Slide 1
The seedless
vascular plants were the second major group of plants to evolve.
Slide 2
The vascular seedless plants are not a monophyletic group but they all share the characteristic of having vascular tissue.
Vascular tissue allows the plant to move water and nutrients around the body of the plant - this reduces the chances of desiccation and allows the plant to grow larger.
In fact, some of the early vascular seedless plants were tree sized - ferns up to 30 feet tall as seen as the artists recreation of what earth may have looked like about 300 million years ago.
Not only were ferns huge at this time - insects such as dragon flies were as well. Recent research suggests that this is because oxygen levels in the atmosphere were significantly higher and this enabled larger organismal growht.
Despite the presence of vascular tissue which prevents desiccation - the ferns and other vascular seedless plants retain the primitive characteristics of flagellated sperm so they still require water for reproduction.
Slide 3
Vascular tissue is specialized tissue that transports water and nutrients around the body of the plant.
Xylem is the tissue that transports water and mineralse from the soil to other parts of the plant.
Phloem transports the sugars that are produced by photosynthesis. These sugars are transported all over the plant and can be stored as starch in the roots.
The vascular plants have true roots, leaves, and stems because they have vascular tissue. Compare this to the rhizoids of the mosses - rhizoids are not true roots because they do not have vascular tissue.
Lignin is a structural polysaccharide that is associated with xylem. It provides structural support to vascular plants - highly lignified tissue becomes "woody" and strong. Plants with a large amount of lignin can become quite large.
Slide 4
Spores are reproductive structures that are adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Typically spores are haploid and unicellular and they are produced by the sporophyte.
Just like the nonvascular plants - the vascular seedless plants do not produce pollen, flowers, or seeds and they rely on spores for dispersal.
As we saw earlier, many of the mosses are heterosporous - they produce megaspores which produce female gametophytes and microspores which produce male gametophytes.
Many of the vascular seedless plants are homosporous - they produce a single spore type which gives rise to a bisexual gametophyte.
Slide 5
The vascular plants have a prominent sporophyte generation in their life cycle (in comparison to the nonvascular plants that have a prominent gametophyte).
While the sporophyte is prominent the gametophyte is still independent (we will see that this changes when we get to the seed plants). An independent gametophyte means that it lives independently for part of the life cycle and is capable of photosynthesis.
The nonvascular plants are the only ones with a prominent gametophyte. All other plants have a prominent sporophyte so there must be a selective advantage to a prominent sporophyte. What do you think this advantage might be? The hint is that it has to do with UV radiation.
Remember, the sporophyte generation is diploid - it has
two copies of each gene while the gametophyte generation is haploid and only
has one copy of each gene.
UV radiation is a mutagen - it mutates DNA. An organism that has two copies of each gene is protected against the negative effects of UV radiation more than an organism with just one copy of each gene.
The second copy is insurance against mutation.
Slide 6
Examples of the vascular seedless plants are the ferns as well as the club mosses and equisetum - also known as horsetails.