Plant Adaptations
Ephemeral Leaves - leaves lasting a markedly brief time, sometimes only a day. Found in very dry climates that have a very short rainy season, like the desert.
Xerophyte - A plant adapted to living in a dry arid habitat; a desert plant.
Mesophyte - A land plant that grows in an environment having a moderate amount of moisture.
Hydrophyte - A plant adapted to grow in water.
Pleating - Edges of a plant arranged in parallel folds (ex. Ocotillo cactus).
Pubescence - A covering of short hairs on certain plants; usually found on desert plants.
Secondary Metabolites - compounds made by plants as a form of defense against predators. There are six classes - alkaloids, cyanogenic glycosides, saponins, cardiac glycosides, tannins and phenolics.
Convergent Evolution - The adaptive evolution of superficially similar structures, such as cacti and euphorbs (Africa), in unrelated species subjected to similar environments.
A cactus (left) and two Euphorbia species from Africa (center and
right)
Adaptations develop over time and generations as a response to the ever changing environment. They allow an organism to reduce competition for space and nutrients, reduce predation and increase reproduction. There are however, several factors that can limit these adaptations: availability of water, light, predation and temperature. Because we live in a desert, this lab will focus mainly on the adaptations to a xeric environment.
Desert plants have adapted to the extremes of heat and aridity by using both
physical and behavioral mechanisms, much like desert animals.
Plants that have adapted by altering their physical structure are called
xerophytes. Xerophytes, such as cacti, usually have special means of storing and
conserving water. They often have few or no leaves, thus reducing the amount of
transpiration.
Phreatophytes are plants that have adapted to arid environments by growing
extremely long roots, allowing them to acquire moisture at or near the water
table. The term, phreatophyte, literally means water-loving plant.
Other desert plants, using behavioral adaptations, have developed a lifestyle
patterned after the seasons of greatest moisture and/or coolest temperatures.
These types of plants are usually (and inaccurately) referred to as perennials,
plants that live for several years, and annuals, plants that live for only a
season.
Desert perennials often survive by remaining dormant during dry periods of the
year, then springing to life when water becomes available.
Most annual desert plants germinate only after heavy seasonal rain, then
complete their reproductive cycle very quickly. They bloom prodigiously for a
few weeks in the spring, accounting for most of the annual wildflower explosions
of the deserts. Their heat- and drought-resistant seeds remain dormant in the
soil until the next year's annual rains.
Xerophytes
The physical and behavioral adaptations of desert
plants are as numerous and innovative as those of desert animals. Xerophytes,
plants that have altered their physical structure to survive extreme heat and
lack of water, are the largest group of such plants living in the deserts of the
American Southwest.
Cacti are among the most drought-resistant plants on the planet due to their
absence of leaves, shallow root systems, ability to store water in their stems,
spines for shade and waxy skin to seal in moisture. Cacti originated in the West
Indies and migrated to many parts of the New World, populating the deserts of
the Southwest with hundreds of varieties.
Barrel Cactus
Opuntia Cactus
Saguaro Cactus
Cholla Cactus
Cacti depend on chlorophyll in the outer tissue of their skin and stems to
conduct photosynthesis for the manufacture of food. Spines protect the plant
from animals, shade the plant from the sun and also collect moisture. Extensive
shallow root systems are usually radial, allowing for the quick acquisition of
large quantities of water when it rains. Because they store water in the core of
both stems and roots, cacti are well-suited to dry climates and can survive
years of drought on the water collected from a single rainfall.
Many other desert trees and shrubs have also adapted by eliminating leaves --
replacing them with thorns, not spines -- or by greatly reducing leaf size to
eliminate transpiration. Such plants also usually have smooth, green bark on
stems and trunks serving to both produce food and seal in moisture, such as the
Paloverde. Some plants
produce ephemeral leaves during the brief rainy season to help increase
transpiration and photosynthesis. Sometimes these leaves only last for one day.
Phreatophytes
Phreatophytes, like the Mesquite Tree, have adapted to desert conditions by developing extremely long root systems to draw water from deep underground water tables. The mesquite's roots are considered the longest of any desert plant and have been recorded as long as 80 feet.
