It seems to us that we know what trees are, but even at the genetic level it is very difficult to determine what distinguishes them from other plants.
Spiny pineSeveral years ago, after Thanksgiving dinner, which was held at my parents' house in Vermont, lightning struck the maple that stood in our yard. We heard a terrible crash, and the darkness outside the kitchen window for a moment gave way to bright light. And only in the spring, we were able to make sure that the tree had already died.
This maple was young, its trunk in diameter did not exceed the dessert plate. If his life had not been cut short by a catastrophe, he could have lived 300 years. But among trees, death by accident occurs surprisingly often. Sometimes this is due to a blatant human error, as when in 2012 in Florida the
cypress cypress tree over 3500 years old was
destroyed as a result of deliberate arson. More often, trouble comes in the form of bad weather - drought, wind, fire or frost. And, of course, trees are affected by parasites and diseases; Such an attack, like a fungus, can noticeably shorten the lifetime of a tree. But those trees that have managed to avoid such enemies can live for an incredibly long time.
If you force a person to describe what makes a tree a tree, then among the first signs will be a long time of life, wood and height. Many plants have a predictably limited lifetime (scientists call this programmed aging), but this is not the case with trees - many of them have been stubbornly living for centuries. It is this property, unlimited growth, that can serve science as a demarcation sign of trees, even more than wood. But it helps to a certain point. It seems to us that we know what trees are, but they start slipping through our fingers when we try to identify them.
Trees are not grouped together — their lineage differs, and they have acquired different traits, using different strategies to become what we see them today. Take a long time life. A classic example of mafusilovoy life expectancy for trees is the current record holder, a
spinous pine age of 5067 years, growing high in the White Mountains in California. The pyramids in Egypt were built when this tree was almost 500 years old. Scientists assume that hardy spinous pines owe their persistence to the main place of growth: they avoid fires passing through lowlands and parasites that do not survive in harsh subalpine conditions. Giant redwoods, growing a little below those mountains where pines grow, use a completely different approach to longevity. These monsters - whose trunk diameter can reach 10 meters - live for thousands of years, resist fire and parasites with the help of thick and resistant bark and a variety of substances that serve as repellents.
About 400 miles to the east, there are trees that look like spindly panicles, sprouting pine and sequoias in their lifespan, again using a completely different strategy.
Aspen-shaped poplar - a tree that can be hugged, and which rarely grows above 15 m - is extremely successful at releasing new processes from the lower part of the trunk. As a result, huge chains of "trees" appear, in fact genetically being one individual, united by underground roots. It is believed that the colony of the aspen-type poplar in Utah is about 80,000 years old. Then, on the Earth lived Neanderthals.
If you add clones to the review, the trees quickly lose their longevity primacy.
Royal Holly - a brilliant green shrub originating from Tasmania (bushes, strictly speaking, do not belong to the trees, because they lack a central, predominant stem). In the world there is only one population of royal holly, and scientists believe that all this - clones. Although it blooms sometimes, no one has ever seen its fruits. A recent radiocarbon estimate of his age showed that he (them?) Is at least 43,000 years old.
And there is also a
creosote bush growing in the Mojave Desert in California, which is called the "king of the clones", and which is about 11,700 years old. In search of signs that unite the definition of trees, longevity turns out to be completely unsatisfactory, as forester
Ronald Lanner writes in a 2002
essay in Aging Research Reviews.
Andrew Grover , a geneticist from the Pacific Southwest Station of the US Forest Service, based in Davis, California, also ponders trees for a long time. He quickly admits that defining them is quite problematic. “Go to the nursery and you will find there plants divided into categories according to their type and functioning, including the group attributed to“ trees ”, - he writes in the
work of 2005“ Which genes make a tree a tree? ”In Trends in Plant Science. This categorization is intuitive and practical, but unnatural. ”
As an example, Gruver points to wood as the defining characteristic of trees. Wood in “real” trees (we will return to them later) appears in the process of “secondary growth”; it allows trees to grow in thickness, not just in height. For the secondary growth meets the ring of particular cells surrounding the stem. The tissue of these cells is called
cambium , and they are divided in two directions - outward relative to the tree, because of what the bark is made, and inward, because of what the wood is made. Year after year, wood is deposited in all new inner rings, where pulp and a long, hard polymer
lignin are added. After solidification, the wood cells die and almost completely disintegrate, leaving behind only rigid walls.
