The way to foreshorten leaves that are oriented toward you or at a right angle to your line of sight is fairly straightforward. The proportions of the leaf will change but the leaf will remain symmetrical. However, when a leaf (or petal) is foreshortened and the axis is oriented at a 45 degree angle to your line of sight, it becomes asymmetrical. The more that one side of the leaf tip points toward the observer (while the other does not) the greater the asymmetry will be. Once you start looking for this asymmetry you will see it in petals and leaves everywhere.
To help you intuitively understand these changes, download and print the leaf model and follow along as you study this post, comparing what you see in the model with the demonstrations. To get this effect to work with the leaf model, hold the model at eye level, close one eye, rock the leaf back to a steeply foreshortened angle and then (keeping the angle) point the leaf tip over one of your shoulders.
Click on the first image to start a slide-show tutorial.