The Creosote Bush is one of the most successful of all desert species because it utilizes a combination of many adaptations. Instead of thorns, it relies for protection on a smell and taste wildlife find unpleasant (secondary metabolites). It has tiny leaves that close their stomata (pores) during the day to avoid water loss and open them at night to absorb moisture. Creosote has an extensive double root system -- both radial and deep -- to accumulate water from both surface and underground water.
Perennials
Some perennials, such as the Ocotillo, survive by becoming dormant during dry periods, then springing to life when water becomes available. After rain falls, the Ocotillo quickly grows a new suit of leaves to photosynthesize food. Flowers bloom within a few weeks, and when seeds become ripe and fall, the Ocotillo loses its leaves again and re-enters dormancy. This process may occur as many as five times a year. The Ocotillo also has a waxy coating on stems which serves to seal in moisture during periods of dormancy.
Annuals (Ephemerals)
The term "annuals" implies blooming yearly, but
since this is not always the case, desert annuals are more accurately referred
to as "ephemerals." Many of them can complete an entire life cycle in a
matter of months, some in just weeks.
Contrary to the idea that deserts are uniformly hot, dry and homogeneous in
their lack of plant life, they are actually biologically diverse and comprise a
multitude of micro-climates changing from year to year. Each season's
unique precipitation pattern falls on a huge variety of mini-environments.
And each year in each of these tiny eco-niches, a different medley of plants
bloom as different species thrive.
Desert plants must act quickly when heat, moisture and light inform them it's
time to bloom. Ephemerals are the sprinters of the plant world, sending flower
stalks jetting out in a few days. The peak of this bloom may last for just days
or many weeks, depending on the weather and difference in elevation. The higher
one goes, the later blooms come. Different varieties of plants will be in bloom
from day to day, and even hour to hour, since some open early and others later
in the day.
Ephemerals such as the Desert Paintbrush usually germinate in the spring following winter rains. They grow quickly, flower and produce seeds before dying and scattering their progeny to the desert floor. These seeds are extremely hardy. They remain dormant, resisting drought and heat, until the following spring -- sometimes 2 or 3 springs -- when they repeat the cycle, germinating after winter rains to bloom again in the spring. There are hundreds of species of ephemerals that thrive in the deserts of the American Southwest.
One plant adaptation important to the survival and early dominance of flowering plants is the production of secondary plant metabolites. These bad tasting and sometimes toxic compounds have been one of plants most powerful means of defense. These compounds can be divided into six easily identifiable classes based on plant material and extract. The table below lists each compound and how they affect vertebrates in general.
| SECONDARY COMPOUND | HUMAN PHARMACOLOGICAL EFFECTS |
| Alkaloids | Antibacterial, stimulants, |
| sedatives, vaso-constrictors & | |
| dilators, diuretics, | |
| expectorants, antidiarrheal | |
| Cyanogenic glycosides | cough suppressants, treatment |
| of digestive disorders | |
| Saponins | expectorant, diuretic; treatment |
| of skin diseases, anemia & diabetes | |
| Cardiac glycosides | Regulation of heart activity |
| Tannins | Astringent used in treating |
| cuts & burns, antidiarrheal | |
| Simple phenolics | Antihelmenthics, antiseptics |
| analgesics, diuretics |
In this lab, six different New Mexico plants were tested for the presence of alkaloids, saponins, tannins, phenolics, and antimicrobial activity. The plants of interest are bindweed, horsemint, nightshade, sunflower, tansy and willow.
 |
Bindweed Convolvulaceae Family Convolvulus arvensis |
|
Horsemint Lamiaceae Family Monarda punctata |
 |
 |
Silverleaf Nightshade Solanaceae Family Solanum elaeagnifolium |
|
Sunflower Asteraceae Family Helianthus annuus |
 |
 |
Tansy Mustard Brassicaceae Family Descurainia pinnata |
|
Coyote Willow Salicaceae Family Salix exigua |
 |
Review Questions
- Define and give an example of convergent evolution in plants.
- What is the equation for photosynthesis?
- Give an example of a medication that is derived from a plant.
- List four secondary plant metabolites.
- List four plant adaptations discussed in lab and tell how each aids plants in
survival.
- How does photosynthesis work in general?
- Why is CAM photosynthesis a plant adaptation?
- Why do plants produce ephemeral leaves?
- What are the functions of the stomata?
- What is a phylogenetic tree and why is it useful?
- Do plants have DNA?
- Describe the adaptive strategy of cacti producing ephemeral leaves.