Hi, my name is Doug and I’m a Garden Guide volunteer at Gamble Garden in Palo Alto, CA. I’ve been interested in plants for as long as I can remember and I’ve spent a lot of time studying plant biology. I’ve developed this tour to help you understand what plants do to survive in hot, dry environments. Using the succulents, cacti and grasses of Gamble Garden as examples we'll see how plants that live in arid environments have evolved adaptations that conserve what little water is available in desert and Mediterranean areas and how they protect themselves from the heat of the day. You can open this guide on your cellphone and use it to walk around the garden, or you can do an ‘armchair’ tour by reading the text and looking at the pictures in your home. You won’t need to know a lot of science to understand the tour and you can skip over any topics that don’t interest you. It will take about 30 minutes for the tour if you’re walking around the garden.
COVID Note: If you're planning to tour the garden, please wear a mask and respect social distancing guidelines. Also, during the morning the staff may cordon off certain areas so they can perform maintenance. That means some stops on the tour may not be accessible. Better to visit after 2pm, when most areas should be open.
Let’s get started. Walk over to the white greenhouse (#1 on the map below).
Here are some quick links to get you back to the text sections after you've consulted the map:
Let's take a moment to consider how you might prepare for a hike on hot, cloudless day in the middle of summer.
- You’d certainly bring along a bottle of water
- You might wear a hat and loose clothing
- You’d put on sunscreen
- You might carry an umbrella
- You might have a bandanna to cover your nose and mouth
Plants have evolved adaptations that mimic all these human preparations:
- They have strategies to obtain and store water
- They have strategies to minimize water loss from their leaves due to evaporation
- They have evolved sunscreens to protect against UV radiation
- They can cover their ‘mouths’ to minimize evaporation
Why would a plant need to store water?
- They live in a place where there isn’t much water all year round, like a desert. The official definition of a desert is a place with less than 10 inches of rain per year.
- They live in a place where rainfall is seasonal or unreliable – Mediterranean climates like CA where summer rain is unlikely and winter rain varies considerably year to year.
Let’s look at some plants that can store water. Succulents and cacti are the masters of water storage. What is a succulent? In general, they are plants whose leaves, stems, or roots have evolved to store water. However,
- Some people exclude plants with root water storage
- Some people exclude cacti
- Some people exclude bromeliads
The problem is that ‘succulent’ is a characteristic of plants, not a single family. There are about 60 plant families that could be termed succulents, with 1000s of plants. I like the operational definition: “A succulent is whatever a succulent collector collects”.
CAUTION! As we go through this tour, we’re going to encounter various kinds of cacti. For some reason, people like to touch the spines to see if they are sharp. This is a very bad idea. In addition to the big spines you can see, many cacti have small, almost invisible spines called ‘trichomes’. These small spines have barbs on them like a fishhook. Once they are embedded in your skin they are extremely irritating and difficult to remove. My advice is don’t touch any cactus.
Let’s compare the leaves of a succulent (jade plant [Crassula]) with the leaves of a nearby sage plant. You’ll find these two plants near the white greenhouse (the sage is near the rain barrel). Before scrolling down, think about how they are different.
- Jade leaves are thick; sage leaves are thin
- Jade leaves have a waxy coating
- Jade leaves are spongy
Do not pick a jade leaf but look at the picture below.
What happens to the stored water if you tip the leaf? Does it pour out? No, the water is stored inside the leaf cells and it is mixed with sugars and protein to make it less likely to evaporate. If you could squeeze the leaf you’d find that it acts just like a sponge and droplets of water would drip out.
Now, let’s look at an Aeonium on the other side of the path near the fence. What do you notice about this plant?
- Leaves are thick and water filled.
- Also waxy leaves arranged in swirls.
- Leaves may be colored for sun protection
In the same area, you’ll find some Aloes. How would you describe them?
- Thick leaves
- Waxy surface
- Low to ground
- Grows in clumps
- Spikes!
Aloes are familiar plants and many of them have spikes at the sides and ends of the leaves. Why do you think that is? If you are a plant that stores water in a dry environment, you are going to be very popular. Lots of succulents and cacti have evolved spines and thorns to protect themselves from predators. Also, frequently the stored water is mixed with poisons or bitter-tasting substances. One of the most popular aloes, Aloe vera, contains a substance which has been found to reduce skin irritation. But people are discouraged from drinking Aloe vera juice because its therapeutic effects aren’t established.
But sometimes you want to be popular. Many succulents produce fluid nectar in their flowers to attract pollinators like insects, bats, and hummingbirds. If you are in the garden when the Aloes are in bloom, take a moment to watch the plants for pollinators. You might see a butterfly or hummingbird.
