|Gyromitra Esculenta, and my li'l rainbow mascot
"If it's hollow, chew and swallow!"
We've all heard/seen this old chestnut in some form or another. The idea being that, if it looks similar to a Morel, you should bisect the mushroom, vertically, to verify that it is, in fact, a Morel.
This one, like many of these old adages dealing with mushrooms, is less-than-adequate and can even be dangerous.
What is wrong with it?
To begin with, very few of these sorts of adages hold up to any sort of scrutiny, and are best left as footnotes of amusing old wives tales.
This particular one is problematic because it doesn't use any external characteristics for identification of the mushroom in question. It just states that a hollow mushroom must be good to eat.
This is, in fact, incorrect.
Specifically in the case of Gyromitra esculenta, one of the acutely toxic "false Morel" species, this is a recipe for a bad time.
G. esculenta also has a hollow stipe; however, ingestion of this species can be acutely toxic, without proper preparation.
How SHOULD we distinguish Morels from other genera?
Morchella spp. all have somewhat similar external morphology.
They all have well-formed, sterile ridges of tissue, that make up the texture of their pilei, and fertile pits between these ridges, in which the spores are produced.
They may have a sinus between the stipe and the "cap" which can extend up to two thirds of the length of the cap, in the case of Morchella populiphila, M. punctipes, and M. semilibera. This does not make them "false", it is just part of their morphology.
They all have more-or-less granulose* stipes, which are generally mostly hollow, though some species may have tissue within the stipe, giving it a chambered appearance. These species can be easily discerned from Gyromitra species with the previously discussed features.
One dichotomous key that can be of use, when in doubt, is at http://www.mushroomexpert.com/morchellaceae
But what about the hollow stem?
Again, a number of different mushrooms have hollow stipes. As far as I can tell, this adage is designed to help discern Morchella punctipes/populiphila from Verpa bohemica.
As Verpa are perfectly edible, when cooked, the use of this one characteristic, to distinguish between the different genera, really only serves to confuse the issue. If your primary focus is food, and you have not experienced any adverse reaction to Verpa spp. or Half-free morels, then there really is no need to be overly concerned about a hollow, or pith-filled stipe. Just enjoy the mushrooms.
See the upcoming entry on Verpa vs. Morchella - The Morel Madness for more information on Verpa bohemica.
*Granulose means covered with small grains or granules. Basically, slightly rough, as if it has sand on it. Almost all Morchella have some granulose texture on their stipe, especially as they age, unless it has been rubbed off.