Matthew 13:31-35

Mat 13:31 (KJB)
Another parable put he forth unto them, saying, The kingdom of heaven is like to a grain of mustard seed, which a man took, and sowed in his field:

What Jesus was probably speaking of was the seed of the black mustard which is the size of a head of a pin and it was grown because it was used as a condiment. In fact, the seeds themselves were the actual condiment, so they ate some and they planted some. It is a very tiny seed and would not need much of a hole for it to take root. The people would have been familiar with this scenario since many were agricultural in their vocations. Unlike in the previous parable, there are no weeds or other plants in this field where the seed is being sown.

Mat 13:32 (KJB)
Which indeed is the least of all seeds: but when it is grown, it is the greatest among herbs, and becometh a tree, so that the birds of the air come and lodge in the branches thereof.

The mustard plant (Sinapsis Nigra) yields many seeds when it is full grown. It grows to a height of 23.5 inches (60 cm). The head of a pin may be 1 mm which means this seed grows to 600 times the size of the seed itself. It is interesting to note that 600 is easily divided by 30, 60, and 100. Now when the Lord speaks of it becoming a tree, He is not speaking of a comparison to an Elm tree or a Maple tree but He states that it is greatest among the herbs. The majority of herbs grow only a few inches from the ground so the mustard plant would tower over it. Now when it comes to the birds, the seeds attract them because they enjoy the taste of them. In Psalm 104, we have two passages of Scripture which give us a little insight into who the birds can be identified with. (Psa 104:16-17 KJV) The trees of the LORD are full of sap; the cedars of Lebanon, which he hath planted; {17} Where the birds make their nests: as for the stork, the fir trees are her house. The sap is what fills the branches and gives the plants life. It keeps them green all year long. The Holy Spirit fills the believer and keeps them in the power of God permanently. He is like the sap in the trees, He fills the believer and gives them strength to withstand the storms of life. The birds are like the believers whom God calls to be in His kingdom. If you notice, the birds nest in the branches but they do not leave. (Psa 121:8 KJV) The LORD shall preserve thy going out and thy coming in from this time forth, and even for evermore. Once the believer is in the branches, they never leave because the Kingdom of God is the place of permanent residence for the true believer.

This is a little tidbit on Mustard
The Romans are the first to be credited with making mustard in the way we know it today. They mixed the ground seeds with grape juice; the word "mustard" derives from mustum, "grape must," and ardens, "burning," mustum ardens- hence "must ard".

Mat 13:33 (KJB)
Another parable spake he unto them; The kingdom of heaven is like unto leaven, which a woman took, and hid in three measures of meal, till the whole was leavened.

Jesus gives another parable concerning the use of leaven. Leaven was yeast which was mixed in with the dough. In many instances the word leaven was used to convey a negative sense, such as the leaven of the Pharisees and Sadducees. (Mat 16:6 KJV) Then Jesus said unto them, Take heed and beware of the leaven of the Pharisees and of the Sadducees. Here a woman took the leaven and hid it within three measures of meal. The fact that she was making three measures of meal meant she was making it for a large gathering. The emphasis here is on the leaven which produces a gas that makes the dough much lighter. People who are unsaved carry with them a tremendous burden of sin which weighs them down very heavily. (Heb 12:1 KJV) Wherefore seeing we also are compassed about with so great a cloud of witnesses, let us lay aside every weight, and the sin which doth so easily beset us, and let us run with patience the race that is set before us, When one lays aside the weight which besets a person and that besetting is a sin which encloses them or presses them in on all sides, they will begin to lighten their load of sin and guilt and begin to run the race once they know that their sins have been removed thus having the burden taken from them. The woman, who is akin to the Holy Spirit in the believer’s life, is continuing to knead the dough until the entire loaf is leavened. The Holy Spirit continues to work with the believer until the sin which resides in our flesh and weighs us down, is removed little by little. It is a honing of the Christian life in every believer so they may do the work of the Lord without any burden of sin’s leftovers.

Mat 13:34 (KJB)
All these things spake Jesus unto the multitude in parables; and without a parable spake he not unto them:

Jesus spoke all these things to the crowds in parables. Whenever He spoke to the multitudes it was always in parabolic form. This was to teach the truths of the Kingdom of Heaven but it was in a parabolic form and those who became saved, would have the understanding made plain.

Mat 13:35 (KJB)
That it might be fulfilled which was spoken by the prophet, saying, I will open my mouth in parables; I will utter things which have been kept secret from the foundation of the world.

(Psa 78:2-3 KJV) I will open my mouth in a parable: I will utter dark sayings of old: {3} Which we have heard and known, and our fathers have told us. The writer of that portion of Scripture the Lord quoted was Asaph who was considered a seer. (2 Chr 29:30 KJV) Moreover Hezekiah the king and the princes commanded the Levites to sing praise unto the LORD with the words of David, and of Asaph the seer. And they sang praises with gladness, and they bowed their heads and worshipped. Jesus used parables to fulfill part of His calling. He did not come to only to fulfill the Law and the Prophets, but there were many aspects of His coming to earth. Jesus is now revealing in parables the mysteries which were kept secret from the foundation of the world or the beginning of time. One such mystery was Himself. (Rom 16:25 KJV) Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began, The preaching of the Lord Jesus Christ was withheld until the fulness of time, before that all things were in shrouded language because the prophets did not know much of what they were writing. (Eph 3:9 KJV) And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ: Another mystery withheld from the beginning of time was the fact that Jesus was also the Creator, a fact which was withheld in plain language but revealed in shrouded language. (Psa 2:12 KJV) Kiss the Son, lest he be angry, and ye perish from the way, when his wrath is kindled but a little. Blessed are all they that put their trust in him.  In Psalm 2:12, it speaks about “the Son” yet the Pharisees and Sadducees did not study that verse to see if Christ fit the description. If they would have, they would have known that he fulfilled it perfectly.