Adaptations in plants. 

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 The theme for this set of pages is adaptation. An adaptation is a structure or behavior that enhances the survival and reproductive success of organisms that have the structure or behavior. 

So for example, our hands are adaptations that enhance or survival by allowing us to do complex manipulations of objects in our environment.

Adaptations arise through evolution and what's important to realize is that adaptations are a often a compromise between conflicting environmental demands. Here we will examine the plant kingdom from in terms of some of the major adaptations that plants have and the compromises that have arisen in response to conflicting demands of the environment on the plant as an organism.
 
 
What is a plant? 

What we call plants are united by the following set of characteristics.

  1. Photosynthesis. Plants obtain their energy by converting light energy into chemical energy by means of photosynthesis. Photosynthesis involves two basic sets of reactions that take place in the chloroplast. The light dependent reactions involve the absorption of light energy by specialized pigments including chlorophyll. The energy is absorbed by electrons and used to create ATP and a compound called NADPH. These compounds carry energy and hydrogen ions(in the case of NADPH) into the so called light independent reactions. These reactions bring carbon dioxide into the plant's metabolism and convert the carbon dioxide and the hydrogen ions donated by the NADPH into a three carbon carbohydrate called PGAL. In turn this is converted by the plant into glucose and other carbohydrates. More about photosynthesis is here. The plant needs light, carbon dioxide, and water as raw materials for photosynthesis. Many of the adaptations we will discuss relate to the needs of the plant for these three resources. 
  2. Multicellularity. All plants are multicellular and have different types of tissues and organs. 
  3. Cellulose cell walls. All plants along with algae have cell walls made out of the polysaccharide, cellulose. 
  4. Alternation of generations. All plants have life cycles that include multicellular haploid and diploid stages. Along with this, plant gametes are not directly produced by meiosis but rather by mitosis from the haploid multicellular stage. Meiosis instead produced specialized haploid cells called spores. Thus alternation of generations is quite different from the standard animal life cycle. A good comparison of plant and animal life cycles is here.
Other groups of organisms show some of these characteristics. For example, some algae show alternation of generations and many other protists and some bacteria carry out photosynthesis.
Divisions of the Plant Kingdom.

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Currently the plant kingdom is divided into a number of informal groups.

 Avascular plants:
 
 

These are plants that lack vascular tissue. Often these plants are called Bryophytes. These mosses are a good example. A vascular plants are rarely more that a few inches tall. This is because water travels into the plant and through the plant simply by osmosis. Likewise, sugars and other nutrients cannot be dispersed long distances through out the plant. 

Unlike more familiar plants, the haploid stage is the most obvious one. In fact the diploid stage which produces the spores depends on the gametophyte for nutrition.
 
 
Vascular plants: 

These are plants that have vascular tissue, that is specialized tissue consisting of a series of tubes for transporting fluids. Xylem transports water and minerals up the plant stem into the leaves and phloem transports sugars and other organic compounds throughout the plant. The major types of vascular plants are:

1. Seedless vascular plants. These are plants that rely on spores for dispersal. For example the picture here shows the underside of a fern leaf. The little dots on the leaf surface are called sori(sing. sorus). Each sorus has specialized tissue in which meiosis happens. The resulting cells are haploid spores. When they are ripe, the spores are released by the sori and the spores are carried by the wind to a moist habitat where they germinate and produce the haploid stage of the multicellular life cycle, the gametophyte. The spore producing plant similarly is called the sporophyte.

 
2. Seed producing vascular plants. These plants include more familiar plants such as pine trees and flowering plants.

Pine trees and other seedless plants are often called Gymnosperms or naked seed plants. Basically what this means is that the seeds are on the lower surface of highly modified leaves. We call a set of these modified leaves a pine cone as shown here.

What we think of as flowering plants are often called the Anthophyta or literally flower plant. Sometimes flowering plants are referred to as Angiosperms or covered seeds. The seeds are hidden in specialized structures at the base of the flower commonly referred to as the ovaries, rather than being on the surface of leaves as in pine trees and other gymnosperms.

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pgd created 8/11/99