An Online Introduction to the Biology of Animals and Plants

Key Concepts

Section 2

Chapter 3

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 - its 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.  Once there, the pollen sprouts a vascular-tube-like "tunnel," a pollen tube, that a sperm could travel down to get toe the egg cells.  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.  

Seed plants are all vascular plants, with an internal tube system that brings water and nutrients up from the roots through thick-walled xylem tubes, and phloem, that carry fuel back to the roots for use and for storage.  There are two major types of seed plants:  the gymnosperms  (Latin for "naked seeds") and the angiosperms (Latin for "covered seeds," sort of).  This chapter will deal with the gymnosperms.  

GYMNOSPERM GROUPS

There are four groups within the Gymnosperms:  the cycads, which somewhat resemble short palm trees;  the ginkgos, mostly known because some Chinese species have been used in landscaping around the world; the gnetales, which share some traits with angiosperms (they're in next chapter;  and the conifers, the cone-bearing shrubs and trees.  The conifers are the best-known gymnosperms, and our discussion of gymnosperm life-cycles will center on conifers.  The gymnosperms were probably the first really widely-distributed plant group, and with their rise came the rise of a major animal group:  the dinosaurs.

Often, depiction of the age of Dinosaurs shows a lot of bare soil, since modern gymnosperms have no grass-like forms.  It seems naive to think that there were no such forms back then, though - can you imagine such a sunny niche going unoccupied for tens of millions of years?

GYMNOSPERM (CONIFER) LIFE CYCLE

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 the blowing 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 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 multiple 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 may 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.  They also commonly produce an oily waterproof layer, 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.

Informational Links

KEY CONCEPTS -
Click on term to go to it in the text.
Terms are in the order they appear.

Evolution:  Plants, Seed
Pollen
Seeds
Seed Plants
Gymnosperms
Angiosperms
Cycads
Ginkgoes
Gnetales
Conifers
Gymnosperms & Dinosaurs
Grasslike Gymnosperms
Conifers:  Life Cycle
Cones:  Male vs Female
Gender Differences:  Cones
Pollen, Pine
Pollen Tube
Ovule
Seed, Conifer
Zygote, Conifer
Embryo, Conifer
Needles
Cuticle

GO ON TO NEXT CHAPTER - ANGIOSPERMS