Mystery of biology 10: Pollen embryo sac.

Normally pollen grain should develop into male gametophyte but in some abnormal instances pollen grain shows peculiar behavior and may develop into structures other than male gametophyte. One such phenomenon is pollen embryo sac.

POLLEN EMBRYO SAC

Pollen grains usually have two nuclei. Occasionally in some abnormal pollen grains the number nuclei may increase. In some members of Lilliaceae, however the number of nuclei increases to such an extent, that they resembles embryo sac. Such pollen embryo sacs were first reported in Hyacinthus orientalis by Nemec in 1898. Occurrence of pollen embryo sac has also been reported in Ornithogalum nutans.

Hyacinthus orientalis

MATURED POLLEN EMBRYO SAC

Pollen embryo sacs are also called NEMEC PHENOMENON, as Nemec discovered them for the first time. According to Nemec, pollen embryo sacs are produced by the repeated divisions of vegetative nucleus while the generative nucleus degenerates.
Pollen embryo sacs have varying number of nuclei ranging from 4-16. They may or may not be organized. In some cases there is an egg, two polar nuclei and five antipodals. Stow has observed even the fusion of polar nuclei in some pollen embryo sacs.  
                                         

POLLEN EMBRYO SAC

According to Stow, it is not the divisions of the vegetative or Generative nucleus which give rise to the pollen-embryo sacs but those of the microspore nucleus itself. Once the vegetative and generative cells have been differentiated, further development is quite normal and no pollen-embryo sacs are formed. He also reports the existence of large number of dead pollen in association with pollen embryo sacs. He believes that the dead pollen secrete a necrohormone, which initiates abnormality in the surviving pollen.