Growth

V. Thapsus is a biennial. The plant grows its leaves out wide and the roots thick and deep the first year. The second year the plant uses the stored energy from the previous year to grow tall and produce flowers and thereby seeds.

The plant has no secondary growth.
There are no growth rings because the leaves (Which makes up the entirety of the above ground portion of the plant) die with the frost, and is completely regrown each spring. That is why the plant stores it's provision of energy where the frost cannot steal it - in the roots. No newly grown foliage lasts to the next year.

Because of this growth pattern and looking at the picture I posted on the last blog, that the plant has a basal meristem that exists just below/at ground level that produces the next years foliage.

First year growth:

http://4.bp.blogspot.com/-jR679MFCd8w/TbIuG8VI4pI/AAAAAAAADlI/AuEenOIlIVI/s1600/mullein%2B1.JPG

Second year growth:


http://www.ifa.hawaii.edu/info/vis/uploads/images/flora_fauna/mullein.jpg

I could not find any information on secondary growth in the roots, or if the roots die after the two years.