One reason is because the short days and long nights prevent the earth from warming up, another reason could be because the sun's rays in winter are more spread out thus minimizing the amount of energy
The causitive agent of the cohesion-tension model of xylem transport is transpiration. During the process of transpiration, water vapor is lost from the stomata of the leaf. To replace this water, water from adjacent cells is withdrawn. The water molecules stick together due to cohesion and are transported upwards through the stem in the form of a stream.<span />
