Carrot Root Question

David Hershey dh321 at excite.com
Thu Jul 29 16:25:00 EST 1999


Here's a quick question for botany students or as a self-quiz for botany
teachers:

The edible part of a carrot storage root is mainly composed of 
A. Dermal tissue
B. Ground tissue
C. Vascular tissue


The reason why this question is interesting is that it caused a lot of
misconceptions and textbook errors in the past and probably still does.
Over half a century ago, a high school teacher (McMenamin 1948) pointed
out that many textbooks incorrectly labeled mature carrot storage root
sections as having cortex. Yet, in the mature carrot storage root, the
cortex and epidermis have been lost due to secondary growth, being
replaced by phloem and a periderm. Thus, the answer to the above
question is C. The bulk of the carrot storage root is secondary phloem.

I think there are probably several reasons for this misconception. One
is that textbook discussions of secondary growth usually focus on stems,
on woody species, and the preponderance of xylem over phloem in woody
stems. The carrot storage root fits none of those.

McMenamin (1948) suggested a demonstration: 

1. Dry large carrot roots until they are easy to bend
2. Cut an inch off the carrot top to exclude any part of stem origin
3. Cut longitudinally to the cambium the full length 
4. Pry the tubular phloem-periderm away from the vascular cylinder in
one piece
5. Notice the four rows of lateral roots radiating from the vascular
cylinder and the holes they left in the phloem

Another interesting carrot project I have done is to take a large carrot
with a good shoot and growth it in solution culture. Small lateral roots
will develop, the shoot will elongate, and it may eventually flower. I
assume flowering is dependent on it having received enough of a cold
treatment while in the refrigerator. 

Reference

McMenamin, Joseph P. 1948. Teaching the carrot root correctly. School
Science and Mathermatics 48: 47-48.


David Hershey
dh321 at excite.com



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