Hi csxiongacs,
Is this a question on botany a test? Sounds like one. hahaha. There's lots of info on the role of Ca in plants on the interent, if you do a google search. Ca generally has the same basic function in all plants; it makes up a large portion of cell walls (in meristem and storage organs), and it also regulates osmosis through those cell walls. Adequate amounts of Ca in cell walls reduce the possibility of disease entering the cells; it makes them more resistant. I'm not sure how function of Ca is different in cacti than in other plants...I'd really have to think about that. Hope this helps a little.
Lunaflora