Everything at the end of the day is a form, but what determines whether we approach it constructionally (using outlines, drawing through forms, etc.) or as a texture (drawing only the shadows those forms cast on their surroundings) is whether or not that form is bound tightly to the surface of another form. In some cases there's leeway to go with either option, especially when you're looking at forms that exist along the surface of an object but protrude a lot - but with the example you gave, seeds are usually going to be treated more as texture along the surface of a larger ball form.

As to your other question, line weight and the sort of cast shadows that help separate out our forms from one another can be used even when sticking only to construction. It's not part of construction exactly, but it's one of those tools that can help clarify what we've drawn.