Which is the primary source of stem thickening in plants?

The primary source of stem thickening in plants is the vascular cambium. It is a plant tissue located between the phloem and the xylem and in the root of a vascular plant. It is the source of secondary growth or the radial growth of the stem of a plant.

No because the eaeht is on a 23.5 degree axis tilt and so when it is winter the northern hemisphere is pointing away from the sun and the soutubern hemisphere is pointing toward the sun

Your answer is water because there is combined mass of water found on, under, and over the surface of a planet. your answer is water.