An overview of transport in a vascular plant.

Life cycle of Angiosperms

Gametes Development and Pollination

Gametophyte Development and Pollination

Anthers and ovules bear sporangia, structures where spores are produced by meiosis and gametophytes develop. Pollen grains, each consisting of a mature male gametophyte surrounded by a spore wall, are formed within pollen sacs (microsporangia) of anthers. An egg–producing female gametophyte, or embryo sac, forms within each ovule.

In angiosperms, pollination is the transfer of pollen from an anther to a stigma. If pollination is successful, a pollen grain produces a structure called a pollen tube, which grows and digests its way down into the ovary via the style and discharges sperm in the vicinity of the embryo sac, resulting in fertilization of the egg. The zygote gives rise to an embryo, and as the embryo grows, the ovule that contains it develops into a seed. The entire ovary, meanwhile, develops into a fruit containing one or more seeds, depending on the species. Fruits, which disperse by dropping to the ground or being carried by wind or animals, help spread seeds some distance from their source plants. When light, soil, and temperature conditions are suitable, seeds germinate and the embryo carried in the seed grows and develops into a seedling.

Keep in mind, however, that there are many variations in the details of these processes, depending on the species.

Within the microsporangia (pollen sacs) of an anther are many diploid cells called microsporocytes, also known as microspore mother cells
Each microsporocyte undergoes meiosis, forming four haploid microspores, each of which can eventually give rise to a haploid male gametophyte.

A microspore undergoes mitosis and cytokinesis, producing two separate cells called the generative cell and tube cell. Together, these two cells and the spore wall constitute a pollen grain. During maturation of the male gametophyte, the generative cell passes into the tube cell. The tube cell now has a completely free–standing cell inside it (the generative cell). The tube cell produces the pollen tube, a structure essential for sperm delivery to the egg. During elongation of the pollen tube, the generative cell usually divides and produces two sperm cells, which remain inside the tube cell. The pollen tube grows through the long style of the carpel and into the ovary, where it then releases the sperm cells in the vicinity of an embryo sac.

One or more ovules, each containing a megasporangium, form within the chambers of the ovary. One cell in the megasporangium of each ovule, the megasporocyte (or megaspore mother cell), grows and then goes through meiosis, producing four haploid megaspores.

The details of the next steps vary extensively, depending on the species. In most angiosperm species, only one megaspore survives. This megaspore continues to grow, and its nucleus divides by mitosis three times without cytokinesis, resulting in one large cell with eight haploid nuclei. Membranes then partition this mass into a multicellular female gametophyte—the embryo sac. At one end of the embryo sac are three cells: the egg cell and two cells called synergids. The synergids flank the egg cell and function in the attraction and guidance of the pollen tube to the embryo sac. At the opposite end of the embryo sac are three antipodal cells of unknown function. The remaining two nuclei, called polar nuclei, are not partitioned into separate cells but instead share the cytoplasm of the large central cell of the embryo sac. The ovule, which will eventually become a seed, now consists of the embryo sac and two surrounding integuments (layers of protective sporophytic tissue that eventually develop into the seed coat).

Pollination, the transfer of pollen from anther to stigma, is the first step in a chain of events that can lead to fertilization. This step is accomplished in various ways. In some angiosperms, including grasses and many trees, wind is a pollinating agent. In such plants, the release of enormous quantities of pollen compensates for the randomness of this dispersal mechanism. At certain times of the year, the air is loaded with pollen grains, as anyone plagued with pollen allergies can attest. Some aquatic plants rely on water to disperse pollen. Most angiosperms, however, depend on insects, birds, or other animals to transfer pollen directly to other flowers.

Leaf Anatomy

Human Lymphatic System

The lymphatic system consists of lymphatic vessels, through which lymph travels, and various structures that trap “foreign” molecules and particles. These structures include the adenoids, tonsils, lymph nodes, spleen, Peyer′s patches, and appendix. The flow of lymph is traced in steps 1–4.

A vertebrate neurone

Arising from the cell body are two types of extensions: numerous dendrites and a single axon. Dendrites (from the Greek dendron, tree) are highly branched extensions that receive signals from other neurones. The axon is typically a much longer extension that transmits signals to other cells, which may be neurones or effector cells. Some axons, such as the ones that reach from your spinal cord to muscle cells in your feet, may be over a meter long. The conical region of an axon where it joins the cell body is called the axon hillock; as we will see, this is typically the region where the signals that travel down the axon are generated. Many axons are enclosed by a layer called the myelin sheath. Near its end, an axon usually divides into several branches, each of which ends in a synaptic terminal. The site of communication between a synaptic terminal and another cell is called a synapse. At most synapses, information is passed from the transmitting neurone (the presynaptic cell) to the receiving cell (the postsynaptic cell) by means of chemical messengers called neurotransmitters.

Knee-Jerk Reflex

The knee–jerk reflex. Many neurones are involved in the reflex, but for simplicity, only one neurone of each type is shown.