Jesus now teaches with the illustration of sowing and reaping. The image of the harvest is a familiar one. It often applies to the ministry of winning souls. Notice that Jesus gave a visible example of this process with the woman of Samaria. Now He teaches it with a new illustration. Jesus does not jump from one topic to another without completing the picture.
The Parable of the Sower and the Parable of the Tares relate to this theme. http://ref.ly/Mt13.1-30 Paul used it in his letters http://ref.ly/Ro1.13 http://ref.ly/1Co3.6-9 http://ref.ly/Ga6.9. After four months comes the harvest. Jesus tells them that harvest is ready to be reaped. The fields are white and ready. http://ref.ly/Mt9.37 There is a need for people to work in the fields. Is there a certain time to witness? I know I keep bringing this up, but this is what these verses are pointing out. There is a need for people to go out into the world and reap what has been sown.
Jesus goes on in verse 36 that the one who reaps will receive wages. This individual will be gathering fruit, which pertains to eternal life. The sower and the reaper will rejoice together. Souls are being brought into the kingdom of God. One has sown the Word of God into the hearts of those who will hear. Another reaps what has been sown. Another reaps that soul. You may witness to someone, and the seed you plant in their heart another shall see the fruit of it.
It takes faith to sow the seed of eternal life. This must be done in the good times as well in the bad times. We should never allow the enemy to stop us from sowing the good seed of eternal life. Note http://ref.ly/Ec11.4. Take this to heart! We have entered into the labors of others.