Air-Borne Wool Balls or Flakes
At the beginning of summer, a few weeks after the sprouting of new twigs, leaves and flowers of willow ([Salix]) and poplar ([Populus]) trees in natural habitats and in ornamental urban areas, wool balls or flakes become prominent. Bicycle riders are the first to discover this phenomenon; the wool penetrates their eyes and mouth. Those who keep a very clean home and garden complain of the amount of fine wool that settles “everywhere.” Only a few parents, general teachers and nature teachers utilize this phenomenon to teach about seed dispersal. The first reason for this is the fact that the “species” – teachers of natural history -have become an endangered species. If there still are such teachers, only marginal attention was given in schools in the past to the use of this kind of seed dispersal.
During the years I taught Botany at the Hebrew University of Jerusalem I tried to influence my students to pay attention to these wool balls in their free time. The campus of Givat Ram, Jerusalem is rich in poplar trees of several species and when they bloom the air is full of wool flakes and balls. They accumulate on the sidewalks and in spaces between buildings where the wool is protected from winds.
This year I had an interesting surprise when visiting the house of good friends in Berlin. I had been waiting several years to meet them. We had afternoon coffee on their balcony on the 5th floor. Wool balls and flakes arrived constantly with the moderate wind from the nearby Spree River. One kind of trap for the wool was a bucket filled with water. My friend Georg testified that the bucket was empty two days before my arrival and the summer rain of May had filled it. I thought that the “dirt” in the water was wool flakes arriving at their destination. I looked through my magnifying glass and was happy to discover in the wet wool many small “things” 2-3 mm long light green in color. I suspected that these were new seedlings.
I recalled germination experiments I carried out during the first Gulf War, during the days we had to stay at home in the sealed room. A red paper napkin functioned in Berlin as a wet substratum for doing a germination experiment. I folded the wet napkin into one layer and it remained stuck to the inside of the glass during the whole “experiment”. I put fresh wool collected in my friend’s stairway between the transparent glass and the napkin, and left “instructions” to keep the napkin wet by keeping 1 cm of water in the glass. The water rose in the napkin by the capillarity forces following its desiccation. A week later, when I visited my friends, the willow seedlings looked better (Fig. 3.7.1). It is clear now that it is not just wool but a very intensive event of seed dispersal where trees growing near water produce millions of seeds. These seeds are wind-borne and reach the ground as a shower of seeds, but only seeds arriving at locations with non-saline wet ground and with appropriate temperatures will germinate. Thus, a person visiting rivers and fresh water springs may find willow or poplar trees even if they occur far away from sites where these trees normally grow.
I wish to declare that when I tried to germinate “wool” I had collected near my home in Bet Hakerem, Jerusalem, not one single seedling developed. It is possible that the poplar species planted in our neighborhood do not ripen seeds although they produce fruits with wool that assists seed dispersal.
A similar method of seed dispersal, relying on plenty of minute seeds assisted by light hairs, is that of the tamarisk ([Tamarix]) trees, many of which are resistant to saline soils and grow in desert areas.
I find it difficult not to mention here a woman who worked at the Herbarium of the Hebrew University of Jerusalem some 30-40 years ago. The Herbarium was housed in a small building south of the Canada Hall. The woman was standing outside the Herbarium with a dried willow specimen and was blowing the wool out of it. I asked her, “What are you doing?” and she answered: “Someone put absorbent cotton on these sheets and I am cleaning them…”