Angiosperms

 **Plantae- ANGIOSPERMS** Daniel Podlisny



 **DIAGNOSTIC CHARACTERISTICS** Angiosperms are plants whose seeds develop within a surrounding layer of plants tissue called the carpel with seeds attached around the margins. The enclosed seeds and the presence of carpels distinguish angiosperms to their closest living relatives, gymnosperms, in which the seed is not enclosed within a fruit but is exposed the environment. (5 VK) The most prominent characteristic of the angiosperms is the growth of flowers, rings of specialized leaves used in reproduction. Within their stems, they have specialized xylem cells that help to differentiate angiosperms from gymnosperms. In addition to tracheid cells, they have fiber cells that are specialized to help support the plant and vessel elements, thick xylem cells that stack end to end for more efficient transfer of water and nutrients up from the roots. Angiosperms also undergo double fertilization, when two sperm cells fertilize the same female gemetophyte. Also, l eaves can be either simple or compound in form and can alternate with each other going up the stem, can be arranged opposite each other on the stem, or arranged as whorled leaves where more than two originate from the same place on the stem. Simple leaves are all in one piece, and compound leaves are made up of multiple leaflets.(2 AW)

 **HABITATS** Angiosperms can be found in most terrestrial ecosystems worldwide, except nutrient poor environments like mountaintops, extremely dry deserts, or bare rock.

 **MAJOR TYPES** (T2) Angiosperms originated and rapidly proliferated early in the Cretaceous period, which followed the Jurassic period (CS 1). Angiosperms had, until recently, been divided into two classes, monocots and dicots. However, after examining the DNA of various angiosperms, monocots remained as a clade while the dicots were split into eudicots and three other more primitive clades.

Monocots are defined as having one seed leave, or cotyledon. They also have parallel veins in their leaves, complex patterns of vascular bundles, a fibrous root system, and floral parts usually in multiples of three. Examples of monocots include lilies, orchids, yuccas, palms, and grasses. Eudicots and other primitive angiosperms have two cotyledons, branching networks of veins in the leaves, and rings of vascular bundles in the stems, and they often have taproots and floral parts in multiples of four or five. Some common eudicots are roses, peas, sunflowers, oaks, and maples. One of the three more ancient clades includes plants like the Star anise (Illicium floridanum ). A more primitive clade is that of the water lilies. The most primitive of the angiosperms is the genus Amborella, which lacks vessel elements unlike more recently evolved members of the phylum.

 **BASIC ANATOMY** Unlike nonvascular plants (such as Mosses), angiosperms have specialized cells for obtaining nutrients and performing photosynthesis and cell division (EG 1). Angiosperm cells, like all plants, have a cell wall made of cellulose surrounding and protecting the cellular membrane and the organelles. These cells constitute the three major organs of angiosperms, roots, stems, and leaves. All three organs have three layers of tissue, the dermal, vascular, and ground cells. The dermal tissue covers the outside of the plant and aids in both the absorption and retention of water. This retension of water, universal to plants, is what separates them from their cousins, the green algae, who never colonized the land for lack of water. (10RM) Vascular tissue consists of two kinds of cells used for long-distance transport of water and nutrients and travels from the leaves through the stem to the roots. Xylem is made of hollow, dead cells that either overlap as in tracheids or stack end-to-end as in vessel elements. Tracheids carry water through pits in the cell wall where only the primary wall remains. Long and narrow, they also function for support. Vessel elements have thinner walls and are less tapered with perforations on the ends that allow water to pass between them. Phloem is made of living cells called sieve-tube members, which stack end to end like vessel elements. The sieve-tube members lack certain organelles such as the nucleus, ribosomes, and a large vacuole, so they are connected to a companion cell that is nonconductive and has a nucleus and other organelles. The sieve-tube members themselves are connected by sieve plates, specialized cell walls that have pores to allow the sap to flow through them. Ground tissue is all of the non- dermal or vascular tissue, which is used for support, storage, and photosynthesis. It is found both inside the vascular tissue and outside of it.

ROOTS Roots are the subterranean structure of angiosperms used to support the plant and absorb water and nutrients. Monocots have networks of thin, fibrous roots that spread out near the surface of the soil, while dicots have a taproot that grows straight down and has smaller lateral roots branching off it. The taproots of these plants are used in some species to store sugars and nutrients for use in growth and reproduction. On the ends of the lateral roots are thousands of root hairs that drastically increase the surface area of the root for the absorption of nutrients. The hairs are specialized extensions of dermal cells. In some species, adventitious, or extraneous, roots are used to support tall growth.

STEMS Stems provide the aboveground structure and support for plants, consisting of nodes, where the leaves are attached, and internodes, the stem segments between nodes. Between the stem and leaf is an axillary bud. The terminal buds at the end of vegetative branches are where the majority of growth is concentrated. Some specialized stems are stolons, runners that aid in asexual reproduction. Rhizomes are similar to stolons except that they grow underground. Tubers are the specialized ends of rhizomes that are used for storing food, while bulbs are actually the swollen bases of leaves that grow vertically underground.

