After having used The Guide to Herbs for RPGs a while, I became convinced that herb collecting is determined by chance. The herbalist takes what he finds. Even if he is looking for a specific herb, he might find something totally different. This system randomly generates a number of herbs found from your proficiency check.
By rolling a d100 you know what herbs you found. Some herbs that have little use because of the current rules in AD&D were deleted from the list. This includes herbs that stop bleeding.