Pollen in Gymnosperm Reproduction During Climatic Shifts

Prior to the rise of the first seed plants, early gymnosperms, our planet was dominated by seedless vascular plants (i.e., ancestors of modern-day ferns). But when a global climatic shift resulted in cooler and drier conditions, gymnosperms gained an advantage over ferns. The evolution of pollen was a key adaptation allowing gymnosperms to be successful under these conditions. Pollen can be transferred by wind or animal couriers (called pollinators) through diverse terrestrial habitats, allowing these plants to reproduce outside of wet conditions (and to sexually reproduce with individuals further away). For seed plants, pollination is the mechanism of sexual reproduction. Pollen, the male gametophyte, is produced by male structures on a plant and must be transported to the female structures of a plant. Once pollen reaches the female structure, sperm cells can make their way to egg cells for fertilization to take place. The resulting zygote, along with tissues from the female gametophyte, then develop into a seed. Gymnosperms are not the only plants that produce seeds today; the term angiosperm means “seed container” - the seeds of angiosperms are contained within an ovary, which ripens into a fruit, offering the seeds both protection and a means of dispersal. Gymnosperms are considered "naked seed" plants, because their seeds are not enclosed within ovaries. In addition to the terrestrial adaptations shared with other plant taxa, the evolution of flowers accounts for the explosive diversification of angiosperms beginning around 130 million years ago. Flowers, which are unique to angiosperms, are an adaptation for attracting pollinators and achieving pollination. The diversification of angiosperms and of insect pollinators such as bees and butterflies occurred, not surprisingly, during the same period of evolutionary history. 81 Fruits are also unique to angiosperms. All flowering plants produce fruit, although it may not be what we normally think of as fruit. Botanically, pineapples, oranges, and peaches are fruits, but so are tomatoes, cucumbers, and squash. The pods that contain peas and beans are fruits, as are the inedible structures that carry seeds of many wind- dispersed grasses and flowering trees (e.g., maple). There are many ways to classify fruits, but the simplest distinction is based on flower structure and flower arrangement. Another common classification scheme is based on whether fruits are fleshy or dry. Within these broad categories there are many specialized fruit types, based mostly on properties of the pericarp layers. More on this when you get to Exercise B. Keep in mind that plants are a very diverse group of organisms, with incredible evolutionary stories. As such, they have plenty of important and interesting structures; but since we have two hours and not 15 weeks for this lab, we are going to focus on flowering seed plants (angiosperms) and the parts directly related to sexual reproduction for these organisms. Angiosperm Life Cycle The figure below shows the life cycle of an angiosperm. Before you move on to the lab exercises, review this figure and the information on the next page. Everything that you observe in lab today (flowers, fruits, seeds) will relate back to this life cycle. 82 Following pollination of the flower (pollen transferred from an anther to a stigma), a pollen tube delivers two sperm cells to an ovule, where a process called double fertilization happens. One sperm cell fertilizes the egg and the other sperm cell fuses with two polar nuclei to form endosperm (3n). The fertilized egg (now a zygote), endosperm, and ovule tissues develop into a seed, while the surrounding ovary wall matures into a fruit. Now let’s explore flowers, fruits, and seeds up close! Exercise A. Flower Anatomy Flowers are leaves that, through the process of natural selection, have been highly modified in various ways for reproduction. The fertile parts are the stamens and the carpel. The sterile parts consist of the sepals and petals. You will learn more about these in a moment. All these parts are attached to a receptacle, a platform at the top of the flower stalk. Some flowers have all parts (stamen, carpel, sepals, and petals) and are called complete. However, the structure of these parts can be quite different for different species of flowers. Flowers lacking one or more of these basic parts are called incomplete. The parts of flowers are arranged in concentric rings or whorls around the receptacle. From the outside to the inside the whorls are the sepals, petals, stamens, and carpels. The outer ring of sepals protects the flower when it is a bud. The sepals are usually green, but in some flowers, they have been modified to attract pollinators. In most members of the lily family, for example, the sepals are indistinguishable from the petals except by their position in the outermost ring. The next ring consists of the petals, which are generally colorful to attract pollinators. The male sexual structures are called stamens. The stamens consist of two parts: an anther (pollen-producing structure) at the tip, and a filament which attaches the anther to the receptacle. Pollen is the immature male gametophyte. As anthers age, they begin releasing their pollen and take on a dusty appearance. The female structures are called carpels (or pistils). Their function is to produce female gametophytes (ovules), which then produce the female gametes (eggs). A flower may have one or more carpels, and they may be either fused or separate. At the top of the carpel is the stigma, which receives the pollen. The ovary, which encloses the ovules, is at the base of the carpel. Fertilized ovules mature into seeds. The ovary becomes a fruit. A simple fruit is formed from a single ovary (either from a single carpel or several fused carpels) from a single flower. The fruit may be fleshy or dry, and the ovary may have one or more ovules. Aggregate fruits develop from a single flower with numerous ovaries. Each ovary develops independently into fruit, but since the fruits are all attached to a single receptacle, they appear to be one single fruit. Blackberries and raspberries are good examples of aggregate fruits Multiple fruit develops from the ovaries of multiple flowers all attached to a single supporting structure. The numerous flowers grow very close together, and their ovary walls fusing during maturation. Examples of multiple fruits include pineapples and figs. Accessory fruits, or “false fruits”, are a fun bonus category. An accessory fruit contains other floral parts in addition to ovaries. For example, the part of an apple that you may think of as the fruit is actually derived from the receptacle. Likewise with a strawberry, the red fleshy part develops from the receptacle. The diagram below illustrates the differences between simple, aggregate, and multiple fruits. The yellow portions represent ovaries with ovules (that become fruits with seeds). Stamens are not shown. Wikimedia Creative Commons Attribution-Share Alike As the ovary of a fruit ripens and grows, it differentiates into three layers. Collectively the layers are called the pericarp. “Carp” means fruit, so the outer layer of the fruit is called the exocarp, the middle layer is the mesocarp, and the inner layer is the Endocarp. Depending on characteristics of the pericarp, fruits may be broadly classified as either fleshy fruits or dry fruits. Fleshy fruits are made of living cells and are usually juicy and sweet (exceptions include olives and avocados). Dry fruits have mature pericarps made of dead cells and are further classified according to whether they split open to release the seeds or not. Pea and bean pods, as well as milkweed pods, are examples of dry fruits that usually split open to release the seeds. True nuts (e.g, walnuts and pecans), grains, and achenes (e.g., sunflower seed shells and dandelion fruits) are all dry fruits in which the seed usually stays in the intact pericarp. Differences among the pericarps of various fleshy fruits are useful in classifying them further. In this portion of the lab, you will use a dichotomous key to classify fleshy fruits. A dichotomous key is a tool developed by biologists to identify organisms into groups. There are keys for all sorts of things like birds, plants, mushrooms, and seashells. A key is constructed to lead you through a series of comparisons. At each step you will choose a statement that best describes the specimen you are trying to identify.