or Day 16, or when most plants have been in flower for two days. The timing of pollination may vary depending on the environment in which the plants have been growing.
Concluding Activities and Questions
Combine the group data into a class data summary using the AstroPlants Growth Class Data Sheet (page 42). See "CUE-TSIPS Science and Technology" for a review of data analysis (page 18).
If available, use data analysis software to create graphical and statistical summaries of class data. Notice how the various statistical notations (range, mean and standard deviation) change over time from sowing. Have students consider the following:
- From a class frequency histogram and statistical summary, does the measured plant character of height exhibit a normal distribution within the class population as hypothesized?
- Are their individual plants shorter than the average in the population? Or taller?
- Do their individual plant heights fall within one or two standard deviations of the class mean? Would they consider their plant heights to be normal? Why or why not?
- How many plants in the population fall within one or two standard deviations of the class mean?
- Are there any "abnormal" plants?
- There are many other ways that students could measure growth and development. Height is only one. Can they come up with others?
Classes can submit the AstroPlants Growth Class Data Sheet to the Wisconsin Fast Plants Program and then check the WFP World Wide Web site to see where their plant growth data fit into the data set from the larger population of plants grown for CUE-TSIPS in the United States and Ukraine (see page for addresses).
Pollination is the process of mating in plants; it is the precursor to double fertilization. In flowers, pollen is delivered to the stigma through a wide range of mechanisms that insure an appropriate balance in the genetic makeup of the species. In brassicas, pollen is distributed by bees and other insects. The flower is the device by which the plant recruits the bee. Bees and brassicas have evolved an interdependent relationship.
- What is the effect of microgravity on pollination?
- What will be the pollen vector in space? Can a bee fly in microgravity?
What is a flower? In human eyes it is something to enjoy, with color and fragrance. For many plants, flowers are vital organs of reproduction containing both male and female gametes. For bees and other nectar-feeding animals, flowers are a source of food.
Symbiosis is the close association of two or more dissimilar organisms. Such associations can be beneficial to both organisms (mutualistic) or detrimental to one (parasitic). Symbiotic relationships among species occur frequently in nature. When the two or more species in symbiosis evolve in response to each other, they are said to coevolve. Under close examination each symbiotic relationship stands out as an example of miraculous complexity which has emerged. The coevolution of bees and brassicas, each dependent upon the other for survival, is such a relationship.
Most flowers have the same basic parts, though they are often arranged in different ways. The five main parts of a flower are the sepals, petals, stamens, pistil and nectaries. The sepals are the green leaflike structures at the base of the petals that protect the developing flower. The petals are the colored leaflike structures within the sepals.
The stamen has two parts, the anther and the filament. The anther contains the pollen grains, which contain the male gametes.
The pistil usually has three parts, the stigma (which receives the pollen), the style (the neck below the stigma) and the carpel (or ovary). AstroPlants flowers have two fused carpels, separated by a thin membrane. The carpels house the ovules, which contain the female gametes.
Sugar-rich nectar is secreted by the specialized nectary tissues strategically located in the flower to ensure that nectar-gathering animals will receive pollen from anthers and transmit it to stigmas.
Investigations of a Gametic Kind
The Developmental Clock
At about Day 7 in the AstroPlants life cycle, you may have looked down on your plants from the top view and noticed a tightly packed whorl of buds, some of which were larger than others. Each successively smaller bud represented a time interval in the developmental clock of AstroPlants, later marked by the appearance of new flower buds on the shoot.
As your AstroPlants begin to flower, normally the lowest flower on the shoot opens first, followed by the next highest and so forth. By recording the time when the first flower opens and counting the number of successive flowers that open in the intervening 24 hours, you can calculate the average time between successive developmental events that initiate a flower on the shoot apical meristem.
- Can you calculate the number of hours between successive flowers on your AstroPlants?
- How many flowers would you predict will open between the time your first flower opens and when you pollinate your plants?
When is a Flower Open?
The answer to this question is not always as straightforward as it might appear. As you observe the progression of flowers opening, you will notice that as the buds swell the sepals are pushed apart by the enlarging anthers and emerging yellow petals. Eventually the petals (which are slightly rolled) fold outward about halfway up their length, flattening and spreading to reveal their bright yellow color. At this time you might conclude that the flowers were open.
From the perspective of mating, a flower is open when it is capable of providing and receiving pollen. Thus, not until the anthers on the filaments of the stamens open up (dehisce) and release their pollen is a flower functionally open.
