Actually that is an Agave plant, a Parry's Agave if you want to get technical. They are also called Century Plants because they are supposed to bloom every 100 years, don't know that that is accurate tho. It was probably an animal that ate the stalk. An agave is also what tequila and pulque is made from. The indians would also harvest the center stalk and eat them, kinda like artichoke. And if you are careful you can pull the sticker off the end of the leaves and get a needle & thread. When the stalks get full heighth and bloom the stalk will die and dry out. These make nice, lightweight hiking staffs. Just sand them down and varnish or oil and stick a rubber cane cap on the end.
I'm pretty sure the white flower is the Silver Morning Glory.
There is a book you ought to get called "A Field Guide To The Plants Of Arizona" by Anne Orth Epple. It has color pics of all the plants then the facts about them in the back of the book. It ran around $35, I bought it about 10 years ago and when I would take the girls out camping we would go on field trips and learn about the plants. Kept them occupied and we got to try several different ones that were edible. Problem is they would remember them and I would forget and have to learn all over again when we went for walk. They liked to rub it in on old Dad.
