Gastroid and Truffle Fungi
Gastroid means stomach-like (as in Gastro-intestinal). These mushrooms are recognized by being closed up, often like a little ball, and sometimes staying underground. Or, they may resemble a mutated regular mushroom that has only partially closed, with primitive remnants of a cap and stem still visible. As you may have noticed, the more "truffle-like" a mushroom evolves to be, from secotioid, to spherical with a columella, to a uniform interior, the more likely it is to be found underground. Earthballs (Scleroderma) are an exception, and puffballs and earthstars evolved holes (pores) to be able to continue to forcibly eject their spores from above ground.
We used to think that these represented some of the oldest species around, before the current shape of a mushroom fully evolved, but closer examination showed that all of the puffballs and truffles were not related to each other. In fact, many different "regular" mushrooms seemed to have a truffle-like thing related to them and we came to realize that many branches of the mushroom tree of life have evolved a gastroid relative independently. Most interestingly, the gastroid forms are often the newest species, meaning that typically, regular mushrooms are evolving into truffles, not the other way around.
Here's what might be happening, with a little bit of personification. When conditions get dry or cold, a mushroom is afraid to open up and let the air dry out the gills or pores to the point that the spores can no longer be discharged (spores don't just fall off of the gills, they are actively launched in a process that requires moisture as explained here). Eventually, they evolve to never open up at all and remain as little secotioid mushroom buttons their whole lives, being eaten by animals in order to get their spores dispersed, and the gills and pores degrade into a wavy mess. Since they don't need to launch spores anymore, they don't need a stem to lift them off the ground, so they lose their stem and get more and more ball-shaped, like puffballs and earthballs (above ground). If they evolve strong odors at the same time, they don't even need to pop out of the ground, but can stay buried (truffles) and squirrels and voles will find and eat them. If it weren't for squirrels and other animals, many truffles might go extinct. They rely on being found by their strong odor, dug up and eaten, and when the squirrel poops somewhere else in the woods, the spores are spread. The California Red-backed vole is said to eat its weight in truffles every day. I'm jealous. These fungi are way more common than anybody suspected, we just can't see them because they're buried.
While every truffle smells tasty to a squirrel, only a few are tasty to humans (we like the garlic and onion ones, and not the compost and toilet smelling ones, for instance). So only a very few of the truffles you will find will be the expensive edible ones. Usually, only mushrooms that grow out of the ground can fully evolve into truffles (which represents a more recent evolutionary state). Those living and growing on decaying logs (the primitive state) don't have that option. That also explains why regular mushrooms evolved into truffles and not the other way around. You had to be somewhat evolved into an ecology to allow yourself to grow from the ground in order to be capable of evolving into a truffle. The true primitive forms of mushrooms, crusts and jellies, grow on wood. Since terrestrial mushrooms sometimes have mycorrhizal relationships with trees, many truffle species are also mycorrhizal and cannot be grown in captivity, as I explain in my page on ecology. They must be hunted and found in the wild, which is very difficult as they are hidden! Historically, pigs were used, because some truffles give off the same odors as pig pheromones, but that can be dangerous as the pig wants to eat the truffle and will bite your hand off to get at it. Raking for truffles or digging randomly can do severe damage to the forest and finds mostly immature truffles that haven't developed their odors yet and are not very good anyway. The best way is to train a dog to find them for you. It turns out dogs can detect truffles from as far away as 150 feet, but they aren't interested in them, so they never say anything. If you can teach your dog that you will trade it a milk bone for a truffle, she will gladly find a nice ripe one for you. Another possibility is to stalk a squirrel for a while and watch wherever they are digging, or even better yet, learn to spot the signs of where a squirrel has been digging and look carefully near the holes.
If you want to know if a given gastroid mushroom is mycorrhizal or saprophytic, you should look at its closest "normal" relative and assume that the ecology is probably the same.
You might also find a few things that look like gastroid fungi, but really aren't. These are "regular" mushrooms that almost look like they are hatching out of an egg, and when they grow up, they will not be gastroid.
