In the last few blogs I have highlighted two different methods of seed dispersal: myremochory (via ants and eliasome) and epizoochory (via animals). Remember, the sole purpose of a plant and its seeds is to reproduce!
Since I like to read the dictionary, I have been intrigued to find out that botanists have individual names for all kinds of dispersal methods that could awe the reader for years with the minute details of plant reproduction. This quarter I will define and highlight two different kinds of seed dispersal that I recently learned about, both of which surprised me.
Ballochory and barochory are examples of autochory seed dispersal. An autochorous plant disperses its own seed without the need for wind, rain, animals or other help. But because autochorous seed dispersal limits the distance of natural seed spread, the plant world has figured out some work-arounds!
Ballochory: This seed dispersal mechanism is “explosive.” As the inside and outside of the seed pods dry out, there is a tension arising between the hull and the seam of the pod. When the tension reaches it's personal threshold, the pod bursts at the seam flinging seeds feet or yards away, depending on the plant. It is fun to watch when it happens on its own but even more fun to poke each pod to see if they are ready to pop. If it is ready, it will pop like a balloon flinging the seeds so quickly it is difficult to catch any. If it is not ready (ripe), the pod will remain closed. When it does pop, the husk will fold back on itself and often twist into a corkscrew. Great examples of plants with this interesting seed dispersal mechanism are sweet peas, true geraniums, and impatiens. Bedding impatiens are sterile and don't seed. However, there are many forms of native impatiens: tall, short, perennial, annual, all of which have gangly growth habits, beautiful flowers, and plenty of seed pods to play with. Many are considered invasive in parts of the country where it is damp.
Sweet pea seed dispersal
Barochory: Gravity is this mechanism of seed dispersal. Good examples are apples, coconuts, pears, and acorns of all kinds. When we think of fruit trees, we think of harvesting. Ladders, shakers, boxes, bags, and ground nets all help humans remove fruit from trees manually for a systematic harvest. However, if left to their own devices, the fruit will fall out of the tree when fully ripe. Under the right conditions, the fruit will rot, leaving the seeds in a fertile environment for development. But that isn't great for the fruit orchard: too much competition for space, light, and nutrients. Instead, animals eat the fruit and deposit the seeds far afield when they poop. But this propagation method leaves a lot to be desired: the percentage of seed germination is low (below 30%), and offspring are not true to the parent plant, with tasteless or bitter fruit. Ranchers pick up the fallen fruit and dump it or use it as animal feed.
To ensure consistency of fruit type for taste and health, modern-day fruit tree propagation is handled by two-legged animals called humans. By grafting hardy, productive scion wood to stable hardy rootstock, scientists have advanced the cultivation of fruits to feed the world in all manner of climates.
Acorns fall by the thousands straight out of the tree. Anyone wandering an oak forest sees spindly seedlings struggling for light, nutrients, and water, because the mother oaks have priority when it comes to these life necessities. It is lucky for these trees that they have found partners to distribute their acorns to increase the health of their “children” and the forest.
Acorns are picked up by squirrels who hide them and often forget where they are, chipmunks who bury them for winter food in their burrows, birds who carry them to a nice wide branch to eat or drop them inadvertently, and humans who hate them and rake them up to get rid of them. The US Department of Agriculture reports that 5% of acorns on the surface of the ground will germinate, 25% of acorns embedded evenly with the ground surface will germinate and 79% of acorns buried 1 inch below the surface will germinate. With all these partners spreading acorns over acres of forest land, surviving seedlings are more likely to have enough space, light, water and nutrients to grow and flourish.
References
Pell, Susan K, & Angell, Bobbi. A Botanist's Vocabulary. Timber Press, Portland, Oregon, 2016.