Secondary growth is what all branches and roots experience in their second and every year thereafter. Secondary growth increases the girth of these tissues with each year’s growth. This growth is seen as the growth rings in the end of any cut branch or trunk. Secondary growth is what allows trunks and branches to be strong enough to reach great heights. Secondary growth in trees also generates all the wood that is used for millions of different applications from lumber to violins, from firewood to fine art. Every wood product in the world is made from the secondary growth of some species of tree.
Primary growth, as described above, is experienced in the tree’s first or seedling year; after that, primary growth occurs only at branch and root tips. This is the growth of the meristems that we call the SAM and RAM. As described earlier, the SAM breaks up into three different meristems, each with a separate tissue to grow. The protoderm generates the epidermis, the ground meristem generates the cortex and pith, and the procambium generates the vascular transport tissues of the xylem and phloem. Similar but slightly different development occurs in roots. Let’s try to move into the newly generated shoot and see these freshly produced tissues just after they have been generated by their meristems. As soon as the initial break up of the three main tissues occurs, they are still in full contact with like tissue below them; there is no break, full contact of the newest cells and their meristem always occurs, with the meristem ever pushing out and ahead, leaving the new cells behind. Perhaps the image of layers and layers of bricks being laid down helps us see this. The last tier of bricks, the upper or uppermost layer, is the expanding meristem.
Internal hydrostatic pressure in all parts of trees and plants is essential for the plant to thrive and grow. Look at a dried out, drooping houseplant that can quickly be revived and stand up straight again shortly after it receives some water. The same is true for newly developing cells that are young enough, let’s say plastic or still malleable enough, and don't have their cell walls reinforced with cellulose yet. If water availability is not a problem, the hydrostatic pressure, turgor, can help to expand the new cells as much as possible.
Plants and trees that receive abundant water during growth spurts will grow to their maximum capacity. This water, required by the newest cells, is supplied by the adjacent xylem tissue, part of each vascular bundle that contains both the young xylem and phloem tissues. Remember that we are in the primary growth of a new shoot. The pattern that develops as the new shoot continues to grow is always the same. The new shoot is always circular; the outside of the shoot is the epidermis, the tissue inside, the ground tissue, is the cortex with the pith at the centre. Outside the pith, below the epidermis, is a ring of the vascular bundles, each containing xylem and phloem tissues.
These bundles are where the next ongoing magic transformation occurs, the outside edge of the vascular bundles that still contains the procambium meristematic tissue. As the growing shoot ages and we move farther into summer, a great change will occur. The outer sheath, or outer walls of the vascular bundles, will slough off and reach out to touch the sloughing tissue of each adjacent bundle. The outer meristematic surface separates from the bundle and joins with the procambium tissue from its neighbouring vascular bundle. These form together in a ring. What now happens is that the phloem tissue is pushed to the outside, below it the vascular cambium forms and the xylem tissue is positioned to the inside of the newly formed cambium tissue.
Wow! The mature first year shoot is now ready for winter. The pith and cortex are packed with sugar and starch, food for the ongoing winter cellular activity, which never stops. The terminal and lateral buds of the shoot have their winter protection, the bud scales, in place and all is ready for the long winter and then spring, when the new shoot that forms from the SAM contained in the bud can begin the whole cycle of primary growth and then later prepare for its second year.
Secondary growth, the increase in volume and width of shrub and tree branches and trunks and roots, is accomplished by a lateral meristem called the vascular cambium, VC. The VC wraps continuously around the inner body of the tree under the outer bark, just as your winter base layers cover your own body. The VC is very thin, just a few layers of cells, a bifacial meristem that through a complex ongoing process of mitotic cell divisions grows the whole of the tree trunk. From its outer face, it grows all of the tissue of the phloem side, and from its inner face grows all the tissue of the xylem side. Secondary growth allows trees to be what they are, the largest, most numerous and diverse group of land plants.
Whereas there can be thousands of SAMs and RAMs on a tree, there is only one VC running continuously from the roots to the shoots. Knowing on some level that they may live a long time, trees exhibit what can only be called wisdom concerning the functioning of the VC. Imagine how many cell divisions, literally billions, it takes to produce the whole body of a large tree. Mitotic cell division is very complex and intricate; natural mistakes, mutations do occur. This is dangerous, in the sense of producing tissue that deviates from the initial genetic plan. The cambium knows this and has come up with an ingenious method to control it. Rather than endless mitotic divisions of the cambium itself, what it does is make copies of itself. These form another outward layer of meristematic cells on each of its faces, both the outer phloem side and the inner xylem side. These are the phloem initials to the outside and the xylem initials to the inside. The VC needs only to produce new initials several times through the entire growing season. This lets the initials do all the heavy lifting, and protects the viability of the VC as it does it. We call the VC and its initials the cambial zone.