In existing plants, secondary growth probably has a single evolutionary source, although the tiny size of
moss and
horsetails invented their version of this process about 300 million years ago, which allowed the now extinct
lepidodendron , for example, to grow to 35 meters in height. But the presence of secondary growth does not lead to the automatic emergence of a tree: despite the single source, “woodenness” arises here and there, across the entire family tree of plants. Some groups of plants lost their ability to form wood, and sometimes this ability reappeared in those evolutionary branches where it had once disappeared. Apparently, this ability appears rather quickly in the course of evolution after the plants colonize the islands. For example, in Hawaii there are trees of violets, and in the Canary Islands there are trees of dandelions.
The concept of the tree itself is quite flexible, which goes against the literal rigidity of this concept - remember the hard stalks of sage or lavender. It is not a question of presence or absence, it is a question of degree. “Non-like plants and large trees with wood are the two ends of a multitude, and the degree of tree demonstrated by certain plants can be influenced by environmental conditions,” Hoover and his colleague wrote in an article in 2010 in the journal New Phytologist. “Indeed, the terms“ grassy ”and“ tree-like ”, although quite practical, do not recognize the enormous anatomical diversity and the presence of different degrees of tree in plants that belong to one or another class.”
Molecular biology offers certain ideas about why the ability to generate wood is maintained and so often appears in the process of plant evolution. Genes associated with the regulation of the growth of shoots directed upward, providing the "main" growth of trees and other plants, are also active during the secondary growth that produces wood. This suggests that the new genes that regulate the appearance of wood, in the process of evolution, have absorbed the already existing and critically important shoot growth genes. This may also explain why the ability to produce wood is retained in plants that do not have it, and why, from an evolutionary point of view, re-enabling this ability is so easy.
However, in order to be a tree, it is not necessary to produce wood. Among
monocots , a huge group of plants that have lost their ability to grow again, there are some tree-like representatives that are not “real” trees, but definitely look like trees. Bananas grow to a great height with the help of something resembling a trunk, but in reality - on a “pseudo-stemming” mass of densely packed and overlapping bases of leaves. A true banana stem appears only during flowering, pushing and protruding from the leaves. However, banana trees can reach 3 meters in height. The family of palm trees, also belonging to the monocotyledonous, grow in height, growing an initial thick sprout, on the top of which a huge bud appears (note that the trunks of palm trees do not thicken during growth).
Given all this, it is not surprising that a recent analysis of the genome of trees can say little about the defining features of a tree. David Nial, a geneticist at the University of California, Davis, and his colleagues studied the genome of 41 plants (including grapes) that were sequenced starting from
black poplar in 2006. Their analysis, published last year in the journal Annual Review of Plant Biology, showed that trees that produce edible fruits in the genome often have an abnormally many genes devoted to the production and transfer of sugars, compared to trees that do not have edible fruits. But they have tomatoes. Some trees, such as spruce, apple, and some eucalyptus trees, have expanded sets of genetic tools to combat problems such as drought or frost. But some herbaceous plants did the same, including spinach and
rezuhovidka Tal , which is a plant-growing biology-like plant that looks as little like a tree as possible.
So far, no outstanding gene or set of genes characteristic of trees, as well as any particular property of genes, have been found. Complexity? No: gene duplication (which is often used as a sign of complexity) is present throughout the plant kingdom. Genome size? No: the largest and smallest genomes in size are found in grassy plants (
Paris japonica and
Genlisea tuberosa, respectively - the first is a spectacular plant with white flowers, and the second is a tiny predator that catches and has the simplest).
A conversation with Niall confirmed that the tree is probably more dependent on which genes are included than on which genes are present in the genome. “From the point of view of the genome, trees, in general, have almost the same thing as grasses,” he said. - Trees are big, have wood, can extract water from the ground and deliver it high. But no particular biology can be seen that would separate the tree from the grass. ”
But, despite the difficulties of classification, there are undeniable advantages to belonging to trees. This allows plants to use height, where they can absorb sunlight and spread pollen and seeds, without being exposed to such obstacles as their relatives growing close to the ground. Therefore, it is possible to start thinking about the word “tree” as a verb, and not as a noun - “wood” or “wood”. This is a strategy, a way of existence, like swimming or flying, although from our point of view, it happens very slowly. “Wood” with no definite finish line - until the ax, parasite, or lightning of Thanksgiving does not destroy you.