Figure \(\PageIndex{1}\): A cross section of a pine stem that is approximately one year old. The epidermis may still be present just outside the layer of periderm that has formed, but will soon slough off. Large (and small) resin canals are present in the cortex region. Just inside the cortex, a layer of more densely packed cells is the secondary phloem. This is separated from the secondary xylem by a thin layer of meristematic tissue: the vascular cambium. The secondary xylem, stained pink, has small blue-stained resin canals traversing it. Some of the cells in the xylem look darker than others. Within this layer of secondary xylem, at the center of the stem, are the large parenchyma cells of the pith.Photo by Maria Morrow, CC-BY 4.0.Figure \(\PageIndex{2}\): A cross section of a pine stem that is 4 years old. The epidermis is gone and several layers of periderm (outer bark) have formed around the outside of the stem. Tears in the periderm form lenticels. Inside the periderm is cortex with embedded resin canals. The secondary phloem is a thin ring of tissue surrounding the secondary xylem. Within the secondary xylem, several growth rings can be distinguished, with regions of more densely packed cells formed in the dry and/or cold season abutting larger cells formed in the following growing season. Photo by Maria Morrow, CC-BY 4.0.
Vascular Tissue
Figure \(\PageIndex{3}\): A close up on the annual growth rings formed in a pine stem. Within the secondary xylem, regions of more densely packed cells formed in the dry and/or cold season are called late wood. These alternate with rings of larger cells formed in the following growing season, called early wood. An annual growth ring is composed of the early wood and late wood formed in a single year. This stem has at least 3 annual growth rings.Photo by Maria Morrow, CC-BY 4.0.Figure \(\PageIndex{4}\): A magnified view of the vascular tissue and, specifically, the vascular cambium. The top of the image is composed of cortex cells. Quite abruptly, the cells become smaller and more tightly packed, appearing in columns. This is the secondary phloem tissue. The vascular cambium is below the phloem cells and appears as a line of flattened, light blue-stained cells. Just below the vascular cambium, the secondary xylem is composed of tracheids with red-stained secondary walls. Lines of parenchyma traverse the phloem (phloem rays) and xylem (xylem rays). Photo by Maria Morrow, CC-BY 4.0.
Periderm
Figure \(\PageIndex{5}\): A magnified view of the periderm (outer bark). The top of the image is composed of cork cells, which are empty and squished together. They are also coated in suberin. The cork cambium can be distinguished as one to two lines of small cells, all with visible nuclei. Just below the cork cambium, large cells that are filled with stained contents make up the phelloderm. These three layers comprise one periderm layer. Image from the Public Domain, sourced from Berkshire Community College Bioscience Image Library with labels added by Maria Morrow.
Wood
Figure \(\PageIndex{6}\): A driftwood stump found at the beach, with annual growth rings indicated by black arrows. This tree may have died from a fungal infection (Leptographium wageneri) that stained the wood. A black stain is present in the most densely packed cells of the late wood, making annual growth rings visible. At least 11 years of growth can be seen, potentially more.Photo by Maria Morrow, CC-BY 4.0.
Figure \(\PageIndex{7}\): The difference between sapwood and heartwood can sometimes be seen distinctly in where moss and algae grow on cut logs. These two logs both have algal growth (and moss, in the second image) in the sapwood region. Yet, it cuts off abruptly as it reaches the heartwood. What might cause this? Photos by Maria Morrow, CC-BY 4.0.
Angiosperms
Figure \(\PageIndex{8}\): A section through an oak tree (Quercus). The outer bark is darker and jagged (A). Just under the outer bark is a thin layer of inner bark (C). B indicates the transitional zone between inner and outer bark. D and E are the wood, but D is still active in water conduction (sapwood). The vascular cambium would be located between C and D. A=Periderms (outer bark), C=Secondary phloem (inner bark), D=Sapwood (active xylem), E=Heartwood (inactive xylem). D+E=wood. Photo by Rbreidbrown, CC BY-SA 4.0, via Wikimedia Commons with labels added by Maria Morrow.Figure \(\PageIndex{9}\): The arrangement of tissues in an angiosperm in secondary growth is similar to a gymnosperm. One way to determine whether you are looking at an angiosperm or gymnosperm cross section is to look for the presence of large, open cells in the xylem (vessel elements). Gymnosperms (excepting the gnetophytes) only have tracheids, whereas angiosperms have both tracheids and vessel elements. Can you tell how old this stem was by looking at the secondary xylem? Photo by Maria Morrow, CC-BY 4.0.Figure \(\PageIndex{10}\): A cross section of the vascular tissue in a two year old oak stem. The cortex is separated from the secondary xylem by large bundles of phloem fibers. Below the phloem fibers are the conducting cells of the secondary phloem. The vascular cambium, indicated by the white arrow, separates the phloem from the xylem. In the xylem, there are two growth rings. A small bit of the pith can be distinguished at the bottom.Photo by Maria Morrow, CC-BY 4.0.Figure \(\PageIndex{11}\): A lenticel in the periderm of an oak. The white arrow indicates a tear in the periderm, called a lenticel, that allows for gas exchange after the epidermis is lost. A thin layer of epidermis, coated in cuticle (stained pink), can be seen hugging the edge of the periderm. This will soon slough off. A lighter strip within the periderm cells (all stained an orange-brown) is the cork cambium. Image from the Public Domain, sourced from Berkshire Community College Bioscience Image Library with labels added by Maria Morrow.
