One Cell in the Sea in the Beginning

Mycorrhizae, the mutualistic relationship between fungus and plant roots, benefits both the fungus and the plant. The fungus that grows on the plant's roots increases the surface area for nutrient absorption of the roots. While plants provide the fungus with organic nutrients, the fungus passes minerals that that the fungus does not absorb to the plant roots. Mycorrhizae are very common - 95% of plants have mycorrhizae and all three phyla of fungi have mycorrhizae-forming members.

There are five main types of simple fruits. Drupes form their skin, fleshy area, and protective seed coating from the ovary wall of a flower with only one ovary and one chamber (example: peach). Pods form from flowers with one ovary and one carpel chamber; the flowers from which pods develop have many ovaries in a single row in the carpel chamber and its seeds are attached to the placenta on one side of the ovary (example: string bean). Achenes are hard protective fruits that develop from flowers with a single-chambered ovary, whose wall becomes hard and protective (example: sunflower seed). Pomes develop from flowers with one multi-chambered ovary. The seeds of pomes are separated in carpel chambers and are attached to the placenta at the center of the fruit, and the fleshy part of pomes develop from the petals, sepals, etc. (example: apple). Berries form from flowers with a singe multi-chambered carpels and a single ovary. Carpel chambers form the segments in which seeds are housed; the seeds are attached to the placenta at the center of the berry (example: tomato). Aggregate fruits develop from a multi-ovary flower, in which each ovary has one ovule, matures into a section of the fruit, and is attached to the central stem (example: strawberry).

Double fertilization in an angiosperm plant is the process by which a zygote and triploid endosperm are created. One sperm nucleus fuses with the eg nucleus, thus producing a zygote. The other sperm nucleus fuses with the polar nuclei, thus producing a triploid endosperm that serves as nutrients and feeds the developing embryo.

The molds of these phylums are similar in that those of phylum Acrasiomycota converge to form a seemingly individual organism from many amoebas, while a mold of the phylum Myxomycota is essentially one large cell that functions as individual cells because the large cell is multi-nucleated. The reproductive cycles of the two phylum differ in that cellular slime mold gives rise to fruiting bodies and release amoebas, while plasmodial slime molds produce gametes that must fuse to and undergo mitosis to form the new plasmodial slime mold.

Organisms in the kingdom Fungi have certain general characteristics. The cell walls of fungi are made of chitin. Fungi have filamentous bodies made of hyphal strands, which are long strings of cells connected end to end by septa. Because the septa only partially separates the cells, cytoplasm flows freely throughout the body of a fungus. A mass of hyphae is called a mycelium. 

Basidiomycota form mushrooms as their fruiting bodies, called basidiocarps, to reproduce. Fungi in phylum Basidiomycota reproduce sexually. Different mating strains meet underground and the cells fuse, however, the nuclei join together without fusing. Dikaryotic hyphae then form and grow into basidiocarps. A basidiocarp is a structure consisting of a stalk, which supports a cap with gills on the underside. The basidiocarp gills give rise to stalks called basidia. Haploid nuclei fuse at the tips of the basidium. each basidium then undergoes meiosis and forms four basidiospores, which are released into the environment and germinate into new hyphae.

The cross section of a dicot root has xylem for water transport arranged in star shape in the middle, surrounded by phloem, which transports sugars, with a ring of pericycle and endodermis surrounding the vascular tissue. A layer of cortex lies between the endodermis and the exodermis. In a monocot root cross section, ground tissue forms a pith in the center of the section, with xylem and phloem in respective rings around the pith. Just as in the dicot root cross section, that of the monocot root also has a ring of pericycle and endodermis, respectively, encompassing the vascular tissue, and a layer of cortex between the endodermis and exodermis.

Due to the evolution of flexible cell membranes, bacterial cell membranes were able to invaginate and form cavities within the cell. The endoplasmic reticulum (ER) arose as a result of the infolding of the cell's pasma membrane. The nucleus is proposed to have evolved from the infolding of cell membrane, which eventually led to the encasing of the cell's DNA within a cavity formed from the invagination of the cell membrane.

Monocot and dicot plants differ mainly in plant structure; however, both have three types of plant tissue – vascular, ground, and dermal – and indeterminate growth due to the presence of apical meristems. External differences include leaf venation – monocots have parallel veins while dicots’ veins are netted. The roots of monocots contain a larger area of ground tissue in the cortex, with the vascular tissue composed of xylem and phloem centered in the middle of the cross-section. Dicot roots have a ring of cambium tissue between the layers of xylem and phloem. Dicot leaves have a layer of parenchyma cells under the upper epidermis, with small vascular bundles in the spongy mesophyll layer between the layer of parenchyma cells and the lower epidermis. Monocot leaves are more condensed to preserve water and have larger vascular bundles. The vascular bundles in monocot stems are scattered throughout the cortex while those in the dicot stems are organized in rings, thus allowing secondary growth. In dicot stems, layers of secondary xylem and phloem are added to either side of the layer of vascular cambium tissue between those of the xylem and phloem. Both monocots and dicots have cotyledons, however, monocots have one while dicots have two inside of their seeds.