I am going to follow the VC and its bifacial activities and deal with the phloem side and the xylem side separately. First the phloem side.
The phloem side initials produce two main types of cells. First, the vertically, axially-aligned cells that are produced by the fusiform initials. They are the phloem sieve tube elements (STE), their companion cells (CC), and the phloem fibers. These pipes and their supporting fibers run in a vertical direction. The second type is the horizontally aligned ray cells and parenchyma cells produced by the phloem side ray initials. These two types of new cells cross and intermingle like the warp and weft of woven cloth to form the three-dimensional tissue to the outside of the VC. The phloem side fusiform and ray initials produce five main types of cells, the STE, the CC, and the fibers, all vertically aligned, and the horizontal rays and parenchyma.
The rays themselves have never gotten the attention they deserve. They are the main conduits that pass all the sugars from the phloem to the VC itself. This movement of sugars continues on deeper inside through the VC and into the xylem parenchyma cells also. Live tissues in the xylem, especially storage parenchyma, receive all the sugars and oxygen they need from these connected rays. The vessels and fibers of the xylem are dead except for the newest formed vessel tissue which hasn’t lost its inner cell contents yet. Most of the vessels and fibers require no energy because at functioning maturity they are dead. Rays are easy to spot in the end of any crosscut branch or trunk section; they are the spokes of the wheel seeming to radiate outwards from the centre.
The xylem side fusiform and ray initials produce all of the new cells, tissue to the inside of the VC, the greater bulk of the trunk. The vertically-aligned cells of the xylem produced by the fusiform initials are the vessels with perforated ends, perforated with tiny holes to allow water flow, the fibers which are reinforced sclerenchyma cells, and the xylem parenchyma cells. Horizontally-aligned cells produced by the xylem ray initials are the xylem rays and parenchyma cells. The fusiform and ray initials of the xylem side produce five main types of cells to form the xylem tissue. Just like the weaving and intermingling of the three-dimensional phloem tissue, the new cells of the xylem side interact to grow the three-dimensional tissue we call wood. The xylem side of the new growth accounts for over 90% of the greater annual growth of the trunk; the phloem side is proportionally less.
Gymnosperms (conifers) do things a little differently, being the older, first modern pattern of how to be a tree trunk. Instead of the vessels that angiosperms (broadleaf trees) exhibit, conifers have water transport cells called tracheids. Longer than vessel elements, they have tapered conical ends that fit well together, strongly overlapping. Tracheid water flow from cell to cell is accomplished by holes called pit pairs. Pits are small holes in the tracheid sidewall and when they line up with the pits of an adjoining tracheid they are called pit pairs. The xylem tissue of conifers consists of three cell types: tracheids, which apart from water conduction function very well as fibers, and parenchyma and rays. On the phloem side of the conifer VC, there are also minor differences from their broadleaf cousins. Phloem tissue of conifers consists of sieve cells and albuminous cells, the conifer style of sieve tube elements and their companion cells that we find in broadleaf trees. Other phloem tissue cells in conifer phloem tissue are rays and parenchyma cells. Simpler in design, a little less flashy, the conifer model is one of endurance, proven by 325 million years of adaptation.
Concurrent with the onset of growth in the spring, the cambium is active and already producing the new necessary xylem and phloem tissues. Early wood, the name for the first part of the expansion of the new growth ring, has especially large vessels and tracheids. Plainly seen in any crosscut wood sample, the larger vessel/tracheid cells in their rings are what we use to count the layered growth rings.
Trees have four separate meristems, all providing necessary tissue growth: the SAM and RAM, providing the primary growth of shoots and roots, the VC described above, and the periderm or cork cambium providing the secondary growth. The periderm is the name for three collected tissues that always grow together in the rings that form the outer bark. They are, from the inside out, the phelloderm, the phellogen, and the phellum or cork. An annually generated tissue, the phellogen, produces all three. Formed anew each year, the phellogen gets its meristematic ability from connected ray parenchyma cells that are genetically programmed to produce the phellogen. The phellogen encircles the cortex to the inside and produces the outer cork, the bark. A new phellogen forms each year from meristematic ray parenchyma cells.
I have studied these matters for decades as a Calgary arborist and Calgary tree doctor. I do it for the betterment of our urban forest, one of the world’s most delicate created biomes.
