From Algae to Angiosperms

By: Rebecca Rose Leitten


 
 
Introduction
Bryophytes:
mosses, liverworts and hornworts
Ferns and Fern Allies
Gymnosperms
Angiosperms

 
Plant Evolution

Around 400 million years ago, in the Silurian Era, the first plants appeared on land. Most similar to what are known today as Bryophytes, they descended from early water dwelling alga. They lacked vascular (circulatory) systems and complex physical characteristics, but their appearance marked a great step in the development of Earth. The world they populated was far different from that which we enjoy today: rocky, exposed and barren, with no sign of the rich and diverse life which was to later make our planet so unique. Only the oceans, from which the first plants came, teemed with organisms. 
As time passed, the environment caused the first plants to change and modify their structures to be able to meet their life needs. Further impetus for evolution arrived with the first terrestrial animals, as the first links in the great chain of animal-plant codependence were forged. With change came specialization and increasing complexity in structure, and it is based upon these characteristics that plants are classified or "ranked" today in the study of phylogenetics. Classification begins with comparing the primitive features of the earliest terrestrial plants to those of other plants. In this method, we can organize terrestrial plants into the following basic divisions, in order of increasing development: Bryophyta, Pteridophyta (ferns and fern allies), gymnosperms (Cycadophyta, Ginkgophyta, Coniferophyta, and Gnetophyta), and angiosperms (Anthophyta, divided into dicots and monocots).
In this tour, we will explore the evolution of the unique kingdom Plantae's terrestrial members. To make matters simpler, we will focus mainly on comparing the reproductive characteristics of each group. Other features also serve to illustrate the evolutionary relationships, but the reproductive differences are vivid and clear markers of the progression of terrestrial plant advancement. As you proceed, consider the increasing complexity of the plants as we progress from the most primitive Bryophytes to the most modern angiosperms. Most of all, enjoy yourself as you delve into the wonderful and fascinating world of plants.


 
 
 
 
Bryophytes

Bryophytes, the mosses, liverworts and hornworts, are the most primitive terrestrial plants surviving today. Many fossils of plants long extinct bear features similar to the modern Bryophytes, particularly, their lack of vascular systems. Based upon genetic analysis, it seems that liverworts are the most primitive of the three kinds, and mosses the closest relatives to vascular plants.
 Bryophytes are the second largest group of land plants in the world today with over 20,000 species. Like their close relatives, the algae, they require very moist conditions for survival, and their life processes, especially reproduction, rely on water for their very execution. 
 


Hornwort
photo courtesy U. of Wisconsin


Liverwort (Marchantia sp.)
photo courtesy U. of Wisconsin


Sphagnum Moss
photo courtesy U. of Wisconsin


 
 
Moss Life Cycle

Reproduction in the Bryophytes is the simplest of all terrestrial plants. In mosses, it begins with the production of spores. The spores, manufactured by mature sporophytes, form in capsules at the tops of  long stemlike stalks called seta. The capsules are protected by the capiltera.  Spores are released when the capsule's lid, or operculum, ruptures. When moistened and in proper conditions, these spores germinate to form protonemas, long fiberlike structures which branch. These protonemas produce gametophytes, small leafy plants, male and female. It is these gametophytes, the leafy plants, which we typically associate with mosses. Sperm is produced within the antheridium of the male gametophyte, eggs are found in the female gametophyte's archaegonium. Water transports mature sperm released by the antheridium to the archaegonium, where fertilization occurs. Fertilization produces a zygote, which grows into an embryo and eventually a mature sporophyte, with seta, calyptra and capsule (sporangium) complete for a new cycle of reproduction.

photos courtesy U. of Wisconsin


 
 
Ferns and Fern Allies
 

Ferns and their allies are the earliest vascular plants. Grouped with the ferns are other primitive vascular plants, among them the horsetails, club mosses and whisk ferns. The fern group is far smaller now than it once was. During the Carboniferous Period, Pteridophytes dominated the plant world. Many of the early ferns grew to immense, treelike sizes, and, when they died, set down layers of organic matter which were to become the coal deposits of today.
 
 

Young fern fronds like the two in this picture are are nicknamed fiddleheads, because they resemble the curved head of a violin. As they mature, they unwind into the mature fronds with which most people are familiar.


Whisk Fern
Isoetes sp.
photo courtesy U. of Wisconsin


Horesetail 
Equisetum sp.


Club Moss
Selaginella sp.


 
Ferns Come in Many Shapes and Sizes


Aspelnium nidus
Bird's Nest Fern

Platycerium sp.
Staghorn Fern


Neprolepsis sp.

