P, Pistillate, s, staminate flowers; h, sterile flowers forming a circlet of stiff hairs closing the mouth of the chamber formed by the lower part of the spathe.
In the case of unisexual flowers, whether monoecious, that is, with staminate and pistillate flowers on one and the same plant, such as many of our native trees - oak, beech, birch, alder, &c., or dioecious with staminate and pistillate flowers on different plants, as in willows and poplars, cross pollination only is possible.
The flowers, which appear in early summer, are in pendulous, slender yellowish catkins, which bear a number of staminate flowers with a few pistillate flowers at the base.
The staminate contain 8 to 20 stamens which produce an enormous amount of dusty yellow pollen, some of which gets carried by wind to the protruding stigmas of the pistillate flowers.
When only one of those organs is present the flower is unisexual or diclinous, and is either male (staminate), j, or female (pistillate), ?
The pistillate whorl is very liable to changes.