Pollen allergy.  Pollen grains of many species cause severe allergies and bronchial afflictions in some people often leading to chronic respiratory disorders, asthma, bronchitis etc. It may be mentioned that Parthenium or carrot grass that came into India as a contaminant with imported wheat, is present everywhere and cause pollen allergy.

Adaptations for self-pollination.
(i) There is synchrony between pollen release and stigma-receptivity; i.e. the anthers and pistil mature simultaneously.
(ii) The anthers and stigma lie close to each other in such a way that pollen from the anther can fall on the stigma.

(i) Mango, Wheat, Maize
(ii) Papaya, Orchids, Watermelon

Polyembryony is the phenomenon of occurrence of more than one embryo in a seed.
(i) All embryos can be isolated and cultured on an artificial medium under aseptic conditions in vitro and through micropropagation, many plants can be raised simultaneously from one seed.  

Parthenocarpic fruits are generally seedless or have non-viable seeds.

In oranges, each seed has more than one embryo, i.e. polyembryony.

Emasculation in a bisexual flower is necessary to prevent the contamination of the stigma with the self-pollen.
Bagging is necessary to prevent contamination of the stigma of the emasculated flower with any other unwanted pollen grains. 

1. Pollen tube carries two male gametes and the vegetative nucleus.
2. Pollen tube grows through the tissues of stigma and style to reach the ovary.
3. It enters the ovule through the micropyle and then enters the embryo sac through the filiform apparatus of one of the synergids.

Advantage
Since cleistogamous flowers are autogamous, there is an assured seed-set.
Disadvantage
Continued self-pollination leads to inbreeding depression.

(i) In Vallisneria, the male flowers are released on the surface of water and female flowers reach the surface by the long stalk.
(ii) Pollen grains are protected from wetting by a mucilaginous covering.

1. It is a case of polyembryony, the phenomenon of occurrence of more than one embryo in a seed.
2. It is possible in the following ways:
(i) Some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryos.
(ii) Synergids of the embryo sac divide and form embryos.

The specialised cellular thickenings of the synergids at the micropylar tips are called filiform apparatus.
They guide the pollen tube into the synergid and to liberate the gametes.