* Cathode rays are streams of electrons. Electrons are negatively charged particles.

* Opposite charges attract. A negatively charged plate will exert an attractive force on the negatively charged electrons in the cathode ray.

* Deflection of the ray. This attraction causes the cathode rays to bend towards the negatively charged plate, changing their trajectory.

In summary:

* Negatively charged plates pull cathode rays towards them due to the electrostatic attraction between opposite charges.

* This deflection is a key piece of evidence that cathode rays are composed of negatively charged particles (electrons).