Dichotomous key (IV) - Pith

Pith is one of my favorite things for some reason. I think its substance remained elusive to me for a long time - I couldn't seem to track down a good answer as to what it was and why it looks the way it does for different species. Over the years, primarily through carving, I've become much more acquainted with pith. Pith is essentially the undifferentiated, highly moldable ingredient of new growth. Also called medulla (which means middle), it also stores starches and water.

Classification of plant cells narrowing down to pith (I didn't subdivide alternate branches of the classification)

• Vascular tissue (transport water, nutrients)
• Dermal tissue (protection, prevent water loss)
• Ground tissue (photosynthesis, food storage, growth/regeneration, structure, protection)
• Sclerenchyma (main structural support of plants)
• Collenchyma (structural, provides extra strength in areas of new growth - think, the long fibrous strands in celery)
• Parenchyma (play a large role in synthesized food and water storage)
• Chlorenchyma (photosynthesis, home of chloroplasts)
• Aerenchyma (large intercellular spaces that hold air for buoyancy and respiration)
• Prosenchyma, or Palisade parenchyma
• Vascular parenchyma (forms either the xylem or phloem)
• Medullary or ray parenchyma
• Conjunctive parenchyma (root parenchyma)
• PITH!

Color
Pith changes in a variety of ways over time, the most obvious of which is color. Most pith starts as whitish in color and changes as it dies and ages to a yellowish or brown. The above photo shows this years growth of staghorn sumac on the left. Note the single growth ring of white adjacent to the egg yolk yellow spongy pith. On the right, below the bundle scars from the previous year, is a two year old section of the branch showing the darker brown aged pith. Note here the white ring of sap wood and the previous year of woody growth in the single ring of yellowish wood.

Turtle-like pattern of poplars

Cottonwood showing fluted ridges that correspond to the vascular bundles connecting to the leaves

Shape
Pith doesn't have a ton of variability in terms of shape in cross-section, so this isn't often a helpful tool for identifying the species. But some are beautifully non-circular. The Populus genus has five-pointed stars in cross section. The above photo shows a cottonwood (Populus tremuloides) twig in cross-section. The five-pointedness of the pith on cottonwoods corresponds quite nicely to the five ridges running parallel lines on outside of the twig.

Chambered pith of white walnut - or butternut (Juglans cinerea) above and black walnut (Juglans nigra) below

Texture
The walnuts have perhaps my favorite pith. Above is both black and white walnut pith side by side. For others the pith disintegrates completely (one way to distinguish between native and non-native honeysuckles is to crack open a twig; non-native honeysuckles are hollow, as shown in the photo at the top of the posting).

From top to bottom, Staghorn sumac, catalpa, elderberry, white ash

Size
Some species have fantastically huge pith. Particularly great examples of these include elderberry, ash, staghorn sumac, catalpa, boxelder are among these species. Other species have nearly invisible pith.