Lab_5-Angiosperms-Spring2024.pdf

Introduction to the Seed Plants— Angiosperms Introduction The flowering plants, commonly called the angiosperms, are the largest group of plants on Earth with at least 300,000 species. Like gymnosperms, angiosperms produce ovules and seeds. In addition to ovules and seeds, angiosperms produce flower...

Introduction to the Seed Plants— Angiosperms Introduction The flowering plants, commonly called the angiosperms, are the largest group of plants on Earth with at least 300,000 species. Like gymnosperms, angiosperms produce ovules and seeds. In addition to ovules and seeds, angiosperms produce flowers, the ovary of which develops into a fruit. Gymnosperms do not produce flowers and ovaries, and therefore cannot produce fruit. Remember that gymnosperms produce their seeds entirely exposed or on the scales of seed cones. Many of the characteristics seen in gymnosperms are exhibited by angiosperms, too. Heterosporous (i.e., producing two different kinds of spores) sporophytes and reduced, dioecious gametophytes are characteristic of the angiosperms. Pollen grains are responsible for moving the sperm nucleus from one flower to another. The pollinators, however, are much more varied than in the gymnosperms, where the wind was the main vector for pollination. Birds, bats, butterflies, mice, possums, ants, water, as well as wind, all transfer pollen for various angiosperms. After pollination, the egg inside an embryo sac (megagametophyte) is fertilized and the ovule will develop into a seed. The zygote will develop into an embryonic sporophyte, the integuments of the ovule become the seed coat, and the ovary (and often, related structures) will mature into a fruit. Many flowers produce several ovules inside an ovary, and the resulting fruit will hold several seeds. Botanists recognize many different types of angiosperm fruit. Some of these fruit types can only be understood by examining the fruit while it is developing from the ovary. What you should be able to do after this lab 1. 2. 3. 4. 5. Recognize terms and understand the floral formula used to describe flowers. Locate pollen and ovules in different flower types. Recognize some general characteristics that distinguish monocots from eudicots. Follow the development of flowers to fruits: ovules to seeds and ovaries to fruit. Become familiar with the terms used to describe different fleshy and dry fruits. Part I. Flowers Virtually all of the critical features of the angiosperm life cycle occur in the flower. These include meiosis to produce the megaspores and microspores, development of these spores into gametophytes, and the production of gametes by these gametophytes. Fertilization of the egg occurs in the flower, too. The zygote grows into an embryo within the seed, the seed is housed inside the fruit, which develops from the ovary, a part of the flower! 1 Floral Terminology Most flowers are composed of 4 layers, called whorls, all of which are attached to a central based, called the receptacle. The outermost whorl of most flowers is known as the calyx and is made up of sepals, which are typically small, green leaflike structures that help protect the flower while it is still in bud and support the petals after blooming, but can be more specialized in some plants. Moving from outside in, the next whorl is the corolla, created by the petals, which are commonly large and colorful and often used to attract pollinators to the flower. Petals are often modified according to the specific pollinator for that flower, and may be reduced in size, or completely absent, if the flower is pollinated by some abiotic factor. Moving further into the flower, the next whorl is made of the male parts of the flower, called the stamens, which are composed of a container, the anther, that contains multiple microsporangia, and a long stalk that supports it called the filament. The innermost whorl is composed of the female parts of the flower (stigma + style + ovary), collectively known as the pistil. Ovaries are divided into one or more chambers called carpels. If the ovary contains only 1 carpel, it is simple, while it is compound if it is made up of 2 or more carpels. Compound ovaries can contain multiple separate or fused carpels. You can identify fused carpels by looking for fused stigmas or fused ovaries. Each carpel will hold one or more ovules. There are many different terms botanist use to describe the characteristics of a flower, and you’ll learn a few of those in this lab. One notable trait is ovary position; that is, where the ovary attaches to the receptacle in relation to other parts of the flower. Ovary position is said to be superior if the ovary sits above where the sepals, petals, and stamens attach, while it is inferior if the ovary sits below the point of attachment of these other structures. Botanists also describe flowers based on their symmetry. If a flower, particularly its sepals and petals, can be divided by more than one line passing through its center and produce two equal mirror images, the flower is radially symmetrical, and is said to be regular. An irregular flower is one which can only be divided by a line passing through the center at one point to create 2 mirror images (it is bilaterally symmetrical). Asymmetrical flowers do exist, but they are less common. While most of the diagrams you will see in lab show a flower with both male and female reproductive parts, that is not the case for all flowers. Flowers that contain both stamens and a pistil, are said to be perfect—they are monoecious. Flowers that only contain stamens or a pistil are dioecious, and called imperfect. 