Well, I am not able to understand the exact difference between megasporocyte and archesporium. Also I'm not quite able to get, how or precisely from where do these arise? Please help.
In relation to angiospermic megasporogenesis, archesporium cell is the precursor of Megasporocyte.
Archesporium is defined as the cell or group of cells from which spore mother cells develops
The megasporocyte is the cell(or the tissue) which directly produces the megaspores.
In most angiosperm, the nucellar tissue within the ovule first differentiates into a single archesporial cell, the ancestor of the Megaspore Mother cell(MMC). In certain species, this archesporial cell may itself act as the MMC undergoing meiosis to produce the megaspores. in such cases, the archesporial cell and the megasporocyte (another name for MMC) is the same.
But in some species, the archesporial cell first undergoes a periclinic division producing two cells which further produce the parietal layer (covering of the megaspore) and a single sporogenous cell. This cell acts as the MMC, and is the megasporocyte.
Thus the archesporial cell may or may not be the same as the megasporocyte. It could be the ancestor of megasporocyte.