Have you ever seen a prickly pear cactus (Opuntia)? (You won’t find this cactus at Gamble Garden; this photo is from the Stanford University Arizona Garden. If you are interested in cacti and succulents, the Arizona Garden is a great place, particularly in the Spring when many plants are in bloom.)
The prickly pear produces a very tasty fruit with hundreds of seeds inside. It gives up some of its precious water to attract seed dispersers. Remember how we talked about the water in the core, stem or leaves of some plants can be bitter-tasting or poisonous? The plant gets no advantage if an herbivore eats its leaves. But plants encourage animals to eat their fruit. While the flesh is delicious, the seeds inside are frequently unpleasant to eat so they aren't chewed. They can survive a trip through the animal's digestive tract and be excreted far from the mother plant.
We’ve seen plants that store their water in stems and leaves. Some plants store water in their roots, but these are mainly found in African deserts.
Once you’ve stored the water, you don’t want to lose it. Here are some ways plants minimize evaporation:
Thick, waxy surfaces (jade plant or the Senecio (suh-knee-see-oh) species below with blue stems) (You’ll find these plants also along the path in front of the greenhouses and in other places at Gamble Garden as well.)
Now walk over to the ‘Learning Laboratory’ which is the open expanse near the Pollinator Garden (#2 on the map). We’ve put out an array of cacti and succulents here on the benches. Take a look at a barrel cactus (we have several):
Where are the leaves on a cactus? In fact, those spines evolved from leaves on an earlier plant. Leaves present an enormous surface area for evaporation. Not having leaves means much less evaporation. The only photosynthetic part of the barrel cactus is the stem.
Do you see the pleats on the stem of the barrel cactus? When water is abundant, the stem can swell up like an accordion to store additional water. During the dry season, the pleats will shrink down as the cactus uses the water.
In this same area, you find a Mammillaria cactus:
Not only does the Mammillaria have large protective spines, but also fuzzy, hair-like coverings which block out the sunlight. In some species covered with dense, fine hairs, more than half the sunlight is blocked out and air movement around the stem is restricted to cut down on evaporation. It’s kind of like wearing a loose, white shirt.
Here’s another Mammillaria on the same bench:
Remember the large Aloe we saw by the greenhouse? Many aloes and cacti grow in large clumps, which act like an umbrella to keep the soil below cooler.
Waxy surfaces, no leaves, reflective hairs and clumping are all strategies which help to minimize evaporation and help these plants retain the valuable water they have collected.
I want to talk a little about plant biochemistry. Even if you don’t understand the chemistry involved, I think you’ll find the general concepts interesting.
Desert plants have a huge problem. Recall the basic formula of photosynthesis:
water + carbon dioxide + sunlight --> sugars + oxygen
This equation describes how plants take in water and carbon dioxide and using energy from the sun convert it into food for the plant (sugars) and oxygen as a byproduct. The earliest plants evolved a form of photosynthesis called ‘C3’ which takes place inside structures called ‘chloroplasts’ in a single type of leaf cell:
(This diagram and the one following are from the Kahn Academy: https://www.khanacademy.org/science/biology/photosynthesis-in-plants/pho...).
While this overall equation looks simple, photosynthesis involves dozens of intermediate steps. It is also divided into two processes: one which can only occur in sunlight, and another which can occur in light or darkness. The left side of the equation above describes the ingredients needed for photosynthesis. Water comes from the roots and we’ve looked at how it is stored. Sunlight is plentiful in the desert.
Carbon dioxide comes from the air and there are little holes (called stomata) in the leaves to let the carbon dioxide in and the oxygen out. But what happens when you breathe through your mouth on a hot day? Your mouth dries out as the moisture is carried away by evaporation. This happens to plants too.
If you cover your mouth, you won’t lose moisture – but you can’t breathe. Some plants, like those in the jade family, have evolved a way to minimize evaporation. They only open their stomata at night when it is cooler. They take in carbon dioxide and store it as an organic acid. Then during the day, they close their stomata, convert the stored acid back to carbon dioxide, and complete the photosynthetic process without losing water to evaporation. So, the two process of photosynthesis are separated in time. (If you are interested, this adaptation is called Crassulacean Acid Metabolism (CAM). It is named for the Crassulaceae, the family of jade plants).
Grasses have evolved yet another way to cope with hot, dry climates. You’ll find lots of examples of grasses at Gamble Garden. This one is in the California garden (#3 on the map):
It turns out that the enzyme which captures the carbon dioxide is short-circuited by oxygen, especially at high temperatures. This makes C3 photosynthesis less efficient when the plant is in a hot environment.
