How many types of elements together make up the xylem tissue ? Name them.

How many types of elements together make xylem tissue? Name them.

How many types of Plant tissues are ?

(a) How many types of blood vessels are there in the human body ? Name them (b) Why does the heart need valves ?

How many types of crystal systems are there? Name them.

How many types of ecological pyramids are there in practice? Name them.