The evolution of vascular plants produced real roots and allowed for the development of complex above-ground structures because now there was a circulatory system that allowed for the transport of everything the plant needed throughout its entire body. This adaptation opened the door to many complex structures and behaviors, but it also allowed for the possibility of many things that could go wrong.

The bryophytes of 500 million years ago initiated an upward ladder of plant complexity starting with ferns. Ferns and their relatives (horsetails, whisk ferns, club mosses, spike mosses, and quillworts) were the dominant plants during the Carboniferous period, approximately 300 million years ago.

Ferns and their relatives made one especially significant change from their mossy ancestors: they developed a simple vascular system. By developing true roots, they could pull water, with all its dissolved nutrients, up from the soil or off a rock. The stems could then take the mineral-rich water and transport it into the fronds. Likewise, the chloroplasts inside the green fronds and stems could take the sun’s energy, convert it to glucose, and then send those simple sugars throughout the entire plant. Unlike bryophytes, not every part of the plant needed to absorb water and perform photosynthesis. They could have specialized parts in their overall structure. Ferns could now grow to be much larger than their mossy predecessors.

There simply were no pollinators present in the Carboniferous period; there were plenty of insects, but there was nothing for them to pollinate.

fern sperm is flagellated and can only reach a fern egg by swimming to it. They are completely dependent on the weather around them to produce new ferns. Reproduction cannot occur during periods of drought unless a layer of dew is produced overnight. More often than not, fern reproduction requires extended periods of moisture produced by rain and high humidity.

The downside to spores is that they contain very little nutrition for the tiny embryonic plant inside. When a spore decides to sprout, the baby plant must find water and nutrients almost immediately if it is to survive its first days. There’s little room for error. For that reason, the overwhelming number of spores produced by ferns and mosses perish before the young adult is even noticeable.

the Permian period, approximately 50 million years later. By this time, plants easily recognizable as gymnosperms (conifers, cycads, gingkos, and gnetophytes) comprised the dominant plant life.

did not encounter flowering plants until sometime late in the Cretaceous period, approximately 90 million years ago.

Gymnosperms produced seeds in protective cones; they produced pollen instead of flagellated sperm to fertilize their eggs; and their fertilized seeds contained large stores of nutrients for the developing embryo inside. They could still cast huge quantities of pollen into the air because, like spores, there was little parental investment in producing it. However, they could focus more attention on the seeds, producing just a few in comparison to ferns and mosses but ensuring that a much higher percentage would actually develop and grow.

Instead of being encapsulated inside a thin casing as spores are, seeds have both a seed coat and a rich food source to protect and nourish the developing embryonic plant inside. Spores are cast out into the world by their parents with few resources on which to draw; seeds pamper the infant, protect and nourish it, and give it time to fully develop before it leaves the seed, sprouts, and becomes a functioning juvenile on its own.

Whereas the simple rounded spores of mosses and ferns require an air current to move them away from their parents, wind-dispersed seeds are often aided by the presence of “wings” to help carry them farther.

plants sense light by using pigments, known as cryptochromes, phototropins, and phytochromes. These are pigments located within the interior (cytoplasm) of plant cells (not in the chloroplasts), though some are located within the nucleus itself. Photoreception is completely different from photosynthesis. The two systems occur in different parts of the plant, rely on different sets of pigments, and use different parts of the visible-light spectrum. While photosynthesis relies primarily on light within the red portion of the spectrum, photoreception uses light in both the red and blue.

Phytochromes are the pigments most involved in maintaining a plant’s biological clock. Not only do plants need to measure the changing seasons through seasonal changes in day length, but they also need to sense the twenty-four-hour day and adjust their activities accordingly. We’ve understood the role this plays in animals for some time. Plants as well as animals maintain an internal biological clock, known as “circadian rhythm,” that helps them determine the passage of twenty-four-hour blocks of time.

The brilliant fall colors that we admire occur in response to the complicated relationship between shortened day length and cooler temperatures. As plants assimilate this information, they quit producing chlorophyll and prepare to drop their leaves for winter.