Delphinidin

In the early stages of development, the primary tissues of stems of young herbaceous dicots, woody dicots, and conebearing trees are all arranged in a similar fashion. In woody plants, however, obvious differences begin to appear as soon as the vascular cambium and the cork cambium develop.

The most conspicuous differences involve the secondary xylem, or wood, as it is best known. Some tropical trees (e.g., ebony), in which both the vascular cambium and the cork cambium are active all year, produce an ungrained, uniform wood. The wood of most trees, however, is produced seasonally. In trees of temperate climates, virtually all growth takes place during the spring and summer and then ceases until the following spring.

When the vascular cambium of a typical broadleaf tree first becomes active in the spring, it usually produces relatively large vessel elements of secondary xylem; such xylem is referred to as spring wood. As the season progresses, the vascular cambium may produce vessel elements whose diameters become progressively smaller in each succeeding series of cells produced, or there may be fewer vessel elements in proportion to tracheids produced until tracheids (and sometimes fibers) predominate.

The xylem that is produced after the spring wood, and which has smaller or fewer vessel elements and larger numbers of tracheids, is referred to as summer wood. Over a period of years, the result of this type of switch between the early spring and the summer growth is a series of alternating concentric rings of light and dark cells. One year’s growth of xylem is called an annual ring. In conifers, the wood consists mostly of tracheids, with vessels and fibers being absent. Annual rings are still visible, however, since the first tracheids produced in the spring are considerably larger and lighter in color than those produced later in the growing season. Note that an annual ring normally may contain many layers of xylem cells and it is all the layers produced in one growing season that constitute an annual ring—not just the dark layers.

The vascular cambium produces more secondary xylem than it does phloem. Xylem cells also have stronger, more rigid walls than those of phloem cells and are less subject to collapse under tension. As a result, the bulk of a tree trunk consists of annual rings of wood. The annual rings not only indicate the age of the tree (since normally only one is produced each year), but they can also tell something of the climate and other conditions that occurred during the tree’s lifetime. For example, if the rainfall during a particular year is higher than normal, the annual ring for that year will be wider than usual because of increased growth.

Sometimes, caterpillars or locusts will strip the leaves of a tree shortly after they have appeared. This usually results in a narrow annual ring, since very little growth can take place under such conditions. If there is a fire not resulting in the death of the tree, it may be possible to determine when the fire occurred, since the burn scar may appear next to a given ring. The most recent season’s growth is directly next to the vascular cambium, and one need only count the rings back from the cambium to determine the actual year of the fire.

It is not necessary to cut down a tree to determine its age. Instead, botanists and foresters can employ an increment borer to find out how old a woody plant is. This device, which resembles a piece of pipe with a handle on one end, removes a plug of wood from the tree perpendicular to the axis, and the annual rings in the plug can then be counted. The small hole left in the tree can be treated with a disinfectant to prevent disease and covered up without harm to the tree.

A count of annual rings has produced some red faces on at least one occasion. The Hooker Oak, which was named in honor of Sir Joseph Hooker, a famous British botanist who once examined it, was located in the community of Chico, California. Until its demise in 1977, thousands of visitors from all over the world visited the huge oak, which provided enough shade for 9,000 persons on a midsummer’s day. A plaque indicating the tree to be over 1,000 years old was located beneath the tree. A count of rings after its death, however, revealed the Hooker Oak to have been less than 300 years old.

When a tree trunk is examined in transverse, or cross section, fairly obvious lighter streaks or lines can be seen radiating out from the center across the annual rings. These lines, called vascular rays, consist of parenchyma cells that may remain alive for 10 or more years. Their primary function is the lateral conduction of nutrients and water from the stele, through the xylem and phloem, to the cortex, with some cells also storing food. Any part of a ray within the xylem is called a xylem ray, but its extension through the phloem is called a phloem ray. In trees such as basswood (Tilia), some of the phloem rays, when observed in cross section, flare out from a width of two or three cells near the cambium to many cells wide in the part next to the cortex.

