Avocado Trees and Fruits: Botanical and Historical Facts
Interesting Plants and Delicious Fruits
Avocado trees produce appealing and nutritious fruits with a buttery texture and flavor. Their fruit is not their only claim to fame. They have some interesting botanical features and a historical background that includes some intriguing facts. The fruits are a popular food in many places, including North America, but avocado plants are worth studying for more than their use as food.
The scientific name of the avocado is Persea americana. (The word "avocado" is used for the plant as well as its fruit.) The plant belongs to the family Lauraceae. The family includes other culinary plants, including the bay laurel (Laurus nobilis), whose leaves are known as bay leaves and are used to flavor food, and trees in the genus Cinnamomum, whose inner bark is used to make the spice known as cinnamon.
Though all avocado trees belong to the same genus and species, breeders have created different cultivars with slightly different features.
The Avocado Tree
Mature avocado trees are often thirty to forty feet tall. The height depends on the cultivar, however. So-called "dwarf" cultivars exist that reach only ten feet in height. On the other hand, some trees may be as tall as eighty feet. Though many avocado trees are cultivated in order to satisfy people's desire for the fruit, wild ones still exist. The trees may have a long lifespan. They can live for seventy to a hundred years or more, at least in their wild form.
The tree's leaves have an elliptical shape, as shown in the photo below. They are often arranged in loose whorls. The leaves are usually attached to the stem in an alternate pattern and have a glossy sheen. The plant is evergreen, but it sheds some of its leaves if it's stressed. Individual leaves are also shed due to old age and then replaced by new ones. New leaves are red at first and become green as they mature. In some cultivars, the leaves have a wavy edge.
The flowers are arranged in branched clusters known as panicles. They are often pollinated by bees. They have an interesting and unusual feature. Each flower has female and male reproductive structures, but depending on the time when it's open, the flower is functionally either a female or a male. When the flowers first open, they are female. When they open for the second and final time, they are male.
When a flower is in its female form, it has stamens with anthers, but the anthers are closed and can't release pollen. When the flower is in the male form, the anthers open up and release pollen, but the stigma of the pistil doesn't accept pollen grains.
Type A and B Flowers
The flowers are classified as either type A or type B based on their specific cultivar. The types behave differently with respect to flowering time.
- Type A flowers open in the morning, when they are female. They close at noon. On the afternoon of the next day, they open again. At this time, they are male.
- Type B flowers are female in the afternoon. They close in the evening. When they open the next morning, they are male.
The two types complement each other when they are grown near one another. When one type is male and releasing pollen, insects can pick up the pollen and deposit it on the stigma of the other type, which is female. The different stages of the flowers promote cross-pollination. Trees that are cross-pollinated produce more fruit than ones that are self-pollinated.
Avocado Fruits and Seeds
The fruits of the different cultivars are pear shaped or globular. Their color ranges from bright green to dark purple. Some versions have a smooth appearance and others a pebbly one. An alternate name for the fruit is "alligator pear." The pale yellow or greenish-yellow flesh is the part that's eaten. The seed is discarded.
The fruit is very nutritious and is rich in monounsaturated fat, vitamins, minerals, and fiber. It also contains omega-3 fatty acids. Monounsaturated fats are healthy for us (when eaten in reasonable and not excessive quantities). The cut flesh of the avocado fruit is popular on its own or as a component of a meal, as a spread in sandwiches, as the major part of a guacamole spread or dip, and even in desserts.
Biologically, the fruit is a berry. It matures on the tree but ripens off the tree. The dropped or picked fruit produces ethylene gas, which causes it to ripen. It contains a single seed, but this seed, or pit, is very large. Some people keep one or more seeds from their fruit in order to grow an avocado plant. A special procedure is needed for this activity. It's described in the Missouri Botanical Garden reference provided at the end of this article.
