X ray effect in photography

While choosing photo for letter 'x' , I was quite confused. I searched google for some suggestions for "x". Me being an amateur , never knew form of  x ray photography( x ray used for other than medical purpose). X ray is form of photography captured from x ray machine camera. There are some amazing photos of flowers,plants on google captured by photographers.

Then in search of 'x' I found x ray effect in photo edit app.
Flower

Coconut tree

Lotus