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Introduction to Plant Science Flower and Inflorescence Structure Introduction

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Introduction to Plant Science Flower and Inflorescence Structure Introduction INTRODUCTION TO PLANT SCIENCE FLOWER AND INFLORESCENCE STRUCTURE INTRODUCTION Seed or fruit production is a major contributor of food for our welfare. There are numerous reasons for this but some of the major factors are the high nutritional value of the seed and fruit we produce as well as the storability of the seed itself. These are products of the process of sexual reproduction. As such, we put an emphasis on the understanding of this process. Plant reproduction is the production of offspring in plants, which can be accomplished by sexual or asexual reproduction. Sexual reproduction produces offspring by the fusion of gametes, resulting in offspring genetically different from the parent or parents. Asexual reproduction produces new individuals without the fusion of gametes, creating genetically identical (clones) to the parent plants except when mutations occur.. This laboratory exercise will cover (a) the diversity of flower types in respect to structure and classification, (b) the differing anatomy associated with the male and female floral structures of both dicot and monocot plants, and (c) the different structural differences with the various types of inflorescences found with economically important species. LEARNING OUTCOMES 1. Understand the process of sexual reproduction of economically important plants and the characteristics related to the process. 2. Be able to correctly use the terminology and identify the structure of the flowers produced by common agronomic and horticultural species. 3. Learn the characteristics and the associated terminology of inflorescences that support the floral structures during the process of sexual reproduction. MATERIALS & METHODS – SEED DIVERSITY 1. Flowering plants of various plant species of both monocots and dicots. The available plants will include economically important agronomic, horticultural, and forage species. Several weed species may also be included. 2. Dissecting materials such as tweezers, forceps, and razor blades. 3. Dissecting scopes and/or hand lens. LIFE CYCLE A determining factor associated with seed production is the life cycle. One of the major types of a plant life cycle is an annual. An annual plant completes the entire life cycle in a single growing season. Other terms associated with this type of life cycle are based on the time of year in which the plant grows and reproduces. Winter annuals utilize parts of two growing seasons in completing their life cycle but still are able to complete the life cycle within a year. In this region, they are planted in the fall and reproduce in the spring. These plant types usually require a vernalization period (cold treatment to induce flowering and can be classified as having an absolute requirement or having a promotion of flowering the following spring) during winter, after which they produce seed and mature the following summer. Biennials require two growing seasons to complete their life cycle. Vegetative growth occurs during the first season and we see the production of stored reserves, and flowering and fruiting occur in the second season. These plant types also typically require a vernalization period and occur during the winter period. Perennials have an indefinite growth period. The length of growth is typically determined by species and environment. They differ from the other growth cycles patterns in that they do not die after reproduction. FLORAL CLASSIFICATION The parts of the flower are important from the standpoint of both function and its usage in identification. In the plant classification lab, you found that the binomial system that is in place today was developed by Linnaeus. One of the major factors used in the description of the genus and species is floral structures. Characteristics of the floral organs are found in figures 1 – 3. One of the parts of the flower we find are the sepals . These may vary in number but collectively they are called the calyx. They are protective, leaf-like enclosures for the flower buds during their development. They tend to be green in color but may sometimes have a low level of pigmenta- tion. Petals are modified leaves that are usually colorful. They may vary in numbers but are collectively called the corolla. They are usually what produces the aesthetic qualities of the flower. They may also contain nectar glands that potentially benefit the process of pollination. The pistil is the female organ of the flower. It is generally located in the center of the flower. It contains three major components. The stigma is the tip of the style that is capable of identifying and receiving the pollen. The process of pollen germination and producing the pollen tube is initiated on this tissue. The style is the connective tissue