Subject: Simple Question

From: [email-masked] (Holt Kernodle)
Date: Thu, 15 Jun 1995 21:00:35 GMT

I know this question may be quite simple to the super-stud botanists,
but I wanted an answer from those in the know.

What is the difference between the secondary growth of trees, which
increases the circumfrence of their stem/trunk, and the increase of
smaller perennial plants' stalk diameter from seedling
measurements....   to put it more simply: ANY plant realizes an
increase in stalk circumfrence from the time it is a seedling.  What,
then, is the difference between primary and secondary growth?

               ----------====----------
                    Holt Kernodle
                [email-masked]
               ----------====----------

---

From: [email-masked] (Carol Brandt)
Date: 16 Jun 1995 14:51:49 GMT

Good Question!! Here's my shot at an answer without having my botany
textbooks next to me. If you take a cross-section of a stem that has
only primary growth you will see small clusters of cells around the
outer edge of the stem: the vascular bundles.  While cell division
occurs in these bundles producing cells both to the inside and outside,
these cells are not contiguous around the complete circumference. They
can contribute to an increase in stem girth, but my guess is that the
cells in the center of the plant, the cortex, are contributing most of
the bulk by both enlarging and going through cell division. The cortex
is made up of cells called parenchyma that are thin-walled and tender,
thus the tender annual stem.

Secondary growth involves the development of a cambium that is
continuous around the complete circumference of the stem and produces
one tissue to the outside (phloem) and another to the inside (xylem). 
The bulk of the tissue in a stem undergoing secondary growth is xylem
which is heavily lignified or "woody".