What is the significance of the fact that the flax and the barley were smitten by the hail, whereas the wheat and the spelt were not?
Ramban #1 (citing R. Sa'adya Ga'on): Moshe was telling Par'oh that when he Davened for the plague to cease, he would not be able to save the flax and the barley. Only the wheat and spelt could be saved. 5
Ramban #2: It was a warning to Par'oh that if, he did not relent, Hashem would send the plague of locusts, which would finish what the hail had left over.
Rashbam: Having said that the hail destroyed everything in the field, the Torah needed to inform us what the locusts still found to eat. 6
Seforno: It is to demonstrate the extent of Par'oh's wickedness - even though he saw that Moshe's prayers had prevented the hail from destroying everything, he hardened his heart and stubbornly refused to give in.
And Pasuk 25 refers to vegetation that was fully-grown and brittle.
Because it grew only later ("Afilos" means late). This also served as a lesson to Par'oh - that someone who stands up to the storm will perish, and it is one who bends before it who will survive.
Because, ripe or not, there is no way that the wheat and spelt could possibly have withstood the velocity of the hailstones coupled with the fire. See Oznayim la'Torah.
To give Par'oh a chance to do Teshuvah and save the land from total devastation. Refer to 9:16:2:1*.
See the Ramban's objection to this explanation. Oznayim la'Torah: Moshe was informing Par'oh that, due to his having begum to do Teshuvah, Hashem, for the first time, stopped the plague in the middle, to give him a chance to do Teshuvah.
Rashbam: Hail tends to destroy whatever is hard, whereas locusts consume whatever is soft.