Soybean and Bees

less prominent and quickly resemble the rest of the cells of the young ovule. Periclinal divisions
in the hypodermal region produce two parietal layers of nucellus between the elongate
megasporocyte and the epidermis of the ovule.
At the time of fertilization, the nucellus still surrounds the embryo sac, but only the epidermis
remains intact at the micropylar end, in direct contact with the outer integument
(Pamplin, 1963). As the seed develops following fertilization, the nucellus ruptures at the
micropylar end, exposing the embryo sac so that the suspensor of the embryo is now in direct
contact with the epidermis of the outer integument. The chalazal end of the nucellus
persists for several days, but continued development of the endosperm finally results in its
complete obliteration by 14 days after fertilization (Pamplin, 1963).
Pollination of soybean flowers
By the time of pollination, the diadelphous stamens have been elevated so that the anthers
form a ring around the stigma. The pollen thus is shed directly on the stigma, resulting in
a high percentage of self-fertilization (Williams, 1950). Natural crossing on soybeans may
reach up to 2%. It has been noted that pollination may occur the day before full opening of
the flower (Dzikowski, 1936).
Fertilization starts when the pollen grain germinates and sends a pollen tube through the
style and into the egg sac, through an opening called the micropyle. The growth of the pollen
tube requires proteolytic enzymes that can digest the sporophyte tissue just ahead of the
lengthening tube. The generative cell within the pollen grain undergoes mitosis to produce
two sperm cells. These cells follow the tube into the egg sac where one sperm unites with the
egg to form a zygote, while the other sperm joins the two nuclei in the central cell to form
the triploid tissue called endosperm.
The wet stigma is overtopped by a proteinaceous film that originates from the cuticle. The
film probably prevents desiccation of the abundant quantities of lipoid exudate present at
the distal end of the stigma and confine the exudate to the stigmatic surface. It may also contain
recognition factors to facilitate the fertilization (Cheung et al., 1995).
Erbar (2003) pointed out that the transmitting tissue of the stigma is made up of papillae with
lateral protrusions that anastomose with each other. Papillae of this type occupy the distal
end of the stigma and secrete most of the stigma exudate. Proximal to them are one to three
whorls of free papillae lacking protrusions, which are also secretory.
SoybeAn and bees
51