Here are two illustrations, illusory cubes; one is persuasive as a cohesive illusion of cube while the other looks like a set of flat distorted “Y’s” arranged on a surface. Why? We make inferences in order to construct visual sense of the world. In the late 19th century Hermann Von Helmholtz discovered a psychology of perception which noticed that we make an endless series of unconscious inferences from our senses and memoies in order to recognize and live in our world. A sensation of coherence is more easily achieved when the edges between territories appear to blend or, do not appear to be circumscribed by borders or lines. Lines around things separate them; the absence linguistically defining lines allow objects to connect, merge and unify their space. Da Vinci discovers how to exploit this predilection of ours in painting by developing a system he called “sfumato”, meaning smoky or blurred edges. The more blurred the edges, the more the territory joined with the surrounding space. 
In Turner’s landscape example(example #2)you may observe that a portion of the horizon, the line which should mark the border between earth and sky has been dissolved (sfumato effect) allowing the distinction between earth and sky to disappear, and therefore; create a stronger sense of space and atmosphere. 
In Monet’s Water Lily painting ( example #3 in NYC’s Museum of Modern Art) the idea of the reflections of trees and sky on water are effectively confused and merged by the dissolution of the border between reflected trees ( dark green area) against the reflected sky ( rolling pink areas). The two territories blur, bleed, and merge. There is no constant linear spearation to encourage legibility in the painting but rather,we have a sensation of a field of merging and co-mingled colors and shapes. The dissolved borders enhance the spacial unity. Finally, in my own painting example of a luminous interior ( the Met Museum’s Great hall) the light dissolves the columns and walls and, even its occupants. Discretely outlined shapes are sacrificed in the pursuit of luminous unity. 
Morris Docktor
David I enjoyed your illustration and examples which explain quite well closed form vs open form drawing.
I am a portrait painter ,Muralist and sculptor,
Occasionally I teach privately to serious students.
When covering closed form drawing vs open form , I like to point out that human sight is very similar to photography in that the retina of the eye perceives light and the absence of light as does the chemicals on photographic film . The line does not exist in reality. The line that we use is merely an intellectual symbol for contrast between one area or form against another . Buy using contrast in tone or value or texture we are showing the brain the delineation that’s existing between the forms we are representing in our image . This is academic representationalism . We as artists certainly can take artistic freedom to close the form with a line to create accented weight , movement , compositional style and a varying number of effects as well as opening the form by softening contrast . As long as we understand that we are starting to abstract the objective light and dark of our academic objectivity .