A. Plant produces flowers
B. Single flower produced per stem . . . . . . . . . . . . . . . . . . . . . . . . . Plant 3
B1. Multiple flowers produced per stem
C. Flowers contain petals . . . . . . . . . . . . . . . . . . . . . . . . . . . Plant 2
C1. Flowers do not contain petals (only stamens) . . . . . . . . . Plant 4
A1. Plant does not produce flowers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plant 1
Thanks Jamie! This is a good key. Have another look at plant 4. There are two different inflorescences shown, one that has only pistils shown. You can see the ovary, style and two-forked stigma, those are the pistillate catkins of willow. On another picture you can see the inflorescences that lack pistils and only have stamens, the staminate catkins of the willow. These are from the same plant species. Willow are dioecious plants with staminate and pistillate flowers found on different individuals. Best, Steffi.