Is it because there’s a lack of pressure in the stem and that causes the stem to lose rigidity?

turgor pressure, to be more specific.
ERS

Imagine a balloon( the long ones). It’s pretty rigid right? Now pop the balloon or cut it in half. The rubber isn’t rigid anymore right?

What do you mean curve unevenly? Is there an even curving? I’m thinking if a plant stem goes limp, it just kinda curves depending on how the flower is balanced.

I see, but y is it curved unevenly? the outer layer becomes shorter than the inner layer…… is it because the outer layer tends to lose water to its surroundings easily?

It kind of sounds like you’re saying the end of the cut stem expands wider, like a trumpet, or bell bottom pants; in other words, the walls curve outward, but the stem as a whole doesn’t curve. If that’s the case then the answer is probably that the inside of the stem is under high pressure, and when the inside is exposed to atmospheric pressure the inside part of the stem expands. This would cause the walls of the stem to curve outwards in the same way that a bimetallic strip bends when the temperature is changed.

I don’t really know, but this would explain it if that’s what you meant.