Here are some small sketches of trees made whilst taking the dog for a run in the nearby woods. A pocket-sized sketchbook and a fibre-tipped pen are ideal for making this kind of drawing and visual notes.  Drawing with this kind of pen is pure pleasure : the fineness of the nib for detail, the blackness of the ink for shading, no pencil sharpener needed, the responsiveness of the pen when drawing fast. The one drawback is the inability to vary the thickness of the line. For this I recommend a broader harder felt nib and vary the pressure, or ,of course, a flexible nibbed dip pen.