An Online Introduction to the Biology of Animals and Plants

Key Concepts

Section 2

Chapter 5

Angiosperms - Flowering Plants 

RISE OF THE FLOWERING PLANTS -
THE ANGIOSPERMS

Reproduction in angiosperms follows a course similar to that in conifers, with the gametophytes confined to a periodically-made part on the main sporophyte.  These gametophytes are parts of the characteristic feature of the group:  flowers.  Although some species have separate male and female flowers, most have both male and female gametophytes in each flower.  Many angiosperm flowers don't look like what you might expect of a flower - this is discussed below in the section on coevolution.

Angiosperms get their name from the fact that the seeds (which are sporophytes embryos packaged with some fuel to sprout with) are produced inside a fruit, a structure used to move seeds away from the parent plant.  All angiosperms produce fruit, although some might not be things you would see as fruit:  fruits can be built to fly through the air (maple trees and dandelions make this type), float across the water (coconuts), stick to passers-by (burrs), as well as be eaten by animals.  Even edible fruit is almost never meant to act as a food source for the seeds inside.

Angiosperms are vascular plants, containing xylem (going up) and phloem (bringing down) in various bundled patterns.  They have true roots, for holding the plants firm as well as for taking in water and nutrients;  they have true stems, for support and for moving materials up and down;  they have leaves, usually flat, for most of their photosynthesis.  Leaves, and sometimes stems, have pores called stomates that can be opened or closed.  When open, carbon dioxide for photosynthesis enters air spaces inside the leaf through which it gets to photosynthetic cells, but water is also lost (this is the main source of transpiration from angiosperms).  By controlling the pores, a plant can get enough carbon dioxide for its needs and minimize its water losses.  Clusters of xylem and phloem, plus a reinforcing sheath to help support the leaf, are called veins.

Angiosperm stems may be green (herbaceous), which are capable of photosynthesis or woody.  Green stems have support limits;  woody stems, with ring after ring of tightly-packed cells with very rigid cell walls, are present in the large angiosperms, the trees.  Gymnosperms also produce trees with ringed stems.

LIFE CYCLES IN ANGIOSPERMS

 
As was true in the gymnosperms, the "main plants" in angiosperms are sporophytes, while the gametophytes are confined to the flowers, usually male and female together.  Male gametophytes are called stamens, female gametophytes are pistils.  The pollen from the stamen has to reach the pistil and make a pollen tube to the base of the flower, the ovary, and the ovules, where the egg cells are.  Once the egg cells are fertilized, the embryos are sealed up with food in a seed and the ovary is converted into a fruit.

The seeds often wait for some environmental cue, such as warmth, moisture, or light periods shifts, before germinating, sprouting.  A type of growth hormone known as auxins, which settled to the bottom of the sprouting root and stem as they emerge, have different effects on those parts:  auxin-soaked root cells grow more slowly, making the top of the root grow faster and curve the root downward;  auxin-soaked stem cells grow faster, curving the stem up.

In a mature plant, auxins migrate away from the sunlit side of the plant - if the light is coming in from the side, the migration causes the stem to curve toward the light as it grows and better orient the leaves to catch light.  Plants produce many different types of hormones that can affect reproduction, overall growth, defenses, fruit ripening, and other features.

Production of new cells in plants happens in a type of tissue called the meristem.  Meristems can be, and usually are, at the growing tips of the plants, where they are called apical meristems.  Most plants add new cells from the tips out, not evenly all over and not from the bottom up - in the other parts of the plant, the cells grow but do not divide.  Leaves, branching stems or roots, and flowers all are produced by apical meristems.  Some growth may also occur along the sides of the plants, such as is found in the rings of trees - these are called lateral meristems.  These make tree ring patterns in ecosystems with growing seasons,  where growth produces big, "light" cells, alternating with seasons of less growth (cold winters or dry periods), which produce smaller, "darker" cells - each light/dark zone is a ring, and the wider, lighter the ring, the better the growth season, leaving a record of year-to-year climate.

