From: Luke 11:42-46
The Hypocrisy of the Scribes and Pharisees (Continuation)
(Jesus said to the Pharisees,)  "But woe to you Pharisees! for you tithe mint and rue and every herb, and neglect justice and the love of God; these you ought to have done, without neglecting the others.  Woe to you Pharisees! for you love the best seat in the synagogues and salutations in the market places.  Woe to you! for you are like graves which are not seen, and men walk over them without knowing it."
 One of the lawyers answered Him, "Teacher, in saying this You reproach us also."  And He said, "Woe to you lawyers also! for you load men with burdens hard to bear and you yourselves do not touch the burdens with one of your fingers."
42. The Law of Moses laid down that the harvest had to be tithed (cf. Leviticus 27:30-33; Deuteronomy 12:22ff; etc.) to provide for the worship offered in the temple. Insignificant products were not subject to this Law.
Rue is a bitter medicinal plant used by the Jews in ancient times. Did it have to be tithed?: the Pharisees, who were so nit-picking, said that it did.
44. According to the Old Law, anyone who touched a grave became unclean for seven days (Num 19:16), but with the passage of time a grave could become so overgrown that a person could walk on it without noticing. Our Lord uses this comparison to unmask the hypocrisy of these people He is talking to: they are very exact about very small details but they forget their basic duty--justice and the love of God (verse 42). On the outside they are clean but their hearts are full of malice and rottenness (verse 39); they pretend to be just, appearances are all that matters to them; they know that virtue is held in high regard, therefore they strive to appear highly virtuous (verse 43). Duplicity and deceit mark their lives.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.