Sexual Reproduction in Flowering Plants

Double fertilisation (one male gamete fuses with the egg and the other with the polar nuclei) occurs in all angiosperms; it is a unique and universal feature of this group.
Vessels are absent in some primitive angiosperms (e.g. Wintera, Trochodendron).
Secondary growth is absent in most monocots.
Some angiosperms are not autotrophic, e.g. the parasite Cuscuta and saprophyte Monotropa.
So double fertilisation is without exception in angiosperms.

The pollen tube grows through the style and enters the ovule through the micropyle.
Guided by the filiform apparatus of the synergids, it enters one of the synergids and releases its two male gametes into the cytoplasm of that synergid.
From there, one male gamete moves to the egg (syngamy) and the other to the central cell (triple fusion).
So the pollen tube liberates its male gametes into the synergids.

In an anatropous ovule (the most common type), the body of the ovule is completely inverted (turned through $180^\circ$) on its stalk, the funicle.
As a result the micropyle lies close to the funicle and the hilum and points downward, towards the placenta, while the chalaza is at the top.
[add image: anatropous ovule with funicle, hilum and micropyle facing downward and chalaza at the top]
So the micropyle of an anatropous ovule faces downward.

After meiosis of the microspore mother cell, the four microspores remain together as a tetrad.
The tetrad (and each microspore) is surrounded by a special wall made of callose, a $\beta$-1,3 glucan, which is secreted by the microspore mother cell.
Later the tapetum releases the enzyme callase, which dissolves the callose wall and separates the microspores, which then develop into pollen grains.
Sporopollenin forms the exine of the mature pollen grain, not the covering of the tetrad.
So the four microspores of the tetrad are covered by a layer of callose.