2 Similarly, some flowers lack sepals and/or petals. When a flower has all 4 whorls, it is said to be complete. When a flower is missing one or more whorl, as is common in wind-pollinated flowers, it is incomplete. Note, imperfect flowers are always incomplete. To reduce the time and space it takes to describe flowers, botanists have developed a shorthand system to explain the number of structures in each whorl and the characteristics of those whorls. That shorthand is called the floral formula and consists of a list of numbers, each describing one whorl, from outside in. When fusion (the joining of typically separate plant parts into a single unit) occurs within a whorl, parentheses “( )” are placed around the number, while brackets “[ ]” are used for fusion of neighboring whorls. A zero is used in the floral formula if a whorl is absent. For example, if a flower had 4 sepals, 4 petals, 8 stamen, and 4 fused carpels, the floral formula would be: 4, 4, 8, (4). Flower Dissection OBTAIN a lily flower. DISSECT it carefully so that you can IDENTIFY the following parts: receptacle, sepal, petal, stamen, filament, anther, pistil, stigma, style, and ovary. SKETCH and LABEL the flower below. Use the figures on p. 153 and 164 of your atlas as a guide. QUESTION #1: How many carpels does a lily flower have? Are they separate or fused? 3 FILL in the following chart below for the lily flower (#1). DISSECT the other two types of flowers in the same way and FILL in the chart with their information. Next, CALCULATE the floral formula for each flower. FLOWER LARGE LILY (#1) Rough sketch of dissected flower: # Sepals # Petals # Stamens # Carpels in the Ovary Fusion? Y or N If Y, which parts? Position of ovary: Inferior or Superior Regular or Irregular Perfect or Imperfect Complete or Incomplete Floral formula 4 SMALL LILY (#2) SNAPDRAGON (#3) Composites Daisies are composite flowers, with stalk containing many, many small flowers called florets, which create a larger floral head called an inflorescence. The outer florets, which bear the larger petals, are ray florets. The inner flowers, which have much reduced petals, are disc florets. DISSECT a daisy and COMPLETE the table below for both types of florets. (See Fig. 9.134 on p. 156 of your atlas.) Use “+” to indicate presence and “-” to indicate absence Ray floret Disc floret Petal Stamen Pistil Monocots and Eudicots Most flowering plants are classified into monocots and eudicots (=dicots). They are distinguished by the number of embryonic leaves called cotyledons (1 or 2) in the seeds that ultimately are produced. They can also typically be distinguished by leaf venation (parallel or net pattern) and the number of floral parts. With the help of p. 129 of your atlas and Table 25-2 (p. 488) of your text, DETERMINE if each type of flower is a monocot or a eudicot by looking at leaves, petals, and sepals. Flower 1: ______________ Flower 2: ______________ Flower 3: ______________ Observation of ovules and anthers EXAMINE the dissected ovary of each flower under a dissecting scope (on the front bench) and look for ovules. Each ovule will produce one functional megaspore, which will develop into the megagametophyte. QUESTION #2: What is the name of the cell that produces megaspores? QUESTION #3: Is it haploid or diploid? 5 In angiosperms, the mature megagametophyte, which is made up of only 7 cells, is also called the embryo sac. It contains the egg and eventually the embryo. QUESTION #4: What part of the flower produces the microspores and the microgametophyte? QUESTION #5: Does the microgametophyte produce sperm cells before or after pollination? OBSERVE the anther under the dissecting microscope and identify the pollen. A characteristic of angiosperms is that both sperm cells produced by the pollen grain participate in fertilization: one sperm cell fuses with the egg cell to form the zygote (2n). The other fuses with the two polar nuclei of the central cell to form the endosperm (3n). This is called double fertilization. (See Figs. 9.143, 9.148, and 9.158 on pp. 158-161 of your atlas.) QUESTION #6: What is the function of the endosperm? QUESTION #7: What tissue in the gymnosperm seed serves a similar function? QUESTION #8: What does the ovule become? QUESTION #9: What does the ovary become? 