[Arisaema-l] Arisaema root structure

[Peter Taggart]

It is a pity that this web page does not have a vertical cross section of a
corm.
The definitions, which I learnt as a toddler helping to weed in the garden,
from my father who lectured on this subject, may well have been muddled by
my young memory or over simplified in order for me to understand them. If
there is someone better versed in plant anatomy than I, I should be glad of
corrections both for my own interest and to keep public information correct,
they are as follows:

A corm
is a stem surrounded by annular, compressed, modified leaves. it has a corm
tunic, and a top and bottom with the growing tip and roots arranged
respectively. The stem may produce axillary side shoots (or roots) from
between the compressed modified leaves as is common in Crocosmias. [A bulb
cannot do this as it has only a basal plate (which may produce offsets or
stolons)] A corm is dependent on the growing conditions when the  shoot(s)
emerge in order to flower, though a large corm may have sufficient
resources to flower without good growing conditions. However the flower bud
is not created in advance, -an important difference from the flower of a
bulb.

A rhizome
is a swollen stem (ie not a stem surrounded by modified leaves - the stem
its self is the swollen bit).  The rhizome has a growing point at it's
apex, It may also branch side ways and produces roots -usually around the
growing point(s) as in many Iris (Louisiana, Bearded,... etc)

A tuber
is a swollen root which produces stem shoots, like a potato and may also
produce feeder roots as in the tuberous begonias

the difference between a tuber and a rhizome is that one is a swollen root
and the other is a swollen stem.
A bulb and a corm are also types of stem -both surrounded by modified
leaves -unlike a rhizome or tuber.

A bulb
is a compressed stem (basal plate) with roots at it's outer edge and scales
(modified leaves) sitting on top of it. Think of an onion sliced
vertically. The flower bud and growth shoot producing true leaves develops
during the 'dormant' period of the bulb's growth cycle  and matures during
a slow steady period of growth. Hence the recipes for a growing tulips etc
consisting of a hot dry baking ( definite summer dormancy), followed by a
long cold period (extended  Autome / Spring, causing the flowering shoot to
spend longer developing using the resources within the bulb, before being
planted and allowed to emerge as a shoot with a huge flower and little bulb
left to grow on for the following year.

If the 'lump' of an Arisaema is truly a stem, it can only be a corm (or
bulb) if the bulky tissue is modified leaf material surrounding a central
core of 'stem' material.
If the 'lump' is root then it is a form of tuber.

It appears from Tony Avants article that Arisaema's perennial storage
organs are intermediate between root and stem material, and no one has
suggested the presence of any modified leaves, which suggests to me that
both terms 'pseudo tuber' and 'pseudo rhizome' are appropriate. Perhaps
they might be shortened to rhizome or tuber. - I would favour rhizome based
on my understanding of the anatomy.

However the terms 'bulb' and corm 'are' not appropriate.

Peter (UK)




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