I should label this post as another Classroom Adventure, I did this demonstration with my students on a field trip to Alley Pond last week. But we were so caught up in the fun, I forgot to take pictures! So my friend Hubert took me to a spot with a nice stand of cattails to recreate the exercise. (All photos in this post, except one indicated, are by Hubert Urruttia)
We all know that plants are sessile organisms, they don't move around the way animals do. Because of this, plants rely on a variety of methods to ensure that their seeds find a good place to germinate. If seeds just fell from the plants right where they are, the seedlings would crowd around the parent plant and they would all compete for water, nutrients, light and space. None would survive very well. So the parent plant needs the seedling to germinate a distance away. This movement of the seed away from the parent plant is called seed dispersal and plants have evolved several mechanisms for achieving this.
Gravity works, only if the seed can then roll away some distance. We are familiar with animal dispersal, for example, a bird eats a cherry and then poops the seed out far from the parent tree. Some seeds have barbs on them so they stick to animals' fur (or clothing in our case) and then fall off sometime later. Jewelweed and others have spring-loaded seed coats which fling the seeds mechanically (a video for a later post).
These photos are of wind dispersal. The most common example used is the dandelion. Even we city kids grew up picking dandelions and blowing on the seed heads to watch the fluff float away. Each little piece of fluff (called a pappus) carries a seed that will hopefully find a good place to germinate. It is a random process and lots of seeds land in places that are not suitable, so the plant makes many many seeds to guarantee that some will germinate.
My favorite example of wind germination is the cattail. Besides the fact that you end up with tiny seeds attached to fluff floating away, take a look below at just how many seeds are packed into a cattail fruit!
Cattails (Typha spp.) are monoecious, that is, both flower sexes are on the same plant. But the flowers are unisexual, male and female are separate. In the first photo, the brown sausage-shaped spike is the collection of female flowers that have formed seeds. The stick above it used to hold the male flowers. Once the male flowers shed their pollen, they wither and fall away. The second photo is one that I took in Central Park back in June and it shows the male and female flowers.
In the next images, you see how tightly packed are the seeds and just how many there are! Scratch at the surface and they will billow out. This is the part that is fun for kids (of all ages!). Birds use the fluff to line their nests and I would bet that as they perch on the stalk and pull out the fluff, they get the other seeds to start floating away, just as I am doing in these photos. So next time you see a ripe stand of cattails, scratch at one and have some wind dispersal fun!