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THE MAIN GROUPS OF PLANTS


This page explores some problems of plants in Nature.

Adaptations of plants to survive in their natural environment


Green plants may be divided up into a number of different groups, some of which are more common than others. This may be explained by the different adaptations to get round various problems in their natural environment, such as the supply of factors needed by plants.

Let us consider these factors, which may limit the success of plants in growing in certain places, and put them into ecological perspective.

1) The plants we are concerned with here are all green because they contain the pigment chlorophyll which is able to trap energy from sunlight, and thus enable these plants to make their own food by the process of photosynthesis. If they grow larger, they can trap more light. However, with this energy comes another form of energy, heat, which has the bad effect of dehydrating the plant.

2) At the same time, plants need water, and minerals.
Where can they usually get a fairly steady supply of water and minerals from?
   > the soil
Can you explain the contradiction caused by a plant growing in response to problems 1 and 2?
   > soil is downwards, but plant must grow upwards for more light, so plants must grow in 2 opposite directions

Plants need an efficient water absorbing and conducting system to carry water from one part of their "body" to another, as well as to hold the plant up and form a sort of skeleton.

3) In order for plants, (and animals, for that matter), to produce offspring which are slightly different from one another, the best way to reproduce is by sexual reproduction. This variation should result in some types which are better suited to their surroundings than others, so that evolution can occur as only the "fitter" types are able to survive. However, this type of reproduction requires special cells called gametes to be produced, then brought together, and allowed to fuse to form a single cell combining the features of both. All these cells are very fragile and need special protection, and careful distribution processes.

4) Plants tend to grow throughout their lifetime, so that some of these problems may become more complicated as they get bigger. If a plant is unable to solve these problems, it is unable to grow very big or to spread to new environments.

Plants that we know well ("higher plants") tend to be the more successful ones at adapting to these everyday challenges.

[ For the time being we are ignoring simpler groups like algae, fungi and bacteria, all of which are sometimes lumped together with the "higher plants" we are all fairly familiar with.]



Reproduction methods

Simple plants are small (not very efficient?) and reproduce by spores, which are often very small and numerous. Usually these plants rely on wet conditions so that sex cells can swim from one plant to another. They may not tolerate drying out.

Plants that are more successful have these adaptations :

1) the use of specialised spores called pollen which contain sex cells in a protected package so that movement of sex cells does not need water, and seeds which contain an embryo plant together with a food store so that it can grow into a new plant,
2) flowers, which may be attractive to insects, and allow the transfer of pollen (pollination) to be more efficiently carried out, and which also develop into the fruit, which helps to spread or disperse the seeds to new areas.

Green plants are sometimes divided into seed plants and non-seed plants, or into flowering plants and non-flowering plants. Linked with these obvious features are other important characteristics such as efficient water transport systems and specialisation to reduce water loss, so biologists feel that these distinctions are of fundamental importance.

Look at the pictures of the specimen plants and fill in the gaps in the table below, explaining simply the features shown by each group. Give an example from each group, and estimate its size.

SPECIAL FEATURES (ADAPTATIONS)

PLANT GROUP
roots & water-conducting tissue? sex cells that swim? spores? seeds? flowers and
> fruits?
Mosses and
liverworts
(Bryophytes)
e.g. >.......
size >.......cm
no - only
simple
"rhizoids"
yes yes - in
capsules
no no
Ferns and
horsetails
(Pteridophytes)
e.g. >.......
size >.......m
yes - limited yes > yes - on underside of fronds > no > no
Conifers
(Gymnosperms)
e.g. >.......
size >.......m x10
> yes no > pollen = special spores > yes > no
Flowering plants
(Angiosperms)
e.g. >.......
size >.......cm
yes no > pollen = special spores yes yes

Summary


Match the numbered pictures below [NOT TO SCALE] with their descriptions below:
Moss Rose Butter Grass Pine Liverwort Ferns Bluebell

Mosses and liverworts >1 moss > 6 liverwort
- small plants with simple leaves or leaf-like shape
- simple "rhizoids" for water absorption, but practically no water conducting tissue
- found (only?) in damp places
- spores spread by air currents, but sex cells must swim from one plant to another

Ferns (and horsetails) > 7 fern - bracken frond? > 8 hart's tongue fern > 9 horsetail
- slightly larger plants with spreading leaves (fronds), roots and some conducting and strengthening tissue
- mostly found in damp places
- dry spores produced in structures on underside of leaves in autumn, after sex cells swim from plant to plant

Conifers > 5 pine
- larger plants, mostly "evergreen" trees
- needles or small leaves to reduce water loss
- cones for reproduction:
male cones produce pollen that blows in wind from tree to tree, female cones after fertilisation contain seeds that develop slowly - taking several years!
- good at surviving in cold & dry climates, and in poor soil
- grow quite efficiently, planted to produce "softwood"

Flowering plants >2 rose > 3 butttercup >4 grass > 10 bluebell
- many plants, varying in size from small herbs to large trees
- some flowers are conspicuous - large and brightly coloured (probably insect-pollinated)
- others are inconspicuous - small, maybe even green (probably wind-pollinated)
- after fertilisation, seed develops quickly inside fruit
- fruit is usually specialised to help spread (disperse) seed: by animals, wind, water, or by mechanical methods



Further classification of plants


Each of these groups can be subdivided again and again into more precise sub-groups.
Biologists believe that organisms which share certain features of "biological importance" should be put together into groups which are distinct from organisms with other characteristics. This is the basis of classification.

This is very useful to us if it sums up or organises our knowledge of the organisms concerned, and shows them in comparison to other organisms which we may not know so well.

On the other hand, we may use a key to sort out, and perhaps give a name to unfamiliar organisms, but without learning much about their relationships to other organisms.
However, most GCSE syllabuses place great emphasis on keys.

For instance, flowering plants can be subdivided again into two classes:

Monocotyledons (monocots for short)
[grasses, cereals, lilies, daffodils, bluebells and most "bulbs"]
- have only one leaf attached to the embryo plant inside the seed
- mature plants have strap-shaped leaves, with veins running parallel and lengthways
- if you count the various parts of the flower, such as (petals), stamens, etc, you will usually find them in multiples of three (3,6,9,etc)

Dicotyledons (dicots for short)
[most of the other significant plants around us, from small herbs to big trees; perhaps the flowers are not very noticeable to us!]
- have two leaves attached to the embryo plant inside the seed
- mature plants have broader, more complex leaves, with a branching net-like pattern of veins
- often have flower parts in 4's or 5's.

Because these substantial features "go together" we think these are important differences, and classify these plants accordingly.

Each of these groups can be subdivided again, into "families", and then into genera and species, which finally describe only one individual type.
Choose a plant family, for instance the rose family, the mint family, the onion family or the cabbage family, or any other plant family of your own choice. Then find out what plants are lumped together in that family, and their main features of similarity.

> e.g.1 onion family - bulb onions, shallots, spring onions, garlic, chives, leeks: hot oniony taste, whitish bulb with food reserve in modified inner leaves!
e.g.2 cucumber family - cucumber, pumpkins, marrow/courgettes, melons, etc: large watery fruit on straggly plants (separate male and female flowers)


Give three reasons why plants that are useful to Man are mostly from the categories of seed plants, and flowering plants (not simpler ones).

> seeds contain a supply of food for the embryo plant which we can take to feed ourselves
> wood is only obtained from higher plants
> higher plants are more suited to growing in places near to man's habitation, i.e. not in perpetually damp/shady places
> higher plants are bigger and more efficient