As there are many varieties of each, the easiest way to tell the difference is by looking at the leaves and cones. Generally, spruce trees have hanging (pendulous) cones and angular, four-sided needles (see picture on the left). The cones of fir trees are usually more erect and their needles are much flatter. Common spruces are the Blue Spruce Tree (the Colorado Blue Spruce Tree is one of the most beautiful, and often harvested as a Christmas tree), and the Norway Spruce Tree. Common fir trees are the Alpine Fir and the Noble Fir (also often used as a Christmas tree.

Image credit: craterlakeinstitute.com.

Mimosa pudica (also known as the “sensitive plant” or “Tickle Me plant”) is well known for its rapid leaf movement. In the evening the leaflets will fold together and the whole leaf droops downward. It then re-opens at sunrise. This type of movement is called nyctinastic movement. The leaves also close up under various other stimuli, such as touching, warming, or shaking (hence the name, Tickle Me plant).

The stimulus can also be transmitted to neighboring leaves. The scientific term for these are seismonastic movements. The movement is caused by “a rapid loss of pressure in strategically situated cells that cause the leaves to droop right before one’s eyes”. [Wikipedia] This characteristic is quite common within the Mimosaceae family. Gradual loss of pressure can also cause the plant to close its fronds as is sometimes the case when it is exposed to a strong wind, rain or sunlight.

Why the plant’s leaves close up when exposed to these external stimuli is most likely an evolutionary response. As the leaves fold down, sharp thorns are exposed and would make the plant less tasty for herbivore animals that might want to eat them.