Some fungi are predators - the fungi trap protists or small nematodes and absorb N compounds from the prey. Fungi can also be lichens and have mutualistic relationships with algae in which the alga provide nutrients and nitrogen for the fungus, and the fungus provides a suitable, shady, environment that retains moisture and minerals while facilitating gas exchange for the alga.

Pollination in a flower occurs when pollen is transferred from the anther of a flower to the same flower (self-pollination) or another flower’s (cross-pollination) stigma. One of the strategies for pollination of an angiosperm flower is the transport of pollen grains from one flower to the next on the body of a pollinator. When the organism lands on the next flower, the pollen grains that were picked up by the organism sticks to the stigma of the flower. Flowers can also be pollinated by the wind, which carries pollen from one flower to the next. Wind pollination requires many flowers in the same general vicinity to have a good chance of pollen being successfully carried to another flower’s stigma by the wind.

The life cycle of a conifer consists of a distinct diplioid sporophyte generation and reduced haploid gametophyte generation that is retained within the cells of the sporophyte. The adult sporophyte bears two types of cones – pollen-bearing cones as well as ovulate cones. The pollen-bearing cones have scales in which are many microspore mother cells. The microspores undergo meiosis to develop four haploid daughter cells, which mature into pollen grains. The ovulate cone has scales each containing a megaspore mother cell, which undergoes meiosis to form four haploid megaspores, of which only one is functional and the other three degenerate. Pollination occurs when the pollen grain enters the micropyle. The pollen grain then germinates and develops a pollen tube and releases two sperm that travel through the tube to the megaspore. One of the sperm degenerates after the other unites with an egg. The sporophyte embryo then develops inside of the megaspore and the scale containing the megaspore with the developed embryo is released from the mature sporophte and carried by the wind to another location where, if conditions allow, the embryo will develop into an adult sporophyte.

The fern has both a diplid sporotphyte and haploid gametophyte generations in its life cycle. The adult sporophyte has sporangium on the undersides of its leaf fronds. These diploid sporangium undergo meiosis to form haploid spores, which are released from the sporophyte when mature. The spores then undergo mitosis and become independent gametophytes. The gametophyte of a fern has rhizoids to absorb nutrients from the soil, antheridium in which the sperm are housed, and archenonium with one egg in each one. Fertilisation occurs when the sperm from the antheridium come into contact with the egg in the archegonium. The nuclei of the sperm and egg fuse to form a diploid zygote. The zygote then undergoes mitosis to form a sporophyte, which lives off the gametophyte until it reaches maturity. At maturity, the adult sporophtye develops sporangium on the underside of its leaf fronds and thus begins the cycle anew.

Over time, plants adapted to life on land. The rise of vascular tissue (xylem and phloem) allowed transportation of water, nutrients, and ions throughout the plant body. Tracheids, specialized transport cells, in particular allow for more efficient transport of materials throughout the cell by facilitating water and mineral transport. Evolved higher efficiency in the transportation of nutrients from the leaves down to the roots, and water from the roots up to the leaves allowed plants to grow larger. Lignin arose in the walls of xylem cells, an adaptation which helped support the plant's upright structure as well as waterproof the cells. Cuticles, waxy coverings on the outside of plants, helps plants retain water as well as keeping air out. Because of the cuticles, gas exchange (which is essential to photosynthesis and respiration) is limited. The adaptation of stomata, tiny openings in the plant epidermis, allowed for gas diffusion in and out of the cell. The evolution of protection for reproductive structures keeps them from drying out. Different ways of fertilization allow eggs and sperm to unite without water, and to protect the embryo from drying out. The development of a root system allows the plant to absorb water and nutrients from its environment for use in plant growth.

The typical angiosperm flower has four whorls, circles of sets of organs organized in a circle around the receptacle (the part that connects the flower to the stem). The first and outermost whorl is called the calyx and consists of the sepals, which are usually green. The second whorl, called the corolla, consists of the petals, which are modified leaves and are usually colourful and attract pollinators. The third whorl, the androecium, consists of the stamens, which are the filaments with stamen made of pollen sacs filled with pollen grain at the tips. The fourth whorl is one or more carpels, which consist of a style with a stigma at the end, connecting to the ovary at the base of the flower. Inside of the ovary are the ovules, which, when fertilized and reach maturity, become seeds in the fruit which will develop from other parts of the flower.

Note: the gametophte is the large butterfly-shaped structure, and the sporophyte is the small heart-shaped structure growing from the notch of the gametophte on the right.