This isn’t a problem for plants in wet, temperate climates, but it is for plants growing on hot, dry plains, for instance. The grasses solved the problem, not by separating the photosynthetic processes in time as in CAM, but by spatially protecting the oxygen-sensitive process by locating it deeper in the leaf, away from the stomata. So, carbon dioxide is again stored as an organic acid in a process which is located in cells near the leaf surface and stomata (mesophyll cells) and is transported deeper in the leaf to bundle sheath cells which are not near the stomata. The acid is converted into carbon dioxide and the rest of the photosynthetic process can occur in an environment which has lower levels of oxygen. If you’d like to follow up on this process, it is called C4 photosynthesis.
Here's a nice diagram that shows the differences in leaf anatomy between C3 and C4 plants:
Do you see how in a C4 plant the bundle sheath cells (where the oxygen-sensitive cycle of photosynthesis occurs) are surrounded by mesophyll cells and spatially separated from the stoma, which are open to the outside air? Because the oxygen concentration in C4 bundle sheath cells is lower, it doesn't short-circuit the photosynthetic process and photosynthesis as a whole is much more efficient at high temperatures.
Did you remember to put on your sunscreen before going out? Plants are out in the sun all the time and can suffer from UV damage just like us. Lots of succulents and cacti have waxy, resinous surfaces, or colored surfaces to protect them. We’ve also seen how many cacti cover themselves with ‘hair’ to cut down on the sunlight. Interestingly, plants also produce sunscreen: ‘sinapate esters’ which are UV-B blockers which protect them from damage.
There’s another strategy that plants can use to beat the heat, especially when there are seasonal rains. Think about native California grasses and poppies and many other plants. (You can find poppies during the Spring in the Learning Laboratory and other places at Gamble Garden.)
What happens to poppies and grasses in the summer after they bloom and set seeds? The seeds or plants go dormant in the hottest part of the summer, until the Fall rains come. The water washes away a growth inhibitor in the seeds and the plants grow quickly during and after the wet season. The plant blooms, flowers and set seed quickly – when our hills turn that glorious green.
Then in the late Spring, the rains stop, and the plants die back and we see our golden hills. The ability to package up seeds that can remain viable through a drought – and in some cases through many years of drought – is an adaptation that made colonization of the driest, hottest places on earth possible.
We’ve seen how plants have adapted to arid environments by using the same strategies that we do:
- Storing (and sometimes sharing) water
- Having a covering like an umbrella or hat
- Putting on sunscreen
- Closing their ‘mouths’ during hot weather
Test what you’ve learned: Quiz time
For each of the plants below, describe which of the adaptations we’ve discussed the plant is using to thrive in an arid environment. Stroll over to the Western Chile part of our Mediterranean Garden (#4 on the map) and look at this Puya:
Puya sp.: Thick, leathery leaves; water storage in leaves; clumping for root shade; spines for protection;
Now, look for this Aloe near the gazebo (#5 on the map):
Thick, leathery leaves filled with water; spines for protection; flowers attractive to pollinators.
Lastly, return to the greenhouse area and look for this Agave Americana (#6 on the map):
Thick, water-filled, leathery leaves; water storage in leaves and in core; spines for protection. This is the species that tequila is made from. It grows very large and the leaves are cut off, leaving a huge core which is ground up, smoked and fermented.
When I give this tour in person, I’m frequently asked about how to grow plants that like arid environments. Succulents and cacti, because they are hardy, make excellent house or outdoor plants, especially in our northern CA climate. The biggest mistake that people make is overwatering them. In our area, watering once a week during the summer and maybe once every two weeks in the winter is plenty. The soil should dry out completely in between waterings. Please check out the Gamble Garden plant sale tables for a variety of succulents and cacti you can purchase. Special cactus planting mix which drains quickly can be obtained from a local nursery. Like all plants, cacti and succulents should be planted in pots that have a hole in the bottom for drainage. Never let the roots stay soaked.
That’s the end of our tour. I hope you enjoyed it. Did you know that Gamble Garden is a 501c(3) non- profit organization? We receive no city, state or federal money and we charge no admission fee. All of our income derives from memberships, donations and special events. Please consider becoming a member and/or donating so that Gamble Garden can continue to be a place of respite, beauty and education for our community. You can find details at https://www.gamblegarden.org/
If you have comments or questions about this tour, please email me using the Contacts tab on the menu at the top of this page.
