This section is from the book "Our Edible Toadstools and Mushrooms and How to Distinguish Them", by W. Hamilton Gibson. Also available from Amazon: Our Edible Toadstools And Mushrooms And How To Distinguish Them.
A fungus may be poisonous in various ways:
1. A distinct and certain deadly poison.
2. The cause of violent digestive or other functional disturbance, but not necessarily fatal.
3. The occasion of more or less serious physical derangement through mere indigestibility.
4. Productive of similar disorders through the employment of decayed or wormy specimens of perfectly esculent species.
5. These same esculent species, even in their fresh condition, may become highly noxious by contact or confinement with specimens of the Amanita by the absorption of its volatile poison, as further described on p. 69.
And lastly comes the question of idiosyncrasy, a consideration which is of course not taken into account in our recommendation of certain well-established food varieties.
"One man's food another man's poison." The scent of the rose is sometimes a serious affliction, and even the delicious strawberry has repeatedly proven a poison. Even the most wholesome mushroom will occasionally require to be discriminated against, as certain individuals find it necessary to exclude cabbage, milk, onions, and other common food from their diet. When we reflect, moreover, that in its essential chemical affinities the fungus simulates animal flesh, and many of the larger and more solid varieties are similarly subject to speedy decomposition, it is obviously important that all fungi procured for the table should be collected in their prime, and prepared and served as quickly as possible. More than one case of supposed mushroom poisoning could be directly traced to carelessness in this regard, when the species themselves, in their proper condition, had been perfectly wholesome.
There can be no general rule laid down for the discrimination of an edible fungus. Each must be learned as a species, or at least familiarized as a kind, even as we learn to recognize certain flowers, trees, or birds. Within a certain range this discrimination is practised by the merest child. How are the robin, the chippy, and the swallow recognized, or the red clover, and white clover, and yellow clover ? Even in the instances of species which bear a very close outward similarity, how simple, after all, does the distinction become. Here, for instance, is the wild-lettuce, and its mimic, the mulgedium, growing side by side - to ninety-nine out of a hundred observers absolutely alike, and apparently the same species. But how readily are they distinguished, I will not say by the botanist merely, but by any one who will take the small pains of contrasting their specific botanical characters - perfectly infallible, no matter how various the masquerade of their foliage. The lettuce has yellow blossoms, and a seed prolonged into a long beak, to whose tip the feathery pappus is attached. The mulgedium has dull bluish flowers, and its pappus is attached to the seed by a hardly perceptible elongation. As with the birds and wild-flowers, so with the fungi: we must learn them as species, even as we learn to distinguish the difference between the trefoil of the clover and that of the wood-sorrel, or between the innocuous wild-carrot and the poison-hemlock, the harmless stag-horn sumach and its venomous congener, the Rhus venenata. There are parallel outward resemblances between esculent and poisonous fungi, but each possesses otherwise its own special features by which it may be identified - variations of gills, pores, spores, taste, odor, color, juice, consistency of pulp, method of decay, etc.