What is the desert plant of US and Mexico that transforms from spiny dry sticks to a mass small leaves scarlet flowers following rainy season?

The plant you're referring to is the Ocotillo, also known as Fouquieria splendens. It's a shrub or small tree native to the Sonoran Desert in the southwestern United States and northwestern Mexico. During dry spells, the Ocotillo can lose its leaves and appear like a cluster of spiny, lifeless sticks. However, when the rains come, the plant undergoes a remarkable transformation. It rapidly produces small, green leaves and scarlet flowers, creating a lush and vibrant display of color. This adaptation allows the Ocotillo to survive in the harsh desert environment, where water is scarce and conditions can be extremely dry for extended periods.