The shedding of pollen is known as anthesis. When you observe a succession of flowers at the shoot apex of your AstroPlants you will observe whether anthesis has occurred by noting the release of the powdery yellow pollen from the anthers. A hand lens can be helpful in detecting anthesis.
Sometimes a flower is inhibited from male function by excessive heat or genetic male sterility. As mentioned in "Understanding the Environment" (page 13) anthers may fail to dehisce at very high relative humidities. This problem can usually be corrected in a few minutes by circulating air over the plant with a fan.
The production of sperm and eggs involves a fundamental sequence of events that characterizes most higher plant and animal life. In plants, meiosis precedes gamete formation and establishes the initial events of sexual reproduction by exchanging and sorting the genes on the chromosomes into the vehicles of genetic transmission and reception that we call gametes: the eggs and sperm.
In AstroPlants the developmental process known as microsporogenesis occurs in the developing anthers when the first flower bud of the apical whorl is about one millimeter in diameter and leads to the production of pollen. Within the anthers specialized tissues undergo meiosis to form the microspores that eventually become pollen grains. Pollen is the immature stage of the microgametophyte, which does not become fully mature until it germinates on a stigma and forms a pollen tube containing two sperm cells and a tube nucleus (see page 53).
At about the same time as the anthers are developing, tissues within the developing pistil of the immature flower bud give rise to a series of ovules. Within each ovule meiotic divisions in the process of megasporogenesis lead to the production of megaspores. Through the process of megagametogenesis the megaspore develops into a mature megagametophyte or embryo sac (Raven, Evert and Eichorn, 1992).
These developmental processes may be subject to perturbations resulting from microgravity, and therefore are be included as part of the CUE experiments.
The Flower and The Bee: Pollination
Brassica pollen is heavy and sticky - unable to be easily wind-borne. For brassicas, bees are marvelously coevolved pollen transferring devices (vectors>/I>).
Bees depend on the flower for their survival. Sugars in the nectar provide carbohydrates to power flight and life activities. Pollen is the primary source of proteins, fats, vitamins and minerals to build muscular, glandular and skeletal tissues. A colony of bees will collect as much as 44 to 110 pounds of pollen in a season.
A worker bee foraging for pollen will hover momentarily over the flower and use its highly adapted legs for pollen collection (see illustration). The bee's three pairs of legs are evolved to comb pollen from the bee hairs and pack it into the pollen basket for transport to the hive.
Each foreleg is equipped with an antenna cleaner, a deep notch with a row of small spines, which is used to
brush pollen from the antennae. Using the large flat pollen brushes on the midlegs, the bee removes the pollen from its head, thorax and forelegs. The pollen is raked off the brushes by the pollen combs and packed into the baskets by the pollen press.
Bees are members of the insect family Apidae, which are unique in that their bodies are covered with feather-like hairs (setae). The bright yellow flower petals act as both beacon and landing pad for the bees, attracting them to the flower and guiding them to the nectaries. The bee drives its head deep into the flower to reach the sweet nectar secreted by the nectaries and brushes against the anthers and stigma. Quantities of pollen are entrapped in its body hairs.
As the bees work from plant to plant, pollen on the bee setae is carried from flower to flower. The transfer of pollen from the anther to the stigma is known as pollination. When pollen is transferred from one plant to another, the process is
As the bee forages, crosspollination occurs and genetic information is widely transferred. Some pollen grains are deposited on the sticky surface of each stigma and each compatible pollen grain sends a tube through the style to the ovule to complete fertilization. Within three days of fertilization, petals drop and the pistil begins to elongate to form a pod as the seeds develop inside.
Are We Compatible?
For AstroPlants and many other brassicas, the act of pollination does not insure fertilization and seed formation. Some brassicas contain recognition compounds called glycoproteins which are unique to each plant. These compounds enable the plant to recognize "self," resulting in the abortion of the plant's own pollen. This genetically controlled prevention of fertilization with "self" pollen is called self-incompatibility. Only "non-self" pollen is able to germinate and effect fertilization.
In order for pollen germination and fertilization to occur, pollen must travel from one brassica plant to the stigma of a different brassica plant in the process of cross-pollination. Bees take care of this problem naturally as they move from plant to plant in search of nectar and pollen. AstroPlants are therefore cross-compatible and self-incompatible.