Key Concepts

Plants, as well as some Protists and Monerans, can take small molecules from the environment and bind them together using the energy of light.  The incoming light energy is transformed into the energy holding the new molecules together, and the organisms use those molecules as an energy "fuel."  The basic process can be represented this way:

CO₂    +    H₂O      light   _>    C₆H₁₂O₆    +    O₂
  Carbon     Water                     Glucose          Oxygen
 Dioxide                                     (sugar)                    

Light can be understood as a combination of energy waves traveling outward from a light source, or as photons, small packets moving from that source at the speed of light (each wave peak would correspond to a single photon).  Light always travels at the speed of light, altering only for the material through which it's moving (it goes slower in water, for instance), so a segment of a light beam with wave peaks more separated (a longer wavelength) would have fewer peaks absorbed by a surface (a lower frequency) in any given amount of time, and would hit that surface with less energy.  This means short wavelength = high frequency = more energy,  long wavelength = low frequency = less energy.  The only reason that this is important is that sunlight contains a fairly wide range of energy frequencies, but only a few are absorbed and used by chlorophyll, the energy-capturing molecule of photosynthesis.

You can tell a few of the frequencies that are not absorbed by chlorophyll (and a few other light-absorbing molecules) by looking at a plant.  That green you see is part of the reflected frequencies of light.  For the most part, absorption of the other frequencies of light is used in an energy conversion process that "spits" electrons through a system from the "excited" chlorophyll molecules.

Although land plants absorb a variety of light frequencies, all frequencies are not equally powerful or useful:  while plants can absorb both red frequencies and purple frequencies, the purple have shorter wavelengths and carry more energy.  This is one of the reasons why "plant lights" may be distinctly purple.

It is not unusual for land plants to use molecular supplements to absorb some frequencies that chlorophyll can't, and feed more energy into the photosynthesis process;  these pigments are commonly types of carotenoids.  The colors of leaves in the autumn reveals the carotenoids that have always been there but have been covered by huge amount of chlorophyll.  

The many varieties of carotenoids can serve multiple roles:  they can be photosynthetic aids, but they may also minimize light damage (animals use pigments, like the human tan-producing molecule melanin, for similar protection) or even function in fighting disease.  Land plants may concentrate pigments, including carotenoids, in structures that need to stand out, such as the colors of flowers or mature fruits.  These colors signal animals that a food bribe is available, and then the animals are used to carry pollen or seeds.
 

Plants, like all living things, are descended from ancient organisms that evolved and developed in the oceans.  The ocean-living plants that represent land plants ancestors are simple, so much so that they are often classified as Protistans - little multicellular complexity was needed to float in the light.

Even in the ocean, though, there are a number of different niches available for plants - in the shallows, many algae varieties exist that can attach to submerged objects, probably a feature that helped the land plants' direct ancestors to stick out of the water into the air.  But niches also exist in the deeper water, where there is a functional connection between the photosynthetic pigments used and the nature of how light interacts with water.

The surface of the water reflects some light, but the frequencies that get through get absorbed - you already know that the deeper you go, the darker it gets.  What you may not know is that not all frequencies get absorbed equally - the frequencies that "regular" green plants use don't get very far beneath the surface, but frequencies in the bluish and greenish ranges penetrate much further.  This has spurred the evolution of several classes of various-colored algae with different chlorophylls, capable of using these deeper-reaching frequencies to grow where the green algaes can't.  Red algaes especially can occupy greater depths than the others, although Golden algaes can occupy a bit of a middle zone between green and red.  The color of Brown algaes seems more to do with other pigments and is apparently not really depth-related.

FERNS- SEEDLESS VASCULAR PLANTS

The ferns are the best-known of this group but not the only seedless vascular plants, which includes horsetails and club mosses.

