Pollen cell. In flowering plants the vegetative tube cell produces the pollen tube, and the generative cell divides to form the two sperm cells. 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
Flower constancy for certain flower types and constantly return even if other more rewarding flowers are available. This is shown for example in experiments where honeybees remain flower constant and do not attempt to feed on other available flowers that exhibit an alternative color to their preferred flower type. Flower constancy favors flower pollination, that is, pollinators that are flower constant are more likely to transfer pollen to other conspecific plants. Also, flower constancy prevents the loss of pollen during interspecific flights and pollinators from clogging stigmas with pollen of other flower species. Flower constancy can be enhanced when the flowers are