Mossive Disjunctions are from the Birds

Though we may not think about it, plants have migratory capacity. Their migrations are not like those of a wildebeest or neotropical warblers. Instead of moving as individuals, plants migrate via seeds, spores, or pieces of the parent plant that can then grow into a new, albeit genetically identical individual. Either way, long distance dispersal events have long puzzled ecologists. It has been demonstrated time and again that even modest barriers can inhibit propagule movement. Still, it would seem that over the course of time, plants have managed to overcome such boundaries. One way or another, plants have made some impressive migrations.

Some species have really managed to confuse ecologists. Certain mosses and lichens have very curious distributions. There are species that are found only in the Arctic and the very southern tip of South America. Nowhere in between. Why is this? There have been hypotheses regarding wind currents but the genera to which these plants belong originated in the Miocene and Pleistocene, while the Intertropical Convergence Zone (a major barrier between northern and southern wind currents) was already in place.

Recently, researchers have looked towards long-distance fliers like plovers to explain these distributions. These birds breed in the Arctic and overwinter in South America. Could these be the vessels by which these plants migrate? It has long been known that seeds passing through the gut of a bird often have high germination rates. Many plant species gear their fruit specifically for this reason. Birds travel great distances in their search for food and breeding territory, much greater than the average plant can. But birds aren't necessarily eating mosses and lichens. However, they do use them in their nests. Spores and bits of vegetative material can then get stuck in their feathers. After breeding, the birds migrate to South America and begin their molt. The feathers containing spores and plant material are now shed into the wild where they can germinate and grow.

Considering the size of these migrations, it is likely that these migratory shore birds, and possibly many other species of migratory birds, play a significant role in the dispersal of these plant species.

Photo Credit: barloventomagico (http://bit.ly/1p1X2WC)

Further Reading:
https://peerj.com/articles/424/

Cooksonia: A Step Into the Canopy

image.jpg

For plants, the journey onto land did not happen over night. It began some 485.4–443.4 million years ago during the Ordovician. The best evidence we have for this comes in the form of fossilized spores. These spores resemble those of modern day liverworts. Under high powered microscopes, one can easily see that they were indeed adapted for life on land. These early plants were a lot like the hornworts, liverworts, and mosses we see today in having no vascular tissues for transporting water, an adaptation that would not come along for another few million years. 

Without vascular tissues, plants like liverworts and mosses cannot transport water very far. They instead rely on osmosis and diffusion to get water and nutrients to where they need to be, which severely limits the size of these types of plants to only a few centimeters. This growth pattern carried on well into the Silurian. Until then, the greening of our planet happened in miniature. 

image.jpg

Around 415 million years ago, however, plants became vascularized. This changed everything. It set the stage for the botanical world we know and love today. Paleobotanists place the fossil remains of these newly evolved vascular plants in the genus Cooksonia. Based on what we would call a plant today, Cooksonia probably pushes the limits. However, in some species the branching structure is full of dark stripes, which have been interpreted as vascular tissues. It still wasn't a very tall plant with the tallest specimen standing only a few centimeters but it was a major step towards a much taller green world. 

Cooksonia did not have any leaves that we are aware of but some species certainly had stomata (another major innovation for water regulation in plants). Each branched tip ended in a sporangium or spore-bearing capsule. It has been suggested that Cooksonia may not represent an individual photosynthetic plant but rather a highly adapted sporophyte that may have relied on a gametophyte for photosynthesis. This hypothesis is supported by the diminutive size of many Cooksonia fossils. They simply do not have enough room within their tissues to support photosynthetic machinery. Because of this, some botanists believe that vascularization sprang from a dependent sporophyte that gradually became more and more independent from its gametophyte over time. Until an associated gametophyte fossil is found, we simply don't know. 

Photo Credits: Steel Wool (http://bit.ly/1AjLYh8) and Sabrina Setaro (http://bit.ly/16mdyxw)

Further Reading: [1] [2] [3]

Growing Ferns

I am finally having some success intentionally growing ferns from spores. I collected and sowed spores from some interrupted ferns (Osmunda claytoniana) over the summer. They have been hanging out as gametophytes for months now and some are finally starting to grow sporophytes. Here is how it worked for me:

I kept my eye on a batch of adult plants this summer. Once their fertile fronds developed I would flick them every now and then to see if they were releasing spores. Once I saw that they were I shook the fronds over some paper to collect the spores. I then took some old potting soil and sterilized it with boiling distilled water. I use old takeout containers because they are small and have clear lids that form a seal which keeps the humidity high.

Once the soil was cool I sprinkled the spores over it and then placed it on a shelf where it gets a small amount of ambient light every day. The rest they did themselves. You just have to remember to check on them and keep the humidity quite high because they can dry out really fast. They seemed stuck as gametophytes for months. I just noticed the start of these sporophytes the other day.