The exine of pollen grain is made up of

At the time of pollination,how many cells are formed in the pollen grains?

The wall of a microsporangium is generally composed of how many layers?

Among the following statements related to pollens, choose the correct one.
Statement $I$: In $40\%$ of angiosperms, pollen grains are shed at the $3$-celled stage.
Statement $II$: Intine is made of cellulose and pectin, and it is discontinuous with germ pores.

The microspore tetrad of an angiosperm,at the time of formation,is surrounded by a callose wall. The breakdown of this wall is brought about by the enzyme callase,which is synthesized by the:

In higher plants, where does meiosis occur?