(a) Describe any two devices in a flowerings plant which prevent both autogamy and geitonogamy
(b) Explain the events up to double fertialisation after the pollen tube enters one of the synergids in an ovule of an angisoperm.

Explanation

(a) Autogamy : Thranfer of pollen grans form anther to the stigma of same flower Geitonogamy : Transfer of pollen grains from anther to the stigma of same plant
Two devices that prevents both autogamy and geitonogamy are:
(i) Self incomplatibility : The is a genetic mechanism and prevents self pollen from fertilising the ovules by inhibiting the pollen germination or pollen tube growth in the pistil
(ii) Dioeciouis plants : Male and female flower are present on different plants that is each plant is either or female

(b) The events seen after the pollen tubes enters one of the synergids in an ovules are as follows:
(i) Pollen tube after r eaching the ovary enters the ovuele through the microplye and thus enters one of the synergids through filifrom apparatus
(ii) After entering one of synergids the pollen tube releases the two male gamets in to the cytoplasm of the synergid
(iii) one of the male gameties move towards the egg cell and fuses with its nucleus thus results in formation of zygote this is asyngamy
(iv) The other male gametes move towards the two polor nuclei located in the centeral cell and fuses to form triploid primary endosperm nucleus (PEN) Thisinvoles fusion of three haploid nuclei and hence termed as triple fusion
(v) Two types of fusins syngamy and triple fusion takes place in an embroyo and hence the phenonmenon is termed as double fertiliztion
(vi) After fertizliztion PEN beomes the promary endosperm cell (PEC) and develops in to endosperm while zygote develops in to an embryo.