Cross-pollinating?

I've got both marshmallow (Althaea officinalis) and hollyhock (Alcea rosea) plants and I am especially interested in keeping the marshmallow true. Can anyone tell me if these plants will cross? Also, can anyone recommend a good book on this subject? Websites will work, too. ;)
Thanks all. GreenMum

Alcea rosea are known for cross pollination so much so its recommended that you only grow one variety if you want them to stay true. I'm not familar with althaea officinalis.

I'm thinking since the plants are in different genera (the botanical name indicates Genus, species), not just different species, you should be quite safe. In terms of classical taxonomy, I think different species are not supposed to cross (or at least not to produce fertile offspring), but with plants the definitions seem to get a little fuzzy sometimes, and some interspecific crosses occur.

Marshmallow and hollyhock are the same family. I can't remember which one had the genus name changed, but one of them did. Medicinally speaking, they are both used for the same ailments also - this is how close they are botanically. I guess I can let them go for a year and see what happens. The flowers of the marshmallow are the delicate pink and the hollyhocks are black so any crossing is going to be pretty evident in the produced seed. Thanks for your input folks!