Seed-Bearing Vascular Plants - Gymnosperms 

POLLEN - AN EVOLUTIONARY MILESTONE

The bryophytes and ferns did a relatively good job of invading the wetter parts of the land environment, but one critical weakness kept them from spreading farther:  the need for water to get sperm from male to female plants (or plant parts).  Both had developed wind-blown spores to spread asexually, but as we've seen, asexual reproduction limits the speed that organisms can adapt to new environment, and the land was full of new environments.  Still, it's quite likely that ancient forms of ferns and their relatives spread fairly widely - it's not easy to tell because such organisms don't fossilize easily.  We do know that, for a time, the landscape was very foresty, but the trees were really huge tree ferns.  How much the rest of the environment resembled modern versions is harder to guess at, but animals were spreading across the land very successfully, and that suggests lots of food choices, lots of plants to occupy the first level of the food chain.  Also, as often happens, there was apparently a major climate shift away from generally wet to widely drier, providing even more advantages to those best suited to the new conditions.

The adaptation that really led to an evolutionary leap was pollen, a tiny male gametophyte that, in its first versions, could be carried by the wind like spores to the female gametophytes and sprout a vascular-tube-like "tunnel" into them that a sperm could travel down.  No longer would the two genders have to be close to one another and need some sort of open water between them for the sperm to swim through.  Plants would still need water for their chemistry and specifically for photosynthesis, but they wouldn't be limited to environments where open water was periodically available.  These new types of plants could carry gametophytes high above the ground, where sporophyte embryos could be encased with some "starting off" food and also be set off in the wind.  The casings with little sporophytes in them were seeds, and all of the groups to evolve from pollen-bearing plants would also be seed plants.  

When you see a pine tree, or a spruce, or a cone-bearing shrub, the "main plant" is a sporophyte;  the gametophyte form is confined to the cones, which commonly have male and female versions.  In many pine trees, the males cones are smaller and located at the tree tops, while the female cones are larger and found farther down.  The sizes relate to the needs of the two genders:  the female cones will generate seeds, and will need to be bigger, while the male cones just produce tiny wind-carried pollen (and even though a lot of pollen is made, not much room is needed for it).  The location on the trees reflects how the pollen is wind-spread, with the male cones as high as can be for best wind access, and the female cones lower since much of the pollen will settle eventually.

Pollen from the male cones makes contact with the sticky surface of a female cone and adheres there.  It sprouts and grows down into the female cone, forming what is known as a pollen tube;  this may take as long as a year-and-a-half.  The pollen tube eventually makes contact with the part of the female cone, the ovule, that holds the egg cells, and the sperm from the pollen moves down the tube and fertilizes the egg cell.  Many pollen produce pollen tubes and fertilize many egg cells in  a female cone.

The fertilized egg cell, the zygote, grows into an embryo which is encased in a seed.  The seeds have "wings" and are light enough to be carried off in the wind, once the female cone "opens" to release them.  The seeds that settle in hospitable places sprout and grow into new sporophytes.  The sporophytes have true roots, and true stems, and true leaves, although the leaves are usually needles.  These long thin leaves are more effective than flat leaves at conserving water and supporting snow.  They also commonly produce a cuticle, to reduce evaporation from the inside of a needle.

Much of the patterns and even structures in gymnosperms are found again in the angiosperms, with a few exceptions and several additions.

Angiosperms - Flowering Plants 

There is a very good chance that the vast majority of the plants you can see out your window are angiosperms.  They have taken over the land since their rise during the Age of Dinosaurs, pushing the other groups to the ecological fringes.  Gymnosperms still have an advantage in systems that are too cold and/or dry (they become more common with higher altitude and higher latitude), or have nutrient-poor soils, and so haven't been completely pushed out of business.

Reproduction in angiosperms follows a course similar to that in conifers, with the gametophytes confined to a periodically-made part on the main sporophyte.  These gametophytes are parts of the characteristic feature of the group:  flowers.  Although some species have separate male and female flowers, most have both male and female gametophytes in each flower.  Many angiosperm flowers don't look like what you might expect of a flower - this is discussed below in the section on coevolution.

Angiosperms get their name from the fact that the seeds (which are sporophyte embryos packaged with some fuel to sprout with) are produced inside a fruit, a structure used to move seeds away from the parent plant.  All angiosperms produce fruit, although some might not be things you would see as fruit:  fruits can be built to fly through the air (maple trees and dandelions make this type), float across the water (coconuts), stick to passers-by (burrs), as well as be eaten by animals.  Even edible fruit is almost never meant to act as a food source for the seeds inside. 

