texture map and blend

hi,I am a newer of opengl. I want to make a tree with texture map. I draw two rectangles wihch intersect with 90 degree each other and texture map with a tree map. but i can’t erase the fragment of nouse of the tree’s image.how to set the function of glblendfun(,),how to set gltexenv() can solve it?
thanks very much

take a look here:

there’s also an example on how to make translucent tree