Choose from below: here are the various categories of what a gastroid mushroom might look like at various stages of evolution. (I show the background colour of each section to help you not get lost).
Puffball - soft, uniformly marshmallow like inside, turning to dust, may have a base of some sort. May have a pore or slit through which it ejects its spores from above the ground.
Stalked puffball - a puffball on a stick. Only the top half of the exterior of the puffball remains on this stick, the rest has fallen away.
Earthball - like a puffball, but yellow-brown, tougher, quickly turning purple inside and not turning to dust. No pore.
Earthstar - like one ball inside another where the outer sphere has opened up with starfish-like rays to show the earthball inside.
False truffle - at one stage of evolution, they have the remains of a primitive stem (columella) inside and are often found partially underground. Some of those found underground will be uniformly spongy and porous, or solid but gelatinous inside. If the rind is very thick and it is uniformly gooey inside with no columella, it is a true truffle. Some odd false truffles are neither Basidios nor Ascos!
True truffle - unrelated to any other mushroom on this page, these belong to the phylum Ascomycota. They are uniformly solid and marbled inside, or empty or chambered. Occasionally may be gooey inside with a thick rind. Found underground. This group contains the most famous tasty truffles.
Puffballs, earthballs and earthstars - These are uniform inside and grow above the ground so they are easier to find than truffles and more common than secotioid fungi, so they are better understood.
Puffballs are like marshmallows but they can be almost any size and are usually soft and squishy and edible when young and pure white inside. Then the white changes colour and turns to goo, and sometimes powder. In some cases, a hole opens up in the top so the powder can "puff" out when the rain falls (or somebody stomps on them for fun). All of these are related to each other, and they are all closely related to the genus that includes the store bought button mushroom, Agaricus. Now, you can click back and forth all day between this page and the Agaricus page, and it still won't make any sense. It's definitely one of those results that makes you say "Could you please go and DNA sequence those mushrooms again? Just to make sure?"
Lycoperdon - these are smallish (around 5cm) puffballs that usually puff smoke through a pore at maturity. They are thin-skinned and not perfectly spherical, but with a sterile base of some kind made of material that doesn't turn to dust like the rest of them. Lycoperdon is Latin for "wolf fart", so you know some mycologists have a sense of humour. Many have brown warty spores.
Apioperdon (Lycoperdon) pyriforme - the only one to grow on wood or buried wood, but that may not be obvious. Not spiny. Pear shaped, with distinct white base flesh that doesn't turn colour, and white rhizomorphs.
L. perlatum - white or black spines that leave a scar when removed (unlike L pratense). Darkens as it ages, but the stem is always more pronounced than in other Lycoperdons.
L. curtisii - beautiful long recurved spines.
L. nigrescens (foetidum) - black spines that leave a scar when removed when young, but never the pronounced base that L. perlatum has when it gets old and dark. Almost spherical.
L. umbrinum - smaller spines that don't leave a scar when removed. Like L. pyriforme but more spiny and not associated with wood. Dark spines surrounded by a yellowish base colour.
L. molle - not usually yellowish between the spines, red-brown interior at maturity with less of an olive tint?
Bovista - golf ball sized or bigger, smooth, white spherical puffballs with an orange peel like thick outer skin that peels off to reveal a thinner inner skin that eventually turns coppery-lead. Calvatia is either bigger or doesn't have the same kind of double skin. They do not puff, they just split open and the spores can make their way out that way.
B. pila - here you can see the one on the right has had the outer skin fall off. This species has a small cord on the bottom. <10cm
Calvatia - the giant puffballs, although not all of them are huge (most are from 10-15cm). They are mostly round (but sometimes anvil shaped), and mostly with thicker skin and bigger than Lycoperdon, but they lack the lead coloured double skin of Bovista. Like Lycoperdon, some have brown warty spores, and some of them, although matching the description of Calvatia, may actually belong to Lycoperdon.
