To get a tree, you need a seed. Although many plants and trees are grown from vegetative shoots, cuttings, or root suckers, somewhere back in the past there was a seed. A seed is an embryo, literally a miniature tree in a tiny package, a package that is really a time capsule, which requires a preset combination of circumstances to occur before the seed breaks its dormancy and starts to germinate. Germination is a one-way process, and once started, the seed cannot return to its dormant stage. Seeds can lie dormant for ages, waiting; there’s a story of a date palm seed found in King Herod’s tomb that germinated about 2000 years after landing in the tomb.
To go back just a little further, we first need to have a meeting of plant sex cells from the flowers of mature female and male trees. This is called pollination, accomplished either by animals, insects, bees, or the wind; the male sperm cells, pollen, need to be in contact with the ovules in the ovary, the eggs, for fertilization to happen. Once fertilized, the ovule is called a zygote, which continues to develop into an embryo. The embryo, its stored food reserves, and its bomb-proof cover, the seed coat, are the three basic parts that make up the seed. These processes are much more complicated than I have described. This is an outline, designed to pique your curiosity about the world of botany.
The variety of mature seeds is great. All 280,000 known species of seed plants do something different, from the miniature dust-size seeds of some orchids to the giant coconut, which can weigh 60 pounds. Seeds are what give plants the mobility to move into new areas and give them the space they need to live. Whether carried by the wind, water, or animals, seeds can go on incredible journeys, sometimes attached to the fur of a coyote or washed across the sea by a violent storm. Whatever the method, plant seeds can travel nearly as well as we do. All this time, the embryo is alive inside, waiting to land in the right place with the right conditions at the right time.
One of the most amazing qualities of seeds is their ability to control their internal humidity. This is needed to take care of either extreme, too dry, which can kill the embryo, or too wet, which will start the irreversible process of germination before the right time. Dormant seeds usually have a low moisture content, between 5 and 10 percent. With a bean seed, a tiny pore called the hilum functions as the water content control gate. Other types of seeds have similar structures. It is the hilum that forms the black eyes of certain peas.
Most seeds are programmed to endure a period of cold and dry before germination occurs. Some also need to have the seed coat abraded. Germination must happen after the unpredictable cold that could freeze the tender new tissue of the emerging seedling. Temperature and daylight are factors that help determine the right time to begin.
A seed has three essential parts: the embryo, really a miniature plant, a food source, of which the cotyledons are one kind, and a seed coat, its outer armour. The inside of the miniature plant and its three organs can be seen with a microscope: root, stem, leaves, all lined up and ready to go.
Without the outer armour of the seed coat, survival could not happen. Formed of a thin layer of heavily reinforced cells called sclereids, which contain lots of lignin, the seed coat is the outer shell that allows the seed to safely travel through time and distance until it arrives at its new home. And there, the miracle of germination can begin.
When the time and conditions are right, the seed is able to imbibe a lot of water, which starts the whole reaction. Activated enzymes break down the oils and starches in the food reserve, converting them to glucose, the simple sugar which is the fuel of life. It will be needed for the cell expansion that will be the first growth of the radical and the shoot.
The first thing we see on the outside is the splitting of the seed coat and the emergence of the radical, the first root. The second step is the emergence of the embryonic leaves. These are monocots with a single cotyledon and dicots with two cotyledons. We are all familiar with the monocots as the grass-like plants which all cereal crops are part of: wheat, barley, corn, and rice. Dicots are the broadleaf plants, like beans, oak trees, avocados. The stored energy in the cotyledons is converted to simple sugar, which supplies the energy for growth; this is proven by the expansion and emergence of the radical and the tiny shoot, the hypocotyl, which supports the tiny embryonic leaves. As the radical and the shoot continue to expand, they deplete the stored energy in the cotyledons; the young leaves are as yet unable to produce enough energy through the process of photosynthesis. The cotyledons are the first-stage fuel rocket that allows lift-off.
The young leaves, once photosynthetically active, are stage two, and we are good to go! Wow, a baby tree! Given half a chance, it will someday grow into something really beautiful. As far as the discussion of the seed and seedling goes, we are almost done. What we need to discuss now as the seedling tree continues to grow is called primary growth.
Meristems
Primary growth is what all plants do in their first year of life. If they live for only one year, like annuals, or grow for one year and then die back to the root crown, like perennials, the only growth that occurs is primary. Woody perennial plants, that live for years and get thicker every year, perform primary growth at their branch and root tips, and they also perform secondary growth, which thickens branches and trunks with growth rings. These different types of growth are accomplished by meristems. Meristems are unique tissues that can generate new cells that we see as physical growth. A meristem remains intact and is able to generate new cells and remain constant, whole, generating new tissue while maintaining its own integrity.
