“…the emphatic quality of the artist’s brushwork … allows the spectator to recreate how the artist, in touching the depicted object in the act of painting it, imaginatively touches the real object itself, or brings it within reach.”
Source: Interpreting Cézanne by Paul Smith, page 63
If you’re undecided as to the direction brushmarks should go when painting something, imagine holding the object and visualise the direction of your fingers. That’s your answer.
What about a tree, I hear you ask? Well, on a small tree I’m likely to try to see if I can wrap my hand around the whole trunk. On a tall tree, I’m probably first going to put my hand on the trunk with my fingers pointed upwards as you look at how tall it is, then turn my hand sideways and pull it around the trunk as I look at how wide it is.
What about a mountain, I hear you ask? Well, if I were to climb it, my fingers would be going up; if I were walking around the lower reaches, my hands will be at ninety degree to the summit.
What about my coffee cup when I’m holding it by the handle, I hear you ask? Well, take a look and see.