LEAVES The basic form of angiosperm leaves is a flattened blade attached to a stalk, the petiole. However, most monocots do not have this petiole attaching the leaf to the stem. The leaves themselves can be classified by shape, arrangement on the stem, and the pattern of veins in the leaves. Leaves can also be simple or increasingly compound, with a single leaf on the petiole considered simple, multiple leaflets being compound, and doubly compound if the leaflets are then further divided. However, each leaf only has one axillary bud.

FLOWERS Flowers are the reproductive organs of angiosperms. They consist of four rings of modified leaves: the sepals, petals, stamens, and carpel. The sepals are green leaves that protect the flower while it is still a bud. The petals act to attract pollinators and are often brightly colored and patterned. The stamen is the male reproductive organ. It has the anther where pollen is produced on the end of the filament. The carpel has the ovary of the flower at its base, with a slender neck called the style. At the end of the style is the stigma, a sticky pad where pollen is collected. Inside the ovary are one or more ovules where the female gemetophytes develop. These enclosed gametophytes and the existence of carpels in the angiosperm are what distinguish them from a gymnosperm, in which the seed is not protected by a fruit or flower, but exposed to the environment (1 AN).

(EK)

 **TRANSPORT OF MATERIALS** The basic nutritional needs of angiosperms are water, carbon dioxide, and other nutrients. Roots absorb water and nutrients in the soil through the root hairs. Xylem cells, though dead, carry the water up from the roots to the leaves where it is used in photosynthesis and respiration. The leaves are responsible for the intake of CO2 and release of O2 and water through the stomata. Here the CO2 is used to produce glucose as a product of photosynthesis, as well as other important organic molecules for the plant. These chemicals are then carried to other parts of the plant through the living cells of the phloem.

XYLEM Water and the dissolved minerals in it are carried up from the roots through the xylem by two mechanisms. The adhesion of water molecules to the cell walls of the xylem pulls them upward in conjunction with the negative pressure in the xylem caused by the evaporation of water through the leaves during the day. At night, adhesion is assisted by root pressure, which is cause by root cells pumping mineral ions into the xylem, and then water flowing into the xylem to maintain osmotic equilibrium.

PHLOEM Phloem sap is denser than xylem sap and carries saccharides, predominantly sucrose, from source cells to sink cells using osmotic pressure to push the sap quickly around the plant. The source cells pump sucrose molecules into the phloem, decreasing the water pressure. Water from surrounding cells flows into the phloem, pushing the sap down. At the sink cells, the sucrose is pumped into the cell, raising the osmotic pressure and causing the water to flow back into other cells. This creates a hydrostatic pull that draws the sap through the phloem. The water can then be recycled back to the leaves of the plant by the xylem. (JE)

 **REPRODUCTION** Pollen grains are the male gemetophytes of plants. They are formed in the anther, in a process starting with many diploid cells called microsporocytes. The microsporocytes undergo meiosis forming four haploid microspores. These microspores divide to form two cells that together share a tough outer shell and contain the male genetic information. Wind or other pollinators then carry the pollen grains to the stigma of another flower.

In the ovule of a flower, one diploid cell, the megasporocyte, undergoes meiosis to form four haploid megaspores. In many angiosperms only one of these megaspores survives and then divides into eight smaller cells: one egg cell, two synergids for attracting the pollen, three antipodal cells, and two polar nuclei.

When a pollen grain reaches the ovule, it undergoes a distinct process of angiosperms called double fertilization. Inside the pollen grain, the generative cell splits into two sperm. One of these sperm fertilizes the egg to form a diploid cell, while the other sperm fertilizes the two polar nuclei at the same time, forming a triploid cell that will become the endosperm where the seed’s nutrients are stored.

Another important way angiosperms are able to pollinate is through the help of pollinators. These include insects, birds, and the wind. As insects and birds extract nectar from flowers, they pick up and leave behind pollen from flower to flower (CH 6).

Here is a video that explains reproduction in angiosperms: [|Angiosperm Reproduction](2 HL)

(5 DO)

 **ENVIRONMENTAL ADAPTATIONS** Angiosperms evolved from gymnospers around 215 million years ago. (EK13) Angiosperms have several important adaptations that help in protecting against water loss, predation, and parasitism, and also aid in fertilization and seed dispersal. An adaptation that has helped angiosperms live farther inland is the development of upright meiosporangia for dispersal by spores to new habitats. (1DC) The evolution of the cuticle, a waxy outer layer of the dermal tissue that inhibits the loss of water, has also allowed angiosperms to live far away from water. Other plants have thorns, spines, or irritating oils that deter predators. Incredibly, some species of angiosperms that grow in nutrient poor environments actually eat small insects. Another common feature of angiosperms is coevolution with other species: for example, many types of orchids exist in symbiotic relationships with specific species of pollenating insects, and many legumes benefit mutually from the presence of nitrogen-fixing bacteria on their roots. (AR 1)


 1) How is sucrose moved through an angiosperm? (J. Stein) 2) How does an Angiosperm reproduce? And how does the flower part of the organism relate to reproduction? (1-SC) 3) Where did angiosperms evolve in relation to other phyla? What distinguishes them specifically from other phyla? (AS 13) 4) What are the main differences between monocots and dicots? (KA) 5) List and explain three ways in which angiosperms differ from gymnosperms. (J. Sun 13)

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