The Biology of "Pussy Willows"

There is one particular species of plant called the "pussy willow". Pussy willow is the common name for Salix discolor, a member of the willow family, the Salicaceae. But, nearly all willows, as well as some other species in this plant family, have furry little "flower buds" - what people usually refer to as "pussy willows" - arranged along their branches and stems. Most people don't distinguish between "pussy willow", the plant and "pussy willow", the phenomenon of willow flowering in the spring. So, I decided not to, either.

This essay is an exploration of the general nature of reproduction in willows, with apologies to those purists that might object to my non-technical use of the phrase "pussy willow". So, throughout the remainder of this article, pussy willow refers to willow flowers, in the generic sense. Although, I have to admit that part of wanting to do this was to avoid having to get into willow taxonomy and identification. They are not the easiest group to deal with, and I'm no expert at telling all the different species of willows apart.

Pussy willows emerging in spring.

Catkin ("pussy willow") of a related plant,
Trembling Aspen (Populus tremuloides)

In Manitoba there are about 30 native plant species belonging to the willow family: 5 kinds of poplars (Genus: Populus) and around 20 kinds of willows (Genus: Salix). They all have flowers in the form of catkins (also called "aments"). A catkin is defined as a scaly spike of flowers of one sex only. Willow plants have flowers of each sex on separate plants; there are male willow plants and female willow plants. Uh, oh...time for a botany lesson!

Flower Structure 101

The typical "flower" (of a "dicotyledonous angiosperm" ; sorry, but that is the required term! ) that most of us are familiar with, consists of both male and female reproductive parts. The female parts of a flower are the "carpels" (collectively known as the gynoecium - "house of woman"). Each carpel (there may be several or many per "flower")consists of a stigma, style and ovary. The stigma is the receptacle for pollen. The style is the supporting stalk leading from the ovary to the stigma. And the ovary is the site where the ovules (or eggs to use an animal equivalent, though there are some major differences) are produced.

The male parts of the flower are the "stamens" (collectively the androecium - the house of man). Each stamen (there may be many per "flower") consists of a filament (stalk) that supports an anther. The anther produces the pollen (roughly equivalent to "sperm" in animals, with significant differences, as with the ovules). The carpels and the stamens are the true reproductive parts of the flower. The rest of the flower, most of what we see, are the infertile, or supporting structures: the petals, sepals and receptacle. The petals, or in many cases the sepals (lilies for example), are the "advertising" that plants employ to attract insects. Insects, attracted by the "flower" and the reward of pollen and nectar to be had, serve the plants by transferring pollen from one flower to another, thus fertilizing or cross-pollinating the ovules of other members of that plant species.

A typical flower, wild rose (Rosa arkansana).

Thus, a typical flower has both male and female reproductive organs together in the same structure (though they usually require pollen from another plant's flower for fertilization to occur). Some plants, however, have separate male and female flowers on each individual plant. That is, each individual plant has male and female flower structures on it at different sites on the plant. These and plants possessing the more typical "male and female together" flowers are referred to as monoecious (in one house). Then there are plants, like willows, that have separate sexes, just as animals do. They have male flowers on "male" plants and female flowers on "female" plants. They are called dioecious (in two houses). And this brings us back to our original topic.

Back to Pussy Willows

Of course, having just explained what a typical flower is all about, now we have to discuss pussy willows, which do not conform to the typical nature of a flower. As I mentioned before, a pussy willow is a catkin, a tightly bunched arrangement of stamens or carpels. Think of a cob of corn, and think of each anther (or carpel) as one kernel on the cob. That's a pussy willow, a dense "corn cob" of anthers (or carpels). Each anther (or carpel) has its own attending silvery hairs, that viewed altogether, when the flower is still young, present the attractive pussy willow we all know.

A cross section of a female pussy willow, with
the carpels arranged around the central axis.

Most of what people see and think of as pussy willows are, in fact, the male flowering parts, or male catkins. The female catkins tend to develop and open a little later than the males, but they can form attractive pussy willows, too. And what, to us, is the most attractive stage in the pussy willow is actually very early on in the emergence of the catkin. The soft, silvery hairs that we see are the "fur coat" that helps to keep the developing reproductive parts warm. Remember, pussy willows emerge in early spring when it's still quite cold. But when the sun shines, the temperature of the center of the catkin can rise above air temperatures by trapping the heat from the sun with it's insulating hairs. This additional warming aids in the development of the pollen within the anthers and of the ovules within the carpels. The willow flowers are fully "open" when the yellow pollen-bearing anthers are protruding and the stigmas are visible. (My mother always threw out the pussy willows when the yellow pollen started falling all over the place.)

Another way that willows are atypical involves the way that the pollen is transferred from the anthers to the stigma. Most plants that have catkins or other "non-attractive" flowers (as opposed to the brightly petaled structures we normally think of as being flowers) are wind-pollinated. That is, the pollen grains are simply released on the breeze and the plants count on chance to bring some of their pollen to rest on stigmas of their own species. Close relatives of the willows, the poplars (Populus spp.), all spread their pollen in this manner.

Open catkin of Trembling Aspen.

Catkin of Green Alder (Alnus viridis).

Willows, however, do not spread their pollen via the wind. Instead, they rely on insects for pollination, despite having less than gaudy flowers. What they lack in visual cues, they clearly make up for in olfactory ones, producing large amounts of strongly scented nectar. Bees and flies are readily drawn to pussy willows in full bloom. One of the advantages of flowering early in spring is that there is very little competition for pollinators. The willows gain the full attention of the many bees and flies that also awaken early in the spring and are desperate for food.

Once the ovules inside the carpels have been fertilized, the willow seeds begin to form. The carpels on the female catkins swell and grow much larger, producing "capsules" where the seeds develop. When the seeds are mature the capsules break open and myriads of tiny seeds, each with tufts of white hairs are released. The carpels look like they are covered in cotton fluff, but the wind and rains soon disperse the seeds to start the willow life cycle over again.

So, the next time you out for an early spring walk, take a moment to examine some pussy willows, those furry flower buds of willow plants. Then plan another walk to return when the "flowers" are open, then another when the seeds are ripe, and another..... Oh, just keep going for walks and keep you eyes open, the wonders of nature in Manitoba are always there.