Roots are the Rodney Dangerfields of Science. They don’t get much respect! However, you know how Iimportant these are, in terms of individual plants and ecosystems. I have often looked at trees that grow on sheer cliffs in the Rockies and here with wonder. How can they possibly grow there without being lost to gravity? I got a partial answer one day when I noticed a fairly large shrub clinging to a sheer cliff- face at the Pali lookout. It had thick roots which doubtless had secondary growth. These were splayed over the rock face and gave support and anchorage. Smaller roots probably penetrate cracks in the cliff where they anchor the plant as well as absorb nutrients. Mist usually hovers around this cliff. This probably allows this plant to grow in an otherwise barren environment. However, the roots play an amazing and essential role in the survival of this plant.

We have studied secondary growth in stems first because of the great importance of woody stems and because of their familiarity. You will discover that identical processes occur in roots. There is one important thing to keep in mind as you consider roots. Remember that the procambium forms a solid core of vascular tissue in roots. Thus a pith is generally NOT PRESENT and xylem occupies the center of the root. There are exceptions, of course, but we will concentrate on the most central examples before we venture into the variations.

The Stele of a Typical Dicot Root at the end of
Primary Growth

3-D Representation of a root at the end of
Primary Growth

If Secondary Growth Occurs

The Vascular Cambium remains active between the Phloem & Xylem.

It becomes Continuous because cells of the Pericycle are recruited.

The Vascular Cambium operates the same way as it does in Stems.

The Secondary Xylem in the Roots of herbaceous plants usually contains a lot of Parenchyma while that in trees resembles wood.

Medicago (alfalfa) Root.

Secondary xylem consists largely of parenchyma. During secondary growth, cell divisions have occurred in parenchyma cells of the secondary xylem and primary xylem to such an extent that the primary xylem has become somewhat disorganized, and its original position is hard to determine. Fibers are present in both xylem and phloem.

Center of an Alfalfa Root with Secondary Growth.

Secondary Phloem in fleshy roots usually has lots of Parenchyma but it may resemble Phloem in the Stem

Secondary Phloem of Papaya Root stained
with Toluidine Blue.

Outer Part of a Papaya Root
Locate the Periderm!!!

Secondary xylem in the Roots of woody plants can resemble that formed in stems.

Top