C. booniana - the largest of the puffballs, up to 60cm or more, with white scales on the surface.
C. gigantea is equally large, but completely smooth.
C. fumosa - much like Bovista pila, but this puffball stinks like an outhouse when old, and the thick skin doesn't break away like Bovista. Probably a Lycoperdon.
Lycoperdon subcretaceum (Calvatia subcretacea) - golf ball sized or larger with thick skin like C. fumosa but with black scales and no tail (and no odor).
Calvatia subsculpta - baseball sized, with more prominent warts than C. booniana. Probably a Lycoperdon.
Stalked puffballs are also related to Agaricus and are basically a small puffball (<5cm) on a stick, although sometimes the bottom half of the puffball breaks away and only leaves an empty upper half. They are all rare and prefer desert conditions, where they likely evolved. They also have sometimes coloured spiny spores.
Tulostoma spp. - there are many species here that all resemble a round Lycoperdon on a stick, with a small hole where the spores can puff out. The ball fits on the stem like a ball-in-socket joint.
Battarrea phalloides grp - with a volva at the bottom of the stem (that it "hatched" out of), and a spore case that splits in half, so you may only find the top half of the case (as in this photo) or just the stick.
Chlamydopus meyenianus - also with a volva at the bottom of the stem, but with an intact ball on top that doesn't break away or fall off but opens a pore.
Earthballs (Scleroderma - Boletales) are related to the boletes. They feel much tougher than the puffballs, are usually yellow-brown, and the white inside quickly turns into a brilliant purple instead of olive brown. Long considered poisonous, they are rumoured to be edible if you can find one young enough to not be purple yet, but that is unusual. They are differentiated by the thickness of the skin and the type of scales. They are usually <5cm, and have spiny spores under the microscope.
S. cepa grp (
S. citrinum - erect scales that stick out, very thick skin, and can be quite large - the rest are golf ball sized but this one can be baseball sized.
Pisolithus arhizus (tinctorius) - odd shape, large, related and with amazing purple patterns inside.
Earthstars - Geastrum (Geastrales) - the outer skin
opens like starfish rays to reveal an earthball inside. Sometimes the rays
fold back and lift the ball up on a pedestal! A few are hygroscopic - capable
of closing again if it dries up to save itself for a
wetter period when it will re-open! Geastrum have the distinction of not
being closely related to anything else, but being distinct enough to be
considered in their own order by some mycologists, unlike almost every other
mushroom on this page which is considered to be closely related to some other
more "normal" mushroom. (They are closest to
A few species of Astraeus (Boletales) are related to the boletes, and evolved this interesting behaviour independently from Geastrum. They are also hygroscopic (able to close and re-open when the humidity changes).
Note that Scleroderma polyrhizum and floridanum, earthballs, can open up but do not have anything but spores inside. All are uncommon to rare usually preferring dryer climates.
G. saccatum - 5cm across when open.
G. rufescens - short stalk, pinkish hues on inside of rays.
G. pectinatum - not hygroscopic, strongly striate "beak" and base around the short stalk.
G. quadrifidum - smaller, four or so rays form a pedestal, and the puffball is on a short stalk. There's a definite ring around the "beak".
G. fornicatum - similar, more rays that rest on a cup, no stalk. No ring around beak.
If you are lucky enough to find an unopened earthstar, (or one that lost its rays) this is what it looks like. Because of the large size, this is likely G. triplex.
Secotioid and Truffle fungi
There are many species of secotioid and truffle-like fungi, some of which look the same, and many have probably not even been discovered yet! So I am only going to talk about the different genera and may not always mention with my usual thoroughness every species commonly found here. The generic names of these fungi can be very confusing because they originally were given new genera, but we have since learned that they are closely related to different existing "normal" fungi, and should therefore have the same genus name. To the human eye they may seem quite different, but that's only one point of view. You'll see that some of these have (recently) been renamed and given the same genus as the mushrooms they are related to, but some of them are still in the pipeline to be renamed and currently retain their odd unique genus. Click on the genus name to see the "ordinary" mushrooms they are related to.