LIFE CYCLES IN ANGIOSPERMS

 As was true in the gymnosperms, the "main plants" in angiosperms are sporophytes, while the gametophytes are confined to the flowers, usually male and female together.  Male gametophytes are called stamens, female gametophytes are pistils.  The pollen from the stamen has to reach the pistil and make a pollen tube to the base of the flower, the ovary, and the ovules, where the egg cells are.  Once the egg cells are fertilized, the embryos are sealed up with food in a seed and the ovary is converted into a fruit.

The seeds often wait for some environmental cue, such as warmth, moisture, or light periods shifts, before germinating, sprouting.  A type of growth hormone known as auxins, which settled to the bottom of the sprouting root and stem as they emerge, have different effects on those parts:  auxin-soaked root cells grow more slowly, making the top of the root grow faster and curve the root downward;  auxin-soaked stem cells grow faster, curving the stem up.

In a mature plant, auxins migrate away from the sunlit side of the plant - if the light is coming in from the side, the migration causes the stem to curve toward the light as it grows and better orient the leaves to catch light.  Plants produce many different types of hormones that can affect reproduction, overall growth, defenses, fruit ripening, and other features.

Production of new cells in plants happens in a type of tissue called the meristem.  Meristems can be, and usually are, at the growing tips of the plants, where they are called apical meristems.  Most plants add new cells from the tips out, not evenly all over and not from the bottom up - in the other parts of the plant, the cells grow but do not divide.  Leaves, branching stems or roots, and flowers all are produced by apical meristems.  Some growth may also occur along the sides of the plants, such as is found in the rings of trees - these are called lateral meristems.  These make tree ring patterns in ecosystems with growing seasons,  where good growth produces big, "light" cells, alternating with seasons of less growth (cold winters or dry periods), which produce smaller, "darker" cells - each light/dark zone is a ring, and the wider, lighter the ring, the better the growth season, leaving a record of year-to-year climate. 

ANGIOSPERMS AND ANIMALS -
COEVOLUTION IN ACTION

Since a significant part of any organism's ecosystem is the other organisms it shares that system with, it should come as no surprise that much of evolution is driven by interactions between individuals of different species, sometimes driving long-term relationships between the species.  The interactions can be cooperative, as happens in a symbiosis, or more of an ongoing battle, as happens between predator and prey or plant-eater and plant.  The evolutionary process where two species adapt to each other over time is called coevolution.

Coevolution is obvious throughout the angiosperms.  The most well-known is the use of animals as pollinators, carriers of pollen from the male parts of a flower to the female parts of another flower.  These angiosperms have evolved ways to make their flowers stand out in the environment to their pollinators, both visually and with smells, and often present "rewards" such as food to bring pollinators back over and over.  The construction of a flower is often a clue to its pollinators.  Many flowers without pollinators have no colors or odors, and pollen bearing and catching parts stick out in the air:  these are wind-pollinated, common in many grasses.  Some flowers are broad, with stumpy parts:  these are often pollinated by crawling insects.  Some flowers are tall and narrow, with a central platform:  these are commonly pollinated by flying insects, who press past the anthers on the way in and pick up pollen, then land on the next flower's pistil, leaving pollen there.  Flowers can give clues about their pollinators:  the reason that red flowers rarely use bees (and often use beetles) as pollinators was a clue toward the discovery that bees don't see red as a color (although they can see ultraviolet as a color, and some bee-pollinated flowers are ultraviolet-colored).  Fruits also have often evolved to use particular types of animals as seed-carriers, although the relationships are rarely as carrier-specific as those that have developed with pollinators.

Other types of coeveolution exist in angiosperms:  many defenses, such as thorns and poisons, are set against particular types of enemies.  Many of the drugs that humans use recreationally are derived from the poisons plants have developed to fight their insect enemies:  the active ingredients in tobacco, cocaine, and marijuana are all neurotoxins, natural equivalents of Raid^TM.  

One of the lesser-known theories about the decline of the dinosaurs hypothesizes that the plants that served as the basis of their food chain adapted defenses against them and effectively put them "out of business."  It seems very unlikely, especially given that our plant-eating relatives haven't been fought off as well. 

A very weird thing:  Photosynthesis the Movie.

And The Photosynthesis Song.

How to get many different seed varieties to germinate.