In radial section, rays may be from 2 or 3 cells to 50 or more cells deep, but the majority of rays in both xylem and phloem are 1 or 2 cells wide. Ray cells can be seen in cross section if a woody stem is cut or split lengthwise along a ray. Another view of rays (in tangential section) is obtained when the stem is cut at a tangent (i.e., cut lengthwise and off center). As a tree ages, the protoplasts of some of the parenchyma cells that surround the vessels and tracheids grow through the pits in the walls of these conducting cells and balloon out into the cavities. As the protoplasm continues to expand, much of the cavity of the vessel or tracheid becomes filled. Such protrusions, called tyloses (singular: tylosis), prevent further conduction of water and dissolved substances. When this occurs, resins, gums, and tannins begin to accumulate, along with pigments that darken the color of the wood.

This older, darker wood at the center is called heartwood, while the lighter, still-functioning xylem closest to the cambium is called sapwood. Except for giving strength and support, the heartwood is not of much use to the tree since it can no longer conduct materials. A tree may live and function perfectly well after the heartwood has rotted away and left the interior hollow. It is even possible to remove part of the sapwood and other tissues and apparently not affect the tree very much, as has been done with giant trees, such as the coastal redwoods of California, where holes big enough to drive a car through have been cut out without killing the trees.

Sapwood forms at roughly the same rate as heartwood develops, so there is always enough “plumbing” for the vital conducting functions. The relative widths of the two types of wood, however, vary considerably from species to species. For example, in the golden chain tree (a native of Europe and a member of the Legume Family), the sapwood is usually only one or two rings wide, while in several North American trees (e.g., maple, ash, and beech), the sapwood may be many rings wide.

Pines and other cone-bearing trees have xylem that consists primarily of tracheids; no fibers or vessel elements are produced. Since it has no fibers, the wood tends to be softer than that of trees with them and is commonly referred to as softwood, while the wood of woody dicot trees is called hardwood.

In many cone-bearing trees, resin canals are scattered not only through the xylem but throughout other tissues as well. These canals are tubelike and may or may not be branched; they are lined with specialized cells that secrete resin into their cavities. Although resin canals are commonly associated with cone-bearing trees, they are not confined to them. Tropical flowering plants, such as olibanum and myrrh trees, have resin ducts in the bark that produce the soft resins frankincense and myrrh of biblical note.

While the vascular cambium is producing secondary xylem to the inside, it is also producing secondary phloem to the outside. The term bark is usually applied to all the tissues outside the cambium, including the phloem. Some scientists distinguish between the inner bark, consisting of primary and secondary phloem, and the outer bark (periderm), consisting of cork tissue and cork cambium. Despite the presence of fibers, the thin-walled conducting cells of the phloem are not usually able to withstand for many seasons the pressure of thousands of new cells added to their interior, and the older layers become crushed and functionless.

The parenchyma cells of the cortex to the outside of the phloem also function only briefly because they too become crushed or sloughed off. Before they disappear, however, the cork cambium begins its production of cork, and since new xylem and phloem tissues produced by the vascular cambium arise to the inside of the older phloem, the mature bark may consist of alternating layers of crushed phloem and cork.

The younger layers of phloem nearest to the cambium transport, via their sieve tubes, sugars and other substances in solution from the leaves where they are made to various parts of the plant, where they are either stored or used in the process of respiration. This sugar content of the phloem was in the past recognized by native Americans. Some stripped the young phloem and cambium from Douglas fir trees and used the dried strips as food for winter and in emergencies.

Specialized cells or ducts called laticifers are found in about 20 families of herbaceous and woody flowering plants. These cells are most common in the phloem but occur throughout all parts of the plants. The laticifers, which resemble vessels, form extensive branched networks of latex-secreting cells originating from rows of meristematic cells. Unlike vessels, however, the cells remain living and may have many nuclei.

Latex is a thick fluid that is white, yellow, orange, or red in color and consists of gums, proteins, sugars, oils, salts, alkaloidal drugs, enzymes, and other substances. Its function in the plant is not clear, although some believe it aids in closing wounds. Some forms of latex have considerable commercial value. Of these, rubber is the most important. Amazon Indians utilized rubber for making balls and containers hundreds of years before Pará rubber trees were cultivated for their latex. The chicle tree produces a latex used in the making of chewing gum. Several poppies, notably the opium poppy, produce a latex containing important drugs, such as morphine and heroin. Other well-known latex producers include milkweeds, dogbanes, and dandelions.