Remember, there are more species than just the ones I'm showing here, and it is good fortune to run across any of them!
Gastroid Agarics - gilled mushrooms that have closed up, with oddly formed gills that you usually can't see until you slice the mushroom in half. (Sometimes hard to tell apart from the gastroid boletes that come next). They are still found above ground, but mostly in the desert climates where the evolutionary pressure was placed on them. If you don't recognize the shape of a particular mushroom, you may have to rely on the "gill" colour at maturity (after the spores have matured and changed the colour of the young gills) to help identify the mushroom. Don't bother to try and take a spore print, though. Most have lost their ability to drop spores. All of these except for Cortinarius pinguis are quite rare. They are close in size to the "normal" mushrooms in their genera.
Agaricus inapertus - dark chocolate spores, a gastroid Agaricus albolutescens. Was Endoptychum.
Agaricus deserticola (Longula texensis) - thick stem! Southern species.
Chroogomphus albipes - greyish black spores when mature. Was Brauniellula.
C. loculatus - more colourful cap? (darkening in iodine), stronly interveined to pore-like gills.
Lactarius silviae (Gastrolactarius camphoratus) - a secotioid candy cap that needs a new name. Produces milk!
L. crassus/lactarioides - paler, odorless southern species.
Leratiomyces cucullatus - dark brown spores, was Weraroa.
Pholiota nubigena - a secotioid Pholiota, unusual for growing on wood. Brown spores. Was Nivatogastrium.
Podaxis pistillaris - a gastroid Coprinus.
Montagnea arenaria - another gastroid Coprinus, with short cap and volva, gills exposed by breakdown of cap.
Russula (Macowanites) subloculata - pale spores, another (M. chlorinosmus) smells like bleach. Brittle flesh like Russula.
C. magnivelatus - huge cap, thick, white elastic veil that hardly ever breaks. Partially underground but not misshapen.
C. wiebeae - similar, gills browner.
C. saxamontanus - similar but yellow, yellow veil
C. verrucisporus - yellow, shorter yellow stem
C. bigelowii - yellow cap, thin white veil, shorter stem. (See Cortinarius).
Gastroid Boletes - fleshy pored mushrooms that have closed up, with oddly formed pores. It can be hard to tell if a secotioid mushroom had gills or pores because they are so contorted! Smaller than their "normal" counterpart. Another gastroid bolete is found under the false truffle section. They are found partially underground, usually <10cm. Originally all called Gastroboletus, they are being moved into the following proper genera:
Boletus subalpinus - entirely pale brown when young, pores yellow- to olive-brown. Related to Boletus regineus.
'Neoboletus' (Gastroboletus) turbinatus var. flammeus - Blues quickly, and has yellow flesh, red or yellow pores and a blackish-brown cap, for quite a colour combination. May be a group.
'Pulchroboletus' (Gastroboletus) vividus/
False Truffles with Columellas - these have completely closed up into spheres and you can't tell what they are until you cut them in half (make sure to cut from top to bottom if you can figure out how it was oriented). Inside you will see the columella (remnants of a stem) and contorted gills, pores or other hymenial tissue (whatever the spores grew on in the "normal" mushroom). They may be found above ground, partially underground or fully underground. Discerning between these groups can be very subtle because there are of course intermediate forms where you can kind of tell what it is before cutting it open, or there will only be the vaguest hint of a columella inside and it will look like a puffball or truffle. All of these distinctions are artificial... we have created these categories to make it easier for you to find your species by sorting all these pictures into groups. The true taxonomic picture is that each one of these is related to different "normal" mushrooms and that's all there is to it, except to marvel at the wide variety of forms that have evolved. Fairly small, ranging in size from <2.5cm to <5cm.
Chamonixia caespitosa - turns blue! A bluing trufflized Leccinum. Mostly underground.
'Neoboletus' (Gastroboletus) ruber - reddish exterior, yellow interior with a whitish 'stem', bluing. Partially underground.
Trappea darkeri - as Hysterangium, but the green interior is often surrounded by a white layer of sterile cells, and it does not have a skin that can flake or be peeled. <6cm.
