World Builders™
World Builders™
Session Eight --  Land Plants  
Session Eight  -- Land Plants  
 

 


How Plants Reproduce
How Plants Reproduce

Plants have two methods of reproduction, asexual reproduction and sexual reproduction.


Asexual reproduction is cloning. A piece of a plant may root or sprout and grow into a new plant which is genetically identical with the parent. 

  • Sexual reproduction results in a new plant which contains genetic material from two parents, but which is not genetically identical with either one.   These new plants may be able to adapt more successfully to environmental changes than their parents could.

     To live and multiply on land, plants needed to evolve new structures and methods for reproduction.  An important advance was the development of the flower, a structure which allowed widespread scattering of its pollen and yet provided a stable, nurturing environment in the ovary for developing seeds.  Pollen could be carried from flower to flower by insects, the wind, or, sometimes, birds. 

     The seed was another important innovation, as significant for plants as the amniotic egg was for land animals.  Seeds would form when the chromosomes of the parents were united in the ovary.  A seed is a compact package made of a cell capable of growing into a plant, food to help the plant get started, and, in angiosperms, a seed coat that protected the seed from dehydration  and  damage in the environment.  Properly protected seeds could survive until conditions favored their development.  In the deserts today, some seeds may lie dormant for many years until the rains come and provide conditions in which the seeds may germinate and grow into plants.  Some seeds also are contained in, or attached to, structures which help the seeds to be carried to other places that may be suitable for them.


Asexual Reproduction

     Many plants are able to reproduce asexually. Some plants send up shoots from their roots and form big clumps of stalks and leaves. Some can grow leaves from pieces of root.

     Many of our house plants can grow from a leaf or a short piece of stem.

 .   Some plants store food in underground tubers and then grow new plants from the tubers: potatoes do that. Some plants grow from bulbs which can grow small new bulbs at the root level.

     Some plants can grow from cut off leaves or stems. Many of our house plants are shared this way. Some plants even grow little plants on their leaves or root when a piece of stem is buried.   

     All these methods of reproduction work well. However, they do not make it possible for plants to move to new locations. They result in plants with the same characteristics as the parents: the same resistance to the same diseases, the same responses to flood, drought, heat and cold, and the same schedules of growing. They are vulnerable to everything that might destroy the parents. A viable community needs members with diverse strengths and vulnerabilities.

     In addition to reproducing asexually, most land plants also reproduce sexually.


Sexual Reproduction

     Once plants got out of the water and onto the land, they faced real challenges. One of those problems was to invent ways to share and to scatter genetic material. Some of the early plants, ferns, for instance, found ways to exchange reproductive material in water, and later released many tiny spores into the air. However, much of the earth is too dry for this strategy. Plants had to find ways to deal with the dry air.


Coniferous Trees

     A group of plants called gymnosperms developed wind borne pollen. These were trees -- cycads, ginkos and needle-bearing trees such as pines and redwoods. Tall plants get more wind than those close to the ground, and the gymnosperms developed small male pollen-bearing cones. Their pollen was released into the air and drifted to other trees in the forest. Today, if you are in a pine forest in spring, you can often see a golden haze of pollen grains in the air.

     The plants also developed female cones which are essentially ovaries. The pollen falls directly on the female cones, and the pollen grains grow tiny tubes into the ovary to find the chromosomes and join with them. The female cone grows into the cones that we are familiar with, and the seeds are tucked safely between the bracts.

Gymnosperm means naked seeds.  These seeds have only a very thin covering that probably does not offer them much protection.

     Wind pollination seems rather extravagant, as surely most of the pollen grains never find a female cone. However, the system works, and has worked for millions of years. Gymnosperms grow in forests and groves, where the tall individual plants grow close together, so there are many potential targets for the pollen grains.


The Development of Flowers

     A new group of plants, the angiosperms, appeared about 110 million years ago. These plants had developed a number of structural innovations, the most striking of which is the flower. Flowers enchant us with their beauty, delicacy, and variety of form, but they represent a very practical development. Plants, by growing flowers and fruit, formed partnerships with animals who provided transportation for pollen and seeds.

     Some flowering plants still use wind to transfer their pollen to other plants. The grasses, growing thickly together in meadows and on plains, continue to rely on wind pollination.

     Many flowering plants, however, use insects or birds to distribute their pollen. Insects can be lured to the flowers by a few drops of sweet nectar. Brightly colored petals guide the insects toward the nectar and pollen. Bees can fly for several miles in a day, and, if all members of a plant species come into bloom at about the same time, the bees spread their pollen far and wide. Pollen sticks to the hairy bodies and legs of insects, and is easily carried away. Many of the fruits that we eat are dependent on insect pollination.

     Another invention of the Angiosperms is the development of the seed coat on their seeds.  Each seed is enclosed in a tough little covering to help it to survive in the world until conditions favor successful germination and development.. 


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© 1996,1997, 1998, 1999, 2000, 2002, 2003, 2005..   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at eviau@earthlink.net .