Megasporogenesis

Ovule development in an orchid generally occurs only after and in response to the stimulation of pollination , carried from the stigma to the ovary via auxins.

The very first visible reaction of a flower to the process of pollination is the wilting and fading of sepals and petals . This happens within 8-10 hours after pollination . Exceptions to this rule are also reported in the Brazilian Miltonias (Hayes , 1968), Phalaenopsis (Curtis , 1963, Duncan & Schubert, 1943; Ringstorm, 1968 , etc.) Where the perianth segments persist and turn green and later dry up . Instead of wilting , in a few cases the perianth segments change pigmentation as in Epidendrum tampense (Ames , 1947), Cymbidium (Duncan, 1947) . Removal of pollinia also brings about similar reactions . The South Indian species of Calanthe masuca invariably shows change of colour from mauve to orange—pink in the lip upon removal of the pollinia .

This phase is followed by a swelling of the column and the sides of the stigma grow inwards to enclose the pollen deposited on the stigma .

These two phenomena ,namely wilting of the perianth parts and the swelling of the column are brought about by chain of chemical reactions set in motion inside the plant tissues by the pollen deposited on the stigma . As early as 1909 Fitting reported that a watery extract of the pollen ,when applied to the stigma produced wilting of floweres and prevented abscission . Thimman (1934) observed that it is the indole-acetic acid present in the extract which is responsible for the reaction thus produced . Burg (1964) found that the pollen germinating on the stigma released indole-acetic acid ,which in turn, induced synthesis of ethylene in plant tissues . Ethylene gas is known to produce fading and wilting of flowers. It may have functions other than mere wilting and fading like hastening of pollen tube growth ,swelling the column cells etc . (Dijkman & Burg , 1970).

Immediately after pollination , cells of the three placental ridges in the ovary begin to divide profusely producing dichotomously branching filaments . The end cells finally form a row of 5-7 cells covered by a single layer of epidermal cells. The terminal cells may persist without division ,while cells still below this initiate integuments –the inner multi-layered and the outer single –layered .

The megaspore mother cell undergoes meiosis to form haploid diad cells. The ell at the micropylar end disintegrates while the chalazal cell divides to give rise to two megaspore functions as the embryo-sac.

The most usual case among orchids is the monosporic Polygonum type embryo-sac. The bisporic Allium type is less common while tetrasporic of Adoxa type is very rare . Where tetrasporic type of embryo-sac is formed no diad cells are produced .

Among the monosporic embryo-sac, the most prevalent is the 8 –nucleate one. Here ther nucleus undergoes three successive divisions to form eight nuclei. Of the four micropylar nuclei, three form the egg apparatus while the fourth moves to the centre of the sac to form the micropylar –polar . Similarly at the chalazal end, three nuclei form the antipodals , while the fourth moves to the centre to form the chalazal –polar .
Deviations from this conventional pattern are also seen in orchid embryo sacs are met with in the various species . These variations result from the abnormal behaviour of the primary chalazal nucleus . This nucleus may sometimes divide only once, thus producing a six-nucleate embryo-sac. The two daughter nuclei behave variously . They may remain at the chalazal end or one or both may move towards the centre of the embryo-sac behaving as polar nuclei.

Sometimes the primary chalazal nucleus may not divide at all and a 5-nucleate sac results. Here again , the behaviour of the chalazal cell varies , either remaining at the chalazal end or moving towards the centre. The 5-nucleate condition of the embryo-sac may also arise by the fusion of the two daughter cells of the primary nucleus.

It is doubtful whether reduction occurs beyond the five –nucleate condition in orchid embryo sacs.

Bisporic embryo –sacs are the rule in all the species belonging to Diandrae , studied so far. In Monandrae the incidence of bisporic types is low. Tetrasporic embryo-sacs are a very rare phenomenon in orchids and are reported so far in 5 species of Neottieae .