Hysterangium occidentale - interior pinkish rather than green, much like Hydnangium. Southern species. Outer layer flakes off. All are usually found underground. <5cm.
Hydnangium carneum - a trufflized Laccaria, recognized by the overall pink colour and folds, resembles Gautieria. <2.5cm. Eucalyptus. Usually underground.
Gautieria monticola - thin skin that lets you see inside to the hollow chambers without cutting it. Nauseating. Related to the coral Ramaria botrytis. Several similar odorless species. Usually underground. May bounce. <10cm.
Schenella simplex (Radiigera
Geastrum fuscogleba - yellow brown mature gleba.
Schenella pityophilus - with very interesting radiating peridioles that contain the spores. The "bird's nest" Sphaerobolus (smaller containing only a single peridiole) is related. Partially underground. <2.5cm.
Truncocolumella citrina - actually common, a trufflized Suillus. If your false truffle with a columella is yellow, it's probably this one. Partially underground.
False Truffles without Columellas are uniform inside and spongy or porous (never solidly marbled, nor hollow, nor with large hollow chambers), or they are gooey with a thick rind. Remember, the milky ones may not exude milk if they are dry, so you'll have to note the different textures and colours of the interior. Again, some of these names might change soon if the truffle is determined to be close enough to the mushroom it is related to. Many of these may be much more common than is generally believed, but their underground habitat means they are seldom found. Small, usually <5cm.
I apologize in advance that I am glossing over Rhizopogon, but to paraphrase Christian Schwarz paraphrasing Tom Bruns: "No one can identify "pogies". If they say they can, they're lying".
Also included in a second section are truffles in other phyla (neither "basidios" nor "ascos")!
Rhizopogon spp - related to Suillus with ~125 known species here. Spongy (they bounce!) filled with little pores that turn olive-brown at maturity. Surrounded by mycelial threads. As Tuber is the classic true truffle, this is the classic false truffle. 1-10cm.
Rhizopogon occidentalis - one especially common pogey is yellowish, staining yellow and red in age. With pine.
Rhizopogon parksii - another is pale staining pinkish then brown then eventually black. With douglas fir.
Rhizopogon ellenae - another especially common pogey is this one, pale staining mostly vinaceous. With mixed conifers, especially pine.
Melanogaster tuberiformis - almost marbled inside but the consistency is of jelly, not solid like a true truffle. Black inside. They are related to Paxillus, a gilled bolete. Oily-metallic odor.
Alpova diplophloeus grp - paler colours than Melanogaster. Red-brown exterior and red-brown jelly inside with fake marbling. Bruising red. Under alder. Fruity odor. Some Alpova may be related to Melanogaster and some to Rhizopogon.
Rhizopogon (Alpova) alexsmithii - dark exterior and pale interior. It sure doesn't look like a pogey, though.
Destuntzia rubra - thick rind, black jelly like Melanogaster. Stains pink and blue. Rotten fish odor. Southern species. Related to Gomphus. <2.5cm
Leucogaster microsporus - paler yellow to pink exterior.
Leucogaster rubescens - white becoming brick red outside, viscid.
Leucogaster and Leucophleps are similar to Endogone.
Leucophleps magnata - similar macroscopically, related, usually whitish with less milk. West of Cascades.
Leucophleps spinospora - east of Cascades.
Zelleromyces (Arcangeliella) scissilis - exudes white milk when fresh, in fact a Lactarius. Cinnamon-yellow inside at maturity.
Russula (Gymnomyces) gilkeyae - white with white interior, not staining quickly. Southern species.
R. orsonmilleri (G. subalpinus) - similar, eastern species in our region.
Russula marshallorum (Cystangium (Martellia) vesiculosum) - also a Russula, white, macroscopically similar to but more widespread than R. gilkeyae and R. orsonmilleri.
Russula subabietis (Gymnomyces abietis) - white with orange-yellow interior at maturity from darker Russula spores.
Mycolevis siccