Questions about Discursive Texts

After reviewing Foucault’s What is an Author, I find that one concept that I think was meant to perplex me as his reader was the fourth, unorthodox author mentioned--the one who takes part in “discursive practices.” 

Foucault defines discursive authors as ones who are “not just the authors of their own works,” but who open up a direction for future texts. They create a new rhetorical situations through their texts and allow other writers to fall into the categories they created. Foucault used the example of Karl Marx as a discursive author because he established an “endless possibility of discourse.” Still, the concept of discursive practice and discursive authors is a little confusing to me. I have four questions regarding discursive practices:

1. As “discursive authors” are not just authors of their own texts, but indirect authors of future ones as well, is the author an agent of appropriation? If so, do they have their own agency at all? 
2. In Herrick’s background of Foucault, it is said that Foucault believed that “systems of discourse come to control how we think and what we claim to know.” In light of this statement, do Foucault’s works, in our current system of discourse, act as discursive texts as well? 
3. According to Herrick, Foucault considered knowledge as a “matter of social, historical and political conditions under which, for example, statements come to count as true or false.” If so, aren't most texts discursive if they can be analyzed and reanalyzed through the lens of the “social, historical and political conditions” as they change over time? 
4. What did Foucault mean when he argued that the discursiveness of scientific texts is a different story compared to that of literary ones? 

Herrick, James A. The History and Theory of Rhetoric: An Introduction, Third Edition. Boston, MA: Pearson, 2005.“Foucault” (246-252).

Foucault, Michel. “What Is an Author?” The Critical Tradition: Classic Texts and Contemporary Trends,Third Edition. Ed. David H. Richter. Boston, MA: Bedford/St. Martins, 2007. 904-914.