Notice that this instruction to take no thought of the morrow is addressed to those who are called to spend all of their time and energy preaching the gospel. They are not to become involved in the routine concerns of running a business or providing for one's daily necessities. In a moment after saying this Jesus will turn from the twelve disciples to the multitude. Notice that he doesn't tell them not to take any thought of tomorrow, because that's one of the first laws of temporal existence.