THE TWO MAJOR SUBGROUPS OF ANGIOSPERMS

 
The flowering plants are generally treated as two separate groups, named for a feature in their seeds called a cotyledon.  The monocots have a single-piece seed (corn is a good example of a monocot), while the dicots have a two-piece seed (a peanut is a clear example of the seed, but a maple may be a better overall example of a dicot).  There are several general differences between the two subgroups:

Vascular bundles are collections of xylem and phloem, laid out in different patterns in the two groups.  In woody plants, the outermost rings are the active xylem, with active phloem on the underside of the bark.  Peeling a strip of bark from all the way around a tree will cut off the roots' food supply (phloem, remember, carries fuel from photosynthesis down to the lower parts) and eventually kill the tree.

ANGIOSPERMS AND ANIMALS -
COEVOLUTION IN ACTION

Since a significant part of any organism's ecosystem is the other organisms it shares that system with, it should come as no surprise that much of evolution is driven by interactions between individuals of different species, sometimes driving long-term relationships between the species.  The interactions can be cooperative, as happens in a symbiosis, or more of an ongoing battle, as happens between predator and prey or plant-eater and plant.  The evolutionary process where two species adapt to each other over time is called coevolution.

Coevolution is obvious throughout the angiosperms.  The most well-known is the use of animals as pollinators, carriers of pollen from the male parts of a flower to the female parts of another flower.  These angiosperms have evolved ways to make their flowers stand out in the environment to their pollinators, both visually and with smells, and often present "rewards" such as food to bring pollinators back over and over.  The construction of a flower is often a clue to its pollinators.  Many flowers without pollinators have no colors or odors, and pollen bearing and catching parts stick out in the air:  these are wind-pollinated, common in many grasses.  Some flowers are broad, with stumpy parts:  these are often pollinated by crawling insects.  Some flowers are tall and narrow, with a central platform:  these are commonly pollinated by flying insects, who press past the anthers on the way in and pick up pollen, then land on the next flower's pistil, leaving pollen there.  Flowers can give clues about their pollinators:  the reason that red flowers rarely use bees (and often use beetles) as pollinators was a clue toward the discovery that bees don't see red as a color (although they can see ultraviolet as a color, and some bee-pollinated flowers are ultraviolet-colored).  Fruits also have often evolved to use particular types of animals as seed-carriers, although the relationships are rarely as carrier-specific as those that have developed with pollinators.

Other types of coeveolution exist in angiosperms:  many defenses, such as thorns and poisons, are set against particular types of enemies.  Many of the drugs that humans use recreationally are derived from the poisons plants have developed to fight their insect enemies:  the active ingredients in tobacco, cocaine, and marijuana are all neurotoxins, natural equivalents of Raid^TM.  

One of the lesser-known theories about the decline of the dinosaurs hypothesizes that the plants that served as the basis of their food chain adapted defenses against them and effectively put the "out of business."  It seems unlikely, especially given that our plant-eating relatives haven't been fought off as well.

Informational Links

 
Way more than you ever wanted to know about auxins.

A page on how tree rings are used to study climate history.

A more in-depth treatment of plants, talking mostly about their tissues.

How to get many different seen varieties to germinate

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

Gymnosperm-Dominated Environments
Flowers
Angiosperm Seeds
Fruit
Angiosperm Roots
Angiosperm Stems
Angiosperm Leaves
Stomates
Veins
Life Cycle
Sporophytes
Gametophytes
Stamens
Pistils
Pollen Tube
Ovary
Ovules
Fruit Formation
Germination
Auxins & Sprouting
Auxins and Light
Hormone Effects
Meristem
Apical Meristem
Lateral Meristem
Tree Rings
Using Tree Rings as Climate Record
Cotyledon
Monocots vs Dicots
Vascular Bundles
Bark
Coevolution
Pollinators
Flower Form & Function
Plant Defenses

GO ON TO THE NEXT SECTION - ANIMALS
(CHAPTER 1, PROTOZOANS)