Unlike most plants, cactuses absorb carbon dioxide from the air mainly at night. Night air contains greater quantities of certain types of carbon than does daytime air. Consequently, these types of carbon are more common in cactuses than in most other plants. Quintral plants, however, which grow as parasites on cactuses, contain nearly the same proportions of these types of carbon as cactuses. Therefore, quintral plants probably take significant quantities of carbon from their cactus hosts, since ______.