between the stigma and the ovary. The ovary is the basal part of the pistil and contains 1 – many PISTIL SEPALS (CALYX) PETALS (COROLLA) STAMEN PETALS (COROLLA) PISTIL STAMEN A A B B C C Figure 1. Several example of a complete-perfect flower: (A) tomato and (B) blackberry. ANTHER OVARY STIGMA STIGMA STIGMA Figure 2. A typical incomplete-perfect flower found in a monocot with the individual florets attached to the rachis. The spikelet and the individual florets are encased within a palea and a lemma. ovules that will develop into the seed. The ovary will also develop into the fruit. The stamen is the male floral organ. It is comprised of two parts: the anthers and the filament. The anthers are the pollen- producing organs. The stalk that supports the anthers is called a filament. Another term used in the description of the floral structure is the receptacle. This is the enlarged tip of a stem on which the flower is found. The peduncle is the term used for the stalk that supports a flower that is borne singly. The term is also used to refer to the main stem of an inflorescence. There are several classifications for flowers and they are based on (1) the absence or presence of the four specific parts of the flower and (2) the presence of male and/or female parts. A flower classified as complete will have all four floral parts: sepals, petals, stamens, and a pistil. An incomplete flower will be lacking one or more of the floral parts. The next classification is based only on the presence of the male or female parts. A perfect flower will contain both male and female parts, whereas an imperfect flower will lack either the male or the female part. If it only has the female part, the flower is referred to as being pistillate. If the flower contains only the male part, it is called staminate. There are other variations to the above descriptions. One of these is found within the monocots. The floret of a grass plant that is classified as incomplete-perfect is typically composed of a lemma and a palea . These replace the sepals and enclose the reproductive parts of the plant during development. A flower type that has a major significance in agriculture is one found in the legume family (Fabaceae). It is comprised of three unique petal types (Figure 4). Another unique tissue associated with the floral structures of dicots is the involucre. This is a whorl of bracts or leaf-like structures. Since they are associated with the floral structure they are often overlooked. These are typically found with flowers from the oak and sunflower. PLANT REPRODUCTION Sexual reproduction in plants occurs when the pollen from an anther is transferred to the stigma. Plants can self-fertilize. This occurs when the pollen from an anther fertilizes the egg within the same flower. Cross- fertilization occurs when the pollen is transferred to the stigma of an entirely different plant. This characteristic is important to know for the production of specific plants due to differing genetic characteristics. An example of this would be associated with plants that maintain homozygosity within the planting stock. Many of these plant types are self-pollinated. Another type of genetic characteristic is the hybrid. This is an example where we force cross-pollination for the production of F1 seed. A PETALS (COROLLA) SEPALS (CALYX) PETALS (COROLLA) STIGMA STAMEN B OVARY OVULES STAMENS STYLE C Figure 3. X-section of a cotton flower with (A) the whole flower, (B) the stigmatic surface and the male reproductive part, and (C) the ovary with numerous ovules. Figure 4. (a) This is a A typical flower of a plant in the Fabaceae family (legume). (b) The corolla is made up of five petals of unequal size and include 1 banner petal, 2 A wing petals, and 2 keel A petals that may appear to be fused but are separate. B WING PETAL (2) KEEL PETAL (2) [not fused] BANNER PETAL (1) A B Figure 5. Example of several dicot flowers with (A) an okra flower showing a superior ovary and (B) a pumpkin flower (cucurbit) with a well developed inferior ovary. Figure 6. Example of several dicot flowers with variability in appearance. INFLORESCENCES The structures that support the flowers during the reproductive process are called inflorescences. There is a significant amount of variation found within these structures. These can be classified into rather broad groupings. A single flower is found on an elongated branch or stalk and is typically referred to as the peduncle. Examples of a single flower would be okra, cotton, or tulips. The inflorescence is a general term used for the arrangement of a number of flowers on a plant in a specific arrangement. The stem supporting an inflorescence is referred to as the peduncle, and the stalk of each individual flower within the cluster is a pedicel. In most cases, both the peduncle and the pedicel are green in color. Most are without leaves but may have small leaf-like structures that are typically called bracts.
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