Is speed almost never constant

depends on the situation, but scientifically, Einstein's theory of special relativity sets the speed of light, 186,000 miles per second (300 million meters per second). But some scientists are exploring the possibility that this cosmic speed limit changes. The speed of light is constant, or so textbooks say. (google)

the law of conservation of mass states that in a chemical reaction, the total mass of reactants is equal to the total mass of products. because atoms are only rearranged in a chemical reaction, there must be the same number of sodium atoms and chlorine atoms in both the reactants and products

the leaf is an organ of a typical plant, and the main functions are to complete photosynthesis and allow the plant to exchange gas, just like how we have the respiratory system. leaves have the stomata, which collects maintains co2 and o2 levels. this is necessary for cell metabolism. the leaf also collects sunlight using chloroplasts. succinctly, leaves own a pivotal role in sustaining plant life.