What is a darker contor line usaly every fifth line?

Index contour.

Explanation: Index contours are those elevation line which are represented by a thick or darker line. It is because if we draw every elevation with a simple line, its not so reliable. So making every fifth contour line dark helps us in reading the map elevation clearly without any difficulties.

   This is very commonly used in the maps and are separated at equal heights.

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.

a- all cells come from pre-existing cells