When did Christmas Trees changed shapes?

My sister Becky's Christmas card this year was made from a photo of my grandparent's Christmas tree back in the 30's. The shape of the tree was pretty weird. Here's a photo of a tree from 1921. Notice the shape? This is the exact shape of my grandparent's tree in the 30's.



So when did the Christmas tree from the past evolved into today's cone shaped tree?