6 Part II. Fruits (see p. 164-170 in your atlas) After fertilization, the zygote develops into an embryonic sporophyte, while the ovule containing it matures into a seed. The ovary, made up of one or more carpels, each containing one or more seeds, grows into a fruit. In most fruits, it is only the ovary and its contents that produce the fruit, and the other floral structures are not involved; there are, however, very common exceptions (see below). As the ovary develops into the mature fruit, outer ovary wall matures into the pericarp of the fruit. If you were asked to describe a given fruit, you would almost certainly describe the characteristics of the fruit’s pericarp. In most fruits, the mature pericarp can be divided into 3 layers: the outermost, skin-like layer called the exocarp; the endocarp, the innermost layer that contains the seed(s), and a layer in between, the mesocarp. Fruits that maintain soft, ‘squishy’ mesocarps at maturity are fleshy fruits, while those that develop a dry, often hard or brittle pericarp are dry fruits. The characteristics of the endocarp may be very different from those of the mesocarp, with the endocarp being either hard and stone-like, as in an olive pit, or thick and papery, like that of an apple core. Fleshy fruits commonly lack an endocarp. In FIG 1 below, LABEL the layers of the (fleshy) fruit. Use Figs. 9.180 and 9.182 on p. 168 of your atlas as a guide. FIG 1. Fruit Structure 7 Within the fleshy and dry categories, fruits are divided into three main categories based on the number of flowers and carpels involved in producing the fruit and how many seeds are held in each carpel. A fruit produced by 1 flower, with one or more than one fused carpels is said to be simple, while those developing from 1 flower with numerous unfused carpels is an aggregate fruit, and those created by more than 1 flower are called multiple fruits. TYPE # FLOWERS # CARPELS Simple Aggregate Multiple 1 1 >1 1 or >1 fused > 1 unfused carpel 1 per flower Simple fruits are defined by the number of carpels and seeds in the fruit, while aggregate and multiple fruits are collections of those simple fruits. For instance, a fruit produced by 1 flower with 1 carpel containing 1 seed is a (simple) drupe, while a fruit made by several flowers, each with 1 carpel containing 1 seed is an aggregate fruit, made up of a collection of drupes (here, called drupelets). Some fruits are composed of more than just the ovary and its contents, often including adjacent structures like the receptacle or sepals—these are accessory fruits. The additional components are called accessory tissues and often increase in size or thickness as the fruit matures. Finally, some fruits rupture at maturity to release or expose seeds for dispersal—these fruits are dehiscent. Alternatively, some fruits are indehiscent and do not burst, with the seeds remaining in the fruit, even after it has been shed from the parent plant. Dry fruits are commonly dehiscent or indehiscent, while fleshy fruits are almost always indehiscent. 8 Fleshy Fruits Fleshy fruits maintain a soft, spongy pericarp, with a higher water content, at maturity. In many species, the mesocarp will become sweet or tasty, to lure animals into eating the fruit and distributing the seeds, which will survive digestion, in their droppings. Most fleshy fruits are simple, and there are 3 major categories: Drupe: Each carpel contains a single seed, often with a hardened endocarp (think pit) around the seed Berry: Each carpel contains many seeds Pome: An accessory fruit consists of the mature ovary + enlarged base of the receptacle. The outer edge of the ovary wall is inside and tough. EXAMINE the fleshy fruits available for study. For each, LOCATE the ovary (the fruit wall) and the seeds. FILL in the following chart by finding at least one example for each type (see p. 160170 in your atlas). Type Example(s) Accessory? If yes, which part? Berry Simple Drupe Pome Aggregate Multiple QUESTION #10: How are fleshy fruits dispersed? 9 Dry Fruits Dry fruits develop a hard, brittle pericarp, that has a very low water content at maturity. While some dry fruits rely on animals consuming the fruit to disperse their seeds, they are more likely to rely on other means, like being carried on fur or feathers, or being spread by the wind or floating on water. To increase their chances of dispersal, dry fruits commonly have modified pericarps, which might feature hairs, hooks, or wings. Like fleshy fruits, most dry fruits are simple. While there are many different types of dry fruits, we have narrowed the types you’ll examine to the most common: Follicle: Dry fruit with a single carpel; splits down one seam at maturity Legume: Dry fruit with a single carpel; splits down two seams at maturity Capsule: Dry fruit with 2 or more carpels; splits down two or more seams at maturity Nut: Dry fruit with a stony (hard) fruit wall; does NOT split open at maturity Caryopsis: Dry fruit with the fruit wall fused to the seed coat; does NOT split open at maturity Achene: Dry fruit with a single seed loosely attached to fruit wall; does NOT split open at maturity Samara: Winged achene EXAMINE the dry fruits available for study. For each, LOCATE the ovary (the fruit wall) and the seeds. FILL in Type the following chart by finding at least one example of a fruit for each term. Example(s) Dehiscent or Indehiscent? Follicle Legume Achene Samara Capsule Nut Caryopsis Aggregate *Dispersal: wind, water, explosive ejection, or attachment to animal 10 Dispersal?* Introduction to the Seed Plants— Angiosperms: Terms you should know Receptacle Simple fruit Sepals Aggregate fruit Petals Multiple fruit Filament Dehiscent Anther Indehiscent Stamen Berry Stigma Drupe Style Pome Ovary Legume Carpel Capsule Whorl Achene Superior ovary Samara Inferior ovary Nut Regular Endosperm Irregular Embryo sac Perfect Fleshy fruits Imperfect Dry fruits Floral formula Endosperm Composite flowers Monocots Ray floret Eudicots Disk floret 11