It differs because it is a unicellular plant. Many scientists claim that Volvoxes are protists while many claim that they are plants, and protists are not plants.
Rubidium is an element that belongs to Group 1. As such it will have physical properties similar to the other Group 1 elements. Rubidium is below
Potassium in the periodic table but above
Cesium. As such it would be most like one of those two elements.