 


 
 
Fern Life Cycle

Like the Bryophytes from which they descend, Pteridophytes require moist conditions for survival and reproduction. The principle element of their reproductive cycle is the spore. The spores are produced in sacs called sporangia and held in clusters called sori on the leaf surfaces, usually the undersides. These spores, when mature, are released, and, if the surface they find is moist, they will germinate.
 The germinated spore, called the prothallus, is usually heart shaped and photosynthetic. It has shallow rhizoids on its lower surface for water absorption.  When the prothallus is mature, it produces, in two separate regions, sperm and egg cells. Sperm is released when the prothallus comes in contact with water, often rain. These sperm are attracted by chemicals secreted by the eggs. The eggs are fertilized by the sperm and become zygotes.  Over time, as their cells divide and multiply, the zygote becomes embryos.  The prothallus remains for some time, eventually withering away as the zygote, now called an embryo, grows. The embryo continues to experience growth until it is a mature adult fern (sporophyte) and capable of reproducing, thus beginning the cycle again.
 

photos and drawings courtesy U. of Wisconsin


 
 
 
Gymnosperms

Gymnosperms are a familiar group nearly everywhere on earth. Ranging from the exotic, ancient looking cycads to the towering Sequoias, they area very diverse and widespread group. Today, we use them for everything from Christmas trees to medicinal Gingko extracts to the special flavoring of gin. Within the group, the cycads, or seed ferns, are most likely to have evolved first, and the Gnetophytes, which many consider to be a missing link between gymnosperms and angiosperms, most recently.
  The evolution of the gymnosperms brought an entirely new concept to plant reproduction: seeds. Unlike the ferns and bryophytes before them, gymnosperms had no need for extremely moist conditions for reproduction. Instead, embryonic plants were neatly packages for dispersal and germination in favorable conditions. In other words, gymnosperms rocked the plant world.
 
 

a closeup of the seeds in a cycad cone
Zamia sp.


cycad


conifer
Araucaria heterophylla
 


gnetum
Gnetum gnemon

Gingko biloba


 
 
 
Pine Life Cycle
Unlike ferns, gymnosperms produce their gametophytes (sperm and eggs) in different, more specialized areas. As they are most familiar, we will consider the life cycle of a pine tree. As an adult, mature tree, the pine produces two different types of cones, pollen cones (male) and ovulate cones (female). The male gametophytes exist within special sacs as pollen grains, while the female gametophytes, the eggs, are held within ovules, protected by a layer called the integument. The integument has a single opening, called the micropyle.  An ovule may hold one or more eggs.   When the pollen grains are mature they are released and borne by the wind to the ovules' micropyles. If they are accepted by the ovule, the pollen grains germinate and produce a tube leading into the integument. The sperm, held within the pollen grains, pass down these tubes and meet the eggs, which they fertilize. When the eggs are fertilized, the integument thickens into a seed coat, to protect the embryonic plant within. The seed also has nutritive tissues which supply food for the embryo until the seed can germinate and the plant can produce food for itself through photosynthesis. The seed, when developed and mature, is released in a variety of ways, including wind dispersal. If it finds suitable growing conditions where it lands, the seed will germinate and develop into a new plant, capable on maturity of producing more seeds and continuing the cycle.

photos courtesy U. of Wisconsin


 
 
Angiosperms

Think of how different our world would be if flowering plants had never evolved. Imagine a spring without those lovely monocots, the flowering bulbs.  Or an anniversary sans a dozen dicots; roses.  There would be no fruit, no grains, no need for pollinating insects, no perfumes, and few textile sources.  This short segment of a long list is ample illustration of the vast importance of angiosperms in our lives. 
 The flower is the main reproductive center of the angiosperm. Some angiosperms produce only one type of gametophyte, either male or female, per plant. They are thus termed dioecious, and these plants are either male or female. An example of this type of plant is holly. Others, which are monoecious, produce male and female gametophytes on different flowers of the same plant. An example of this is corn (Zea mays). The last type has both male and female parts in a single flower.


 
 
Flower Parts


 
Monocots vs. Dicots

 Angiosperms are divided into two main groups: monocots and the more primitive dicots. You can see their differences illustrated on the chart at this station. The dicot magnolia family is considered to be the oldest group of angiosperms, the monocot orchids the most recent. 
 

flower parts in 4's and 5's
 
 
 

leaves with "netted" veins
 
 
 
 
 


bean seed
photo courtesy U. of Wisconsin
seeds with two cotyledons
The peanut is another example of a dicot seed

flower parts in 3's
 

leaves with veins that run parallel to leaf edges (margins)


corn kernel
photo courtesy U. of Wisconsin

seeds have one cotyledon


 
 
Angiosperm Life Cycle

The male gametophytes, called microgametophytes, are produced in anther sacs in the anthers of the flower's stamens. Like the male gametophytes of the gymnosperms, the sperm is encased in a protective cover and this complex is called a pollen grain. As the microgametophyte matures, this cover diminishes, leaving behind a two celled male gametophyte.
 The female gametophytes, megagametophytes, are produced in the ovary of the flower. The ovary is made up of one or many carpels, either fused or unfused.  Each carpel contains ovules, immature seeds, singly or in multitudes, depending on the type of plant. The ovary is part of a larger structure, collectively called the gynoecium, which makes up the entire female reproductive system. Aside from the ovary, it consists of a stigma, a sticky surface where pollen grains adhere, and a pistil, a long tube leading from the stigma to the ovary's inside.
 When pollen is mature, it is released for dispersal. Some of it eventually, through various means, reaches the stigma, where it adheres. One of the two cells of the pollen grain begins to work its way down the inside of the pistil to the ovary, forming a pollen tube. The other cell, the sperm, or generative cell, follows. When the generative cell reaches the ovule, it undergoes a series of steps and eventually the ovule is fertilized. The fertilized ovule is now seed containing a developing zygote. The zygote grows and differentiates into an embryo, nourished by stored nutrients in the seed's endosperm. When the seed is mature, it is released through various means. Should it find a fertile growing location, it will germinate and grow to produce a mature plant which in turn can carry on the cycle.

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this page created by Rebecca Leitten