Anyone bringing avocados into their home or who is considering growing an avocado plant should be careful if they have pets or farm animals. The flesh of the fruit is safe and nutritious for humans, but the fruit, the seed, and the leaves contain a chemical called persin that is toxic to some animals.
Origin of Avocados
Avocado plants have had a long history and have been grown for their fruit for a long time. Their origin is uncertain, but it's thought that the first trees that could be called "avocados" appeared in south central Mexico. Exactly when this happened is unknown. The species spread through Central America and eventually into South America. The history of the plant contain contains some interesting highlights. I discuss some of them below.
Avocados and the Pleistocene Megafauna
The Pleistocene Megafauna
Avocado plants seem to have thrived during the time of the Pleistocene megafauna (or the Pleistocene megaherbivores). The megaherbivores were a large group of herbivorous animals that inhabited North and South America, Europe, and other areas during the Pleistocene Epoch. The animals each weighed over a hundred pounds. They would have been impressive to see. Somewhat mysteriously, they became extinct as the epoch approached its ending around 13,000 years ago. The extinction happened at a different time in each area.
Wild Avocados in the Pleistocene
The wild avocado in the time of the megafauna produced a fruit with a large seed covered with a relatively thin layer of flesh. The wild fruits of today still have these features. The thick-fleshed fruits that many people love to eat arose during cultivation.
Avocados probably depended to a large extent on the megafauna for the existence of their species during the Pleistocene. Only big animals with a digestive tract of a large diameter could have safely passed the fruit and its large seed through their body, digested the flesh, and then deposited the seed in a suitable habitat in their feces. The process would have enabled the seed to germinate in a new area and grow into a new plant.
One herbivore in the Pleistocene megafauna group was a giant sloth named Megatherium americanum. It was able to stand on its hind legs to reach vegetation. Its tail helped it to balance in this situation. Eremotherium was a relative that lived in South and Central America and in southern North America.
Extinction of the Megafauna
The Pleistocene is sometimes known as the Ice Age. Glaciers extended through North America and into South America (and through Europe). Not everywhere was covered by ice, however, and the glaciers repeatedly advanced and then retreated as the climate cooled and warmed.
The changes in the climate are believed to have been severe and rapid as the end of the Pleistocene approached. The stresses that occurred during this period are thought to have played a role in the extinction of the megafauna. Some researchers suspect that as the death of the animals started and their effects on their surroundings disappeared, conditions in the environment may have changed in a way that was deleterious for the remaining animals. It's believed that the increasing number of human hunters were also part of the reason for the extinction of the megafauna.
Effects of the Megafauna Extinction
Avocados are sometimes said to be "anachronistic plants" or "ghosts of evolution" because it seems that they should have become extinct long ago when the megafauna disappeared.
Without the aid of the megafauna, the fruits of an avocado tree would have dropped under the parent tree and (assuming nothing else ate their flesh) decayed. If the seeds inside were exposed before they began to decay, they may have been blocked from light by the leaves of the parent tree. Modern avocado trees can produce dense shade. Assuming this was also the case in the Pleistocene, the shade would have interfered with the growth of any seedlings that emerged from the seeds.
The scenario described in the previous paragraph is commonly mentioned and may well have happened, but it can't be the whole story. Avocados didn't become extinct at the end of the Pleistocene, and both wild and cultivated ones exist today. They were obviously able to reproduce and spread in the wild without the aid of their former megafauna helpers, though perhaps to a lesser extent. Exactly how they survived after the ancient megaherbivores disappeared is unknown.
It's possible that humans collected some of the fruits, ate the flesh, and discarded the seeds without eating them. At some point, people decided to deliberately plant the seeds. They likely chose seeds from fruits with the thickest flesh in an attempt to produce more of them.
The avocado's prime as a wild species (as far as we know) was in the time of the megafauna. Today they are abundant as a cultivated species due to the activities of humans.