When we talk of meristems, we are focused on the processes of primary and secondary growth. Primary growth in trees is the annual extension of shoot and root tips to make the plant taller and expand its root system to match the greater growing mass of leaves above. Secondary growth is the expansion of the trunk and bark accomplished by the vascular cambium and the cork cambium. Herbaceous plants, annuals, biennials, and perennials experience only primary growth, whereas woody plants, trees, and shrubs experience both primary and secondary growth.
The only way that plants can grow and get larger is to increase their mass by making more cells. That is accomplished through a process called mitosis. Mitosis takes us back to the cell cycle and the generation of two daughter cells from a mother cell. This dividing action is what meristems are all about, and the position and activity are strongly genetically controlled. The two meristems that perform primary growth are the RAM (root apical meristem) and the SAM (shoot apical meristem). The RAM is located at the root tips and the SAM is located at shoot tips. The SAM produces all the above-ground organs, stems, leaves, and flowers. The RAM produces the root system. Every shoot tip has a SAM and every root tip has a RAM.
The SAM produces three other specialized meristems that each grow an essential part of the expanding shoot tip: the protoderm, the procambium, and the ground meristem. The protoderm (first skin) generates the outer layer of cells that covers the young expanding leaves. After cell expansion is finished, the protoderm matures into epidermal pavement cells, guard cells, and trichomes (leaf hairs). The procambium generates the cells that will differentiate into the xylem and phloem strands. The ground meristem generates the ground tissue, the cortex, and pith in stems and the mesophyll in leaves. This miracle growth system, gene expression, is run by information stored in the cell’s nucleus and orchestrated by proteins carrying out those instructions.
Besides growing the three meristems that produce primary growth of the epidermis (protoderm), ground meristem (pith and cortex), and the vasculature (procambium), the SAM also grows all the exterior lateral organs—the leaves, axillary buds, flowers, and the lateral branches. These lateral external organs begin from the surface of the SAM tip. Shortly after initial growth, the axillary buds may develop their own SAM and continue again as described above, producing their own protoderm, procambium, and ground meristem and leaf primordia. Actively expanding growth regions produce the plant hormone auxin, which inhibits growth of axillary buds. This process is called apical dominance.
Now we need to get back to the RAM, the root apical meristem, and its processes and tissues that grow the root system. The primary growth in length of the roots is accomplished by the RAM where mitotic cell division occurs. The greatest concentration of active cell division occurs at the root tip in the quiescent centre. The quiescent centre is the tissue that, through mitotic division, supplies the mother cells that will divide and divide to produce the new cells needed for root growth.
Just as with the growth we have seen in the SAM, the RAM acts in a similar fashion and produces three types of meristematic tissue that will grow the root. The protoderm produces the epidermis, the procambium produces the vasculature, and the ground meristem produces the cortex; there is no pith in roots. All of this ongoing cell division is happening in the area we call the zone of division, which only occurs in roots.
The zones of division, elongation, and maturation are the three main growth and expansion areas in the newly developing root. Cells are dividing in the zone of division, the zone of elongation extends the length of the root through the soil, and the zone of maturation, which does not extend farther through the soil, is where lateral roots will eventually develop. This genius system never pushes the new lateral roots forward into the soil through rough and sharp particles, which would tear them off. Instead, the root waits to produce new lateral roots and only does so in the zone of maturation, where they will succeed.
Unlike shoots, the roots produce only one lateral organ: new roots. These lateral roots, unlike the root tip, are not initially produced by the RAM. Lateral roots form internally, back from the root tip, and as they extend outward through existing root tissue, they form their own RAM, and head off in search of water and minerals.
The vasculature, the xylem and phloem in a growing root, are contained in a central core called the vascular stele. Lateral roots are generated by mitotic activity of cells at the outer edge of the stele. The developing lateral root tip pushes through the cortex and epidermis and out into the soil. Meristematic cells in the developing lateral root form its xylem/phloem tissue and connect to the existing vasculature in the stele. A root cap develops at the tip of the new lateral root with its own RAM, and the endlessly spreading and exploring life of the roots goes on.
That is a lot about primary growth. I have thought of different ways to help people visualize this process; one is the idea of using a pencil to draw a line. Think of the lead touching the paper as a SAM or RAM. Before you start moving the pencil to draw a line, we could think of that as dormancy or at least no growth going on at the moment. Now start pushing the pencil along the paper, making a line. The line is the new growth; the lead is the SAM or RAM, always at the tip, staying intact and able to create more of the line as needed. This is exactly how a new shoot or root grows, always from the tip, where the cell-generating area always stays ahead of the new tissue it has just formed. The contrail of a jet and the track of a snow tire are two more metaphors that help us to visualize the new growth happening, just as the line of a pencil does.
All of this comes from my decades of work and study as a Calgary arborist, Calgary tree doctor. I do this for the greater good of the urban forest, one of the world’s most delicate created biomes.
