[Cob] purpose of straw in cob

[Damon Howell]

What is the purpose of straw in a cob mix? Nobody seems to "really  
know" what the role of straw is anyway. Is it there to hold the cob  
together while the wall is still wet (like a free form), or to keep  
the wall from crumbling incase it cracks later (like reenforcement),  
or to allow air/water to move through the wall (because straw is  
hollow)? The problem is that nobody knows the reason they used straw  
because they didn't leave behind notes on how and why they built that  
way, and it's been a while since they lived here. What do they do in  
Africa? Do they use straw "in" the cob? Can any other plants be used  
as tensile such as long grasses? I'm almost ready to start cobbing  
but straw is just unavailable in GA right now, and what straw there  
is has a very high price on it. I'm not willing to pay three times  
the price for it if there's a substitution. I would love to just go  
out in the field and get some tall grass if it would suffice. It's a  
heck of a lot cheaper!

Chow,
Damon Howell
North Georgia, US