Anatomy of a Tree: The Vascular System

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By Mark Hall  I loved growing up in the shade of massive, old sugar maple trees, whose mighty branches stretched up to the sky. For many generations, they had stood guard over my parents’ early 19th-century farmhouse and, on countless occasions, had withstood the harshest elements. They seemed more like gigantic statues than living things, ever-changing and growing. Even today, as I study the anatomy of a tree, I am amazed by how much takes place inside a tree, given its dense, rigid nature.  

From our exterior vantage point, we may be tempted to think that very little is happening within a tree. It is wood, after all — hard, thick, unyielding, and securely locked into the ground by its roots. The derogatory expression of one’s lack of intelligence with terms such as “blockhead” and the description of one’s stiff, awkward character as “wooden” only further enhance this false impression of limited activity inside trees.  

Surprisingly, a vast degree of commotion occurs beneath a tree’s hard, protective bark. An intricate labyrinth of machinery, known as the vascular system, is busily working there. It is a large, complex web of tissues that transports water, nutrients, and other support materials throughout the plant.  

This fascinating network is comprised of two main vascular tissues. One of them, phloem, is located on the inside layer of the bark. During photosynthesis, leaves use sunlight, carbon dioxide, and water to produce sugars called photosynthates. Although these sugars are produced only in the leaves, they are needed for energy throughout the tree, particularly in areas of active growth such as new shoots, roots, and maturing seeds. The phloem transports these sugars and water up and down and throughout the tree in separate perforated tubes.   

This movement of sugars, called translocation, is thought to be accomplished partially by pressure gradients that pull the sugars from an area of lower concentration to an area of a higher concentration and partially by cells within the tree actively pumping sugars into areas where they are needed. Although this might sound quite simple on paper, these processes are amazingly complex, and scientists still have many questions despite extensive research on this topic.  

Sugars are also transported for storage purposes. The tree relies on its availability each spring when energy is needed to produce new leaves before the tree can resume photosynthesis. Storage locations can be found in all different parts of the tree, depending on the season and the tree’s growth phase.  

The other major vascular tissue inside trees is the xylem, which primarily transports water and dissolved minerals throughout the tree. Despite the downward force of gravity, trees manage to pull nutrients and water up from the roots, sometimes up hundreds of feet, to the topmost branches. Again, the processes that accomplish this are not entirely understood, but scientists think that transpiration has a role in this movement. Transpiration is the release of oxygen in the form of water vapor through tiny pores, or stomata, present in the leaves. This tension creation is unlike sucking a liquid through a straw, pulling water and minerals up through the xylem.  

Particular xylem provides an intensely sweet breakfast topping that many people, including yours truly, consider essential. Maple trees are tapped in late winter or early spring to collect sugary sap from the xylem. Once boiled down, the thick, sticky solution becomes the delicious maple syrup that covers our pancakes, waffles, and French toast. Although phloem usually moves sugars, xylem transports those stored during the previous growing season. This provides the tree with the energy it needs after a dormant winter, and it provides us with maple syrup!  

A tree’s vascular system is complicated, and researchers still have many questions about exactly how and why it functions.

As trees grow, phloem and xylem expand, thanks to groups of actively dividing cells called meristems. Apical meristems are found at the tips of developing shoots and roots and are responsible for their extension, while the vascular cambium, another type of meristem, is responsible for the increase in the tree’s girth.   

The vascular cambium is located between the xylem and phloem. It produces secondary xylem toward the pith, at the tree’s center, and secondary phloem outwards, toward the bark. The new growth in these two vascular tissues enlarges the tree’s circumference. The new xylem, or secondary xylem, begins to surround the old or primary xylem. Once the primary xylem is completely enclosed, the cells expire and no longer transport water or dissolved minerals. Afterward, the dead cells only serve in a structural capacity, adding yet another layer to the strong, rigid heartwood of the tree. Meanwhile, the water and mineral transport continues in the newer layers of the xylem, called the sapwood.  

This growth cycle repeats every year and is recorded naturally inside the tree. Close examination of a cross-cut trunk or branch section is revealing. Not only can its age be determined by counting the annual xylem rings, but the varied distances between rings can recognize differences in yearly growth. A warm, wet year may allow better growth and display a wider ring. A narrow ring may indicate a cold, dry year or inhibited growth from disease or pests.  

A tree’s vascular system is complicated, and researchers still have many questions about exactly how and why it functions. As we continue to study our world, we increasingly discover fantastic complexity, with a myriad of perfectly placed pieces working together to answer some need or perform some function. Who “wood” have known?!  

Resources  

• Petruzzello, M. (2015). Xylem: Plant Tissue. Retrieved May 15, 2022 from Britannica: https://www.britannica.com/science/xylem  
• Porter, T. (2006). Wood Identification and Use. Guild of Master Craftsman Publications Ltd.  
• Turgeon, R. Translocation. Retrieved May 15, 2022 from Biology Reference: www.biologyreference.com/Ta-Va/Translocation.html  

MARK M. HALL lives with his wife, their three daughters, and numerous pets on a four-acre slice of paradise in rural Ohio. Mark is a veteran small-scale chicken farmer and an avid observer of nature. As a freelance writer, he endeavors to share his life experiences in a manner that is both informative and entertaining. 

Originally published in the November/December 2022 issue of Countryside and Small Stock Journal and regularly vetted for accuracy.