The Grafting Process in Plants
A very common cultivar in North America today is the Hass avocado. The history of Hass avocados involves an attempt at grafting. This process is often performed by commercial growers of fruit trees, including avocados. It involves joining parts of two different plants together in order to increase the probability of rapid production of high quality fruit with uniform characteristics. The parts that are joined are referred to as the scion and the rootstock (or the stock).
Various styles of grafting exist. Many factors are involved in creating a successful or unsuccessful graft, and there are some biological unknowns about what happens during the process. The basic process can be summarized as follows.
- The scion is a cutting that is capable of producing buds and comes from a tree with desirable features. (The rest of the tree is left to continue its life.)
- The rootstock is the part of a plant that produces roots and comes from a plant variety or species that is compatible with the scion. The rootstock is grown from a seed and often contains a short stem and sometimes leaves as well as roots.
- The scion and rootstock are carefully joined together so that they become one plant.
- The stock supplies the new plant with water and nutrients. The new branches, leaves, flowers, and fruit that form from the scion will have the genes and features of the desirable plant.
Grafting is a type of asexual reproduction of desirable plants, albeit one that is controlled by humans. As might be imagined, the scion and rootstock must be chosen carefully, and they must be joined correctly in order to ensure that vascular tissues meld and that a successful union is obtained.
Growing new plants from seeds would be a slower process than grafting and would have an uncertain outcome with respect to the fruit's features. The quality of the fruit wouldn't be known in advance.
History of Hass Avocados
Rudolph Hass (1892–1952) was a US letter carrier who lived in California. In 1926, several types of avocados were grown in California. Hass was already growing avocados in his garden (reportedly the Fuerte cultivar) at the time of the discovery for which he is remembered. The general points in the creation of the Hass avocado are known, but some of the details are a bit fuzzy.
Hass bought avocado seeds of an unrecorded and probably unknown type from a seed collector. He hoped to grow rootstocks from the seeds to join with scions from his favorite Fuerte avocado plant (or plants). He tried more than once to graft a Fuerte avocado scion onto a particular stock created from one of the purchased seeds, but his attempts failed.
Hass ignored the rootstock after his failures and found that it continued to grow by itself and produce a tree. He was surprised that the tree eventually produced fruit with an unusual appearance. The Hass cultivar produces fruit at the relatively young age of two or three, so "eventually" wasn't very long. Hass discovered that he and his acquaintances liked the taste of the fruit.
Hass patented the cultivar, which became very popular. Unfortunately, patenting didn't have as much value then as it does today. Hass made very little money from his discovery.
The tree that Rudolph Hass grew and that was a new avocado cultivar stayed where he planted it throughout its life. The tree died of disease in 2002 when it was over seventy years old.
A Delicious Food and a Resilient Plant
I didn't discover avocados until I was an adult and had lived in Canada for some time. Now I can buy an avocado from my local grocery stores any time I want to. The fruits are always available. I love their taste and texture. The fruits that I eat are often Hass avocados, because they are what my local stores most often sell. I occasionally see other types of avocados, though, and plan to gradually explore them. I enjoy exploring the avocado plant's biology and history as well as the taste and uses of its fruit. It's an interesting and impressive plant in more ways than one.
References and Resources
- Avocado fruit and tree information from the Encyclopedia Britannica
- Avocado flowering and pollination from Gary S. Bender, University of California Agriculture and Natural Resources
- Persea americana facts from the Missouri Botanical Garden
- Nutrients in commercial avocados from SELFNutritionData (The data is obtained from the USDA, or United States Department of Agriculture)
- "Why the Avocado Should Have Gone the Way of the Dodo" from K. Annabelle Smith, Smithsonian Magazine
- Anachronistic fruits from Connie Barlow and the Arnold Arboretum, Harvard University
- Megaherbivores and avocados by Jeffrey Miller, Colorado State University
- Plant grafting information from Jing Wang, Libo Jiang, and Rongling Wu, New Phytologist journal
- "How the Hass Avocado Conquered the World" by Brian Handwerk, Smithsonian Magazine
© 2020 Linda Crampton