The acorn is not the oak tree, but both are physical.
In Corinthians 15:50-54, Paul writes:
"What I am saying, brothers and sisters, is this: flesh and blood cannot inherit the kingdom of God, nor does the perishable inherit the imperishable. Listen, I will tell you a mystery! We will not all die, but we will all be changed, in a moment, in the twinkling of an eye, at the last trumpet."
"For the trumpet will sound, and the dead will be raised imperishable, and we will be changed. For this perishable body must put on imperishability, and thus mortal body must put on immortality. When this perishable body puts on imperishability, and this mortal body put on immortality, then the saying that is written will be fulfilled. "Death has been swallowed up in victory.'"
In Corinthians 15:50-54, Paul writes:
"What I am saying, brothers and sisters, is this: flesh and blood cannot inherit the kingdom of God, nor does the perishable inherit the imperishable. Listen, I will tell you a mystery! We will not all die, but we will all be changed, in a moment, in the twinkling of an eye, at the last trumpet."
"For the trumpet will sound, and the dead will be raised imperishable, and we will be changed. For this perishable body must put on imperishability, and thus mortal body must put on immortality. When this perishable body puts on imperishability, and this mortal body put on immortality, then the saying that is written will be fulfilled. "Death has been swallowed up in victory.'"
Concerning this passage, N.T. Wright states:
"Here Paul states clearly and emphatically his belief in a body that is to be changed, not abandoned. The present physicality—in all its transience, its decay, and its subjection to weakness, sickness, and death—is not to go on forever, that is what Paul means by saying “flesh and blood cannot inherit the kingdom of God.”
"The term “flesh” (sarx) is seldom if ever for Paul a merely neutral description of physicality; almost always it carries some hint both of the corruptibility and of the rebelliousness of present human existence. What is required for God’s future state of affairs is what we might call a “noncorruptible physicality”: the dead will be raised “imperishable” and we—that is, those who are left alive until the great day—will be “changed” (1 Cor. 15:52)."
"As the parallel with 2 Corinthians 5 makes clear, Paul envisages the present physical body “putting on” the new body as a new mode of physicality over and above what we presently know. It is not the mere resuscitation of a corpse, coming back into the same mode of physicality it had before, but equally and emphatically it is not disembodiment."
"[In] the most complex part of the chapter, verses, 35 to 49, Paul speaks of the different kinds of physicality between which there exists both continuity and discontinuity. In verses 36 to 38, he uses the analogy of the seed and the plant: there is both continuity and discontinuity between the one and the other. The oak is, and is not, the same thing as the acorn. Then, in verses 39 to 41, he points out that there are different sorts of physicality appropriate for different kinds of creatures each enjoying its peculiar “glory” (doxa)."