The takes either a singular or a plural subject. A/an only takes the singular.
When we pluralize a noun preceded by an indefinite article, we simply drop the article (sometimes replacing it with some). Why is this?
3 years later:
Whilst on a separate goose-chase, I came across Greg Carlson's 1977 paper A Unified Analysis of the English Bare Plural, which addresses this issue in refreshing detail. It does not answer my question etymologically, but it substantiates the premise that the so-called null determiner is ambiguous:
ABSTRACT. It is argued that the English ‘bare plural’ (an NP with plural head that lacks a determiner), in spite of its apparently diverse possibilities of interpretation, is optimally represented in the grammar as a unified phenomenon. The chief distinction to be dealt with is that between the ‘generic’ use of the bare plural (as in ‘Dogs bark’) and its existential or ‘indefinite plural’ use (as in ‘He threw oranges at Alice’). ‘Ihe difference between these uses is not to be accounted for by an ambiguity in the NP itself, but rather by explicating how the context of the sentence acts on the bare plural to give rise to this distinction. A brief analysis is sketched in which bare plurals are treated in all instances as proper names of kinds of things. A subsidiary argument is that the null determiner is not to be regarded as the plural of the indefinite article a.
The primary distinction is summed up in these examples:
Weeds grow refers to all weeds, or weeds in general. It is not equivalent to Some weeds grow.
Weeds grow in my garden refers to some weeds, and is equivalent to Some weeds grow in my garden.
I understand that context is often sufficient to determine the scope of the noun without a plural indefinite article1 - but that applies to the singular indefinite article a/an as well. In fact, it seems that a/an is even more redundant, since both Dog barks and Dog barks in my garden are equally indefinite, not generic.
1Carlson 2001 is further germane analysis. In here, he gives examples of sentences for which context is not sufficient to determine the scope of the null determiner. For instance, I only excluded old ladies can mean I excluded all old ladies (generic), or that all those whom I excluded happen to have been old ladies (indefinite - some old ladies may have gotten in after all).