The English Walnut (Juglans regia) is the most common and widely distributed walnut tree species in Europe. In Spanish the Walnut tree is called "Nogal" and the Walnut fruit is called "Nuez". This tree is highly valued both for its popular nuts and for its high quality wood although the most sought after Walnut wood comes from the "Juglans nigra L." (Black Walnut) which is a close relative.
Looking at these pictures of the Walnut fruit it is hard to imagine that on the inside it contains the wrinkled, light brown, Walnut that we are used to seeing in the grocery store or market. I visited this tree a few years ago when the ripe Walnuts were falling. The round green ball that you see above turns brown and shrivels up a bit before it falls. To get the nut you collect the fallen fruit and remove the outer husk which comes off pretty easily. At that point what you have is the hard shell that we are familiar with which needs to be open with a nutcracker.
The images above and below are of the male flower of the English Walnut tree.
The Common Walnut (sometimes called Persian or English Walnut as well) has oblong leaflets that are about six inches in length and are arranged odd-pinnately compound on the stem. The venation is also pinnate and the leaf has entire margins.
One distinctive of the Walnut tree is its deeply furrowed bark.