Bulbs,corms and tubers

Jim McKenney jamesamckenney@verizon.net
Thu, 17 Jan 2013 12:23:39 PST

No other source that I’m aware of, even the wonderful PBS
wiki, speaks to my inner bulbist with the authenticity and congeniality of
Ian’s bulb log.  Over and over Ian has
intuitively published just the images and discussions I want to see to nurture my bulb
But I do have one quibble about his posting on bulbs and
corms. While I was glad to see Ian finally settle into the “erythroniums as
bulbs” camp, I was disappointed to see him place colchicums in the bulb camp. Let’s
see if I can persuade him to rejoin the “colchicums as corms” camp. 

Ian proposed the hypothesis that the colchicum “bulb” is a
single-scale bulb. “Scale” in these contexts usually refers to a modified leaf.  He included a pair of images, one of a
colchicum and one of a group of tulips, which call attention to the seeming
similarity of the two : in each case, the image shows a bulb or bulbs with a
sprout or a stem emerging or attached. At first glance, they certainly do look

But give consideration to what is shown: in the case of the
colchicum, one sees a living sprout emerging from the base of the “bulb” inside  the tunic.  In the case of the tulips, one sees dead
annual stems attached to the exterior of the dormant bulbs (i.e. outside the
tunic). In the case of the colchicum, the structure which is emerging is mostly
the true, perennial stem of the corm, a stem whose distal portion will later
differentiate into annual stem, leaves and maybe flowers and fruit. I believe that the solid
mass which forms at the base of this structure is stem tissue, not leaf tissue.
It’s thus a corm, not a bulb. 

The structures shown in the images are not truly homologous
structures: the dried stems of the tulips correspond only to the structures
which will eventually develop at the distal tip of the colchicum sprouts (the
basal part persisting as the corm). 
Medieval botanists gave colchicum the mistaken sobriquet “filius
ante patrem” (i.e. the father before the son, because seeds appeared in spring
but flowers did not appear until fall).  Ian’s
images similarly confuse the chronology of things: the dry stems attached to
the tulip bulbs are the previous year’s annual stems, whereas the sprout on the
colchicum are of the current season.  
If someone can cite a study of the embryogeny of these
structures in colchicum which demonstrates that the tissues in question are
leaf tissues, then I’m ready to join Ian in the “colchicums as bulbs” camp.  Until then, I’ll be glad to welcome him back into
the corm camp. 

Jim McKenney
Montgomery County, Maryland, USA, USDA zone7
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