<P> Pollen is a fine to coarse powdery substance comprising pollen grains which are male microgametophytes of seed plants, which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophytes during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the female cone of coniferous plants . If pollen lands on a compatible pistil or female cone, it germinates, producing a pollen tube that transfers the sperm to the ovule containing the female gametophyte . Individual pollen grains are small enough to require magnification to see detail . The study of pollen is called palynology and is highly useful in paleoecology, paleontology, archaeology, and forensics . </P> <P> Pollen in plants is used for transferring haploid male genetic material from the anther of a single flower to the stigma of another in cross-pollination . In a case of self - pollination, this process takes place from the anther of a flower to the stigma of the same flower . </P> <P> Pollen itself is not the male gamete . Each pollen grain contains vegetative (non-reproductive) cells (only a single cell in most flowering plants but several in other seed plants) and a generative (reproductive) cell . In flowering plants the vegetative tube cell produces the pollen tube, and the generative cell divides to form the two sperm cells . </P> <P> Pollen is produced in the microsporangia in the male cone of a conifer or other gymnosperm or in the anthers of an angiosperm flower . Pollen grains come in a wide variety of shapes, sizes, and surface markings characteristic of the species (see electron micrograph, right). Pollen grains of pines, firs, and spruces are winged . The smallest pollen grain, that of the forget - me - not (Myosotis spp .), is around 6 μm (0.006 mm) in diameter . Wind - borne pollen grains can be as large as about 90--100 μm . </P>

What name is used to designate the tubular protrusion from the pollen grain