I have to point out something, for validity’s sake: I am skeptical of what Lady Nerevar says about the Argonians in Morrowind. I have seen no proof of this, though it is plausible. The two references given are also not valid (one proves nothing, and the other is a reference to her own writing).
My argument against such an idea: there are many Argonians who have never been near Hist sap, however they are not sexless. Considering this, I would say that the idea is merely rumor.
I would like to hear more of this idea, though. Perhaps there is more evidence?
First of, that book I linked isn’t my own writing - it’s a book from ESO. It says “submitted by Lady N” because I’m the one that uploaded it to the Library. The Imperial Library does not include any fan writings outside of the Forum Scholar’s Guild.
Here are various sources for Hist determining Argonian bodies (starting with the two I linked earlier):
It is said that when a Saxhleel emerges from juvenescence, it finds a nearby Hist tree to lick sap from its bole. The elements in the sap quicken the hormonal glands, which sprout appropriate organs from which the Argonian’s gender can be determined. Immediately afterward, an appropriate mate is found and reproduction occurs.
Traveling-New-Woman (MW) again refers to a licking ceremony.
Post by Mark Nelson, of Bethesda, referencing the connection, as well as the Hist’s ability to alter the Argonian reproductive system in general:
There has been talk about how the Hist (and Hist sap) are related to Argonian sexuality. This hasn’t changed…it’s still related.
Never underestimate the adaptability of Argonians, or, more specifically, the power of the Hist to allow Argonians to adapt.
I wouldn’t expect to hear an Argonian born in Skyrim (or on Solstheim, for that matter) mention being hatched. Nor would I expect to hear more transient Argonians (say, members of a small, nomadic tribe) speak about laying eggs. However, in warmer climates, in places with established, stable, and permanent communities, you would likely see a great number of eggs.
GT Noonan, also of Bethesda, on the Hist’s ability to change Argonian bodies:
They may range from either a very humanlike appearance to a crocodile-like appearance. I guess it depends on how many times they decide to lick the tree (refer to the PGE).
Douglass Goodall, yet again of Bethesda, teasing the relationship:
No comment on the exact relationship between the Hist and the Argonian reproductive cycle, but it’s cool.
The in-game Morrowind description of the Argonian race:
At home in water and on land, the Argonians of Black Marsh are well-suited to the treacherous swamps of their homeland, with natural immunities protecting them from disease and poison. The female life-phase is highly intelligent, and gifted in the magical arts. The more aggressive male phase has the traits of the hunter: stealth, speed, and agility. Argonians are reserved with strangers, yet fiercely loyal to those they accept as friends.
Dominion Troops General Order 719a (ESO) includes another vague reference:
The tree has something to do with their mating rituals and life-cycle, so that is understandable.
As for why we don’t see any sexless Argonians: gameplay. Perhaps if we get a game set entirely in Black Marsh, Bethesda would be able to devote the resources necessary to depicting Argonian life phases and physiology in detail. But then again, maybe we’ve already seen Argonians without genitals, but didn’t know it?