#1 or the pith is found in the center of the tree. Generally this only found in very young
trees, as it is crushed with the growth of later xylem layers.
#2 is the first of the two annual growth rings. This oldest ring will in many species be
filled with compounds once it is no longer functional in transporting water (see heartwoodbelow) many years later. Note the close-up in the slide below, which better shows the
difference between the spring and summer wood.
#3 is the second, more recent growth ring, which was recently derived from the vascular
cambium which it abuts.
#4 is an expanded ray phloem section which fills the gap produced when the vascular
cambium was smaller, and dividing off a smaller sections of secondary phloem . See
the close-up below.
#5 includes the remaining cortex which will soon be eliminated as it crushed between
the newer layers of woody xylem and the expanding bark.
Annual Growth rings are made up of vertical xylem vessels and tracheids ( in angiosperms)
and in gymnosperms, tracheids Plus horizontal xylem ray cells for lateral transport.
Ring-porous wood: Early in the season ( spring), trees require lots of moisture to fill out the
vacuoles so leaves can expand and new tissue grow. As the temperature gets warmer,
and less water is available, the diameter of the xylem vessels gets smaller and greaternumbers of inwardly laid secondary walls are formed.
On the top of the image below, note the last of the previous summers growth, and
the new layer of spring xylem being formed. Dendrologists can age trees using this
seasonal change in xylem structure.
In diffuse porous wood (such as finer basswood, maples) little difference exists between
spring and summer wood. To 'read' the rings in these species, you need to stain the cores
with a lignin dye, and then you can differentiate the heavier wall structure of summer wood.
Mixture of large vessels and tracheids
The vascular cambium is the darker layer of cells indicated by the arrow. This meristematic
layer of cells divides inwardly to form new xylem, and outwardly to produce new phloem. The
vascular cells produce a mother xylem and mother phloem, each which divides to produce
new offspring. Note that the ray cells are continuous from the xylem through the vc to the
phloem.
This region of phloem tissue is a complex composite of cells. The creamy pink cells are heavy
duty phloem fiber cells, which help support the thin walled seive tube members/cells which they layer with.
Although alive for the first yea ( see layer of phloem sitting right under
the vascular cambium), unlike the xylem vessels and tracheids which are dead and hollowed to permits
rapid flow, they contain little protoplasm and have no nucleus. Thus the smaller companion cells
associated with each seive cell, expresses its' DNA for the production of necessary proteins/
enzymes needed by both. (you can see these in the second slide below. fibers = 1; seive cells = 2;
companion =4 ). As the phloem usually lasts but one year, the layers of phloem you see between
the fiber layers are now collapsed and nonfunctional.
The expanded v-section of ray parenchyma phloem cells ( #3) fill the gap
which arises as the earlier laid phloem deriving from a then smaller circumferenced vascular
cambium are pushed out with the expanding layers of xylem. They no longer fill this new larger
circumference, and the living ray cells oblige. Depending on tree species, phloem cells may live
just a single year or many; eventually the oldest are crushed along with the original cortex cells
as they pushed into the bark region.
Three cuts of wood... top left is transverse section which shows annual age rings ( note
resin canal in this pine wood sample; remember these pines have only tracheids); radial
cut with rays running parallel with cut of wood and with late wood on left and spring on
right, and on the bottom sample a tangential section with the rays coming out toward you.,
