Re: What does THC do in the cannabis plant?

Plant hormones would not be found in such high concentrations as THC. Plants 
produce a wide variety of compounds that have no direct role in growth and 
development. They are termed secondary products or secondary compounds. Examples 
include caffeine, nicotine, rubber and menthol. Their main functions appears to 
be in protecting the plant from herbivores and pathogens. THC's most likely 
function is thus to protect the plant from herbivores or pathogens.

Reference

Taiz, L. and Zeiger, E. 1991. Plant Physiology. Redwood City, CA: 
Benjamin/Cummings.