Angiosperms

Division/Phylum Angiospermae is sometimes called Division Anthophyta (anthe = flower; phyto = plant) because the common name for this group is the “flowering plants.” Angiosperms are so named because the seeds are enclosed within a fruit of some sort.

Class	Seeds	Leaves	Flowers	Roots	Stem Vascular Tissue
Monocots	one nutrient storage area (cotyledon)	parallel veins	flower parts in 3	many fibrous roots	bundles scattered throughout the stem
Dicots	two nutrient storage areas (cotyledons)	net or branching veins	flower parts in 2, 4, or 5	one main taproot (may have smaller roots branching off)	bundles arranged in a ring

The parts of an Angiosperm include:

1. Roots, which are generally underground and serve to absorb water and nutrients
2. Stems, which come in various types such as:
   • stolon, an above-ground “runner”
   • rhizome, an underground “runner”
   • bulb, a fleshy stem modified for nutrient storage
3. Leaves, which can be either simple or compound in form and which alternate with each other going up the stem or can be arranged opposite each other on the stem or as whorled leaves where more than two originate from the same place on the stem
   
   Leaves can be many shapes from round to heart-shaped to oblong. Leaves which are entire, all in one piece, are called simple leaves, while those divided into multiple leaflets are called compound leaves. Compound leaves with their leaflets arranged like a feather are said to be pinnately compound (pinna = wing, feather) while leaves with their leaflets arranged like a person’s fingers are said to be palmately compound. Leaves that arise from the branch/stem in pairs are referred to as opposite leaves, while those which alternate sides up the stem are referred to as alternate leaves, and if more than two leaves arise from the same spot, those leaves are said to be whorled.
4. Flowers, which are the reproductive structures of an angiosperm and consist of four whorls of modified leaves (from outside in):
   • Sepals (sepi = fence in) (which collectively are called the calyx), which are often small and green but are colored like the petals in tulips and lilies, and which generally enclose the flower before it opens
   • Petals (petal = a leaf, spread out, flat) (which collectively are called the corolla) which are often brightly colored to attract pollinators (insects, birds, etc.) and may be very simple to highly modified
   • Stamens (stam(en) = anything standing upright, a thread), the “male” reproductive organs (they make microspores which turn into male gametophytes), which consist of a stalk (the filament) and a tip (the anther) where the microspores are produced and turn into pollen (anthe = flower)
   • Pistil (note spelling) or carpel (carpo = a fruit), which consists of:

     Ovary (ova, ovi = egg)

     the bottom end where seeds are produced

     Style (styl, stylo = a pillar, stake, column)

     the “stalk” portion

     Stigma (stigma = spot)

     the outer, sticky tip where pollen sticks when it lands or is placed there

   
   

   Botanists group species of plants (or, from the other direction, the monocots and dicots can be subdivided) based on a number of characteristics. Botanists pay particular attention to how the flowers are put together:

   A complete flower has all four layers of parts.
   An incomplete flower lacks one or more layers. These knotweed flowers lack petals (corolla).
   A perfect flower had both “sexes” — both stamens and pistil(s). A monoecious plant has perfect flowers or has both male and female flowers on the same plant (for example, Easter lily, pea, dandelion, and rose).
   An imperfect flower is lacking either the pistil or stamens. A dioecious plant has imperfect flowers on separate male and female plants (for example, marijuana, hops, persimmon, and boxelder). Note that plants such as ginkgo, a gymnosperm which doesn’t produce flowers but which has separate male and female reproductive structures on separate male and female plants, are also referred to as being dioecious.
   A regular flower is radially symmetrical.
   An irregular flower has bilateral symmetry, and is also known as a zygomorphic flower.

   Multiple flowers can be arranged or clustered in various ways, including:
   

5. Fruit, which is a ripened (mature) ovary (in which seeds develop/are found) and which serves as protection and means of dispersal for the seeds
   various types of fruits include:
   • Simple fruits arise from one ovary in one flower. Examples include cucumber, peapod, walnut, tomato, orange, cherry, apple, dandelion, and maple “helicopter.” There are a number of types of simple fruit, each with its own official name.
   • Aggregate fruits arise from several ovaries in one flower. Examples include raspberry and strawberry.
   • Multiple fruits arise from ovaries in several, tightly-clustered flowers which grow together into one “fruit.” Examples include pineapple, mulberry, and breadfruit.
   

Angiosperm Life Cycle

Angiosperms have alternation of generations with the 2n sporophyte being the dominant generation. The anthers, which are the equivalent of microsporangia, produce microspores by meiosis, and the microspores develop into male gametophytes (= pollen).

The ovaries, which are the equivalent of megasporangia, produce megaspores which grow into female gametophytes, each of which then produces an egg.

Note that technically the “sex organs” of a plant aren’t because they produce spores (micro- or mega-) which turn into male or female gametophytes. The gametophytes bear the true sex organs, such as they are, and are where eggs or sperm are actually produced.

By some means (wind or an animal pollinator), the pollen is transferred to the stigma of the pistil, and a pollen tube grows down into the ovary. Eventually, two sperm nuclei travel down the pollen tube. Pollination is the transfer of the male gametophyte (pollen) to the stigma of the female, while fertilization is when the sperm nucleus and egg nucleus unite

Angiosperms have an unusual thing called double fertilization. When the sperm nuclei reach the female gametophyte, one sperm nucleus and the egg cell unite to form a new 2n zygote (which grows into an embryo). The other sperm nucleus and two nuclei from the female gametophyte join to form 3n endosperm which often serves as food for the embryo.

The embryo sporophyte consists of:

1. one or two nutrient-storage areas called cotyledons which are in contact with (and absorb nutrients from) the 3n endosperm. Seeds of some species store their nutrients primarily in the endosperm, having very small cotyledon(s), while others have most of their nutrients stored in their cotyledons and the endosperm is very small.
2. the epicotyl (epi = upon, over), which is the region above the cotyledon(s), and which will become the stem and leaves,
3. the hypocotyl (hypo = under, beneath), which is the region under the cotyledon(s). The lower end of the hypocotyl, which becomes the root system, is called the radicle (radix = root) and will become the roots.

In general, monocots tend to store food in their endosperms, and nutrients are transferred to the cotyledon only as needed. In contrast, many (not all) dicots tend to store food in their cotyledons with the endosperm being reduced to a papery coating around the embryo.

Photographs of various angiosperms
Wildflower indentification practice “game”
Further technical information on plant families.
For more information on plant taxonomy, check out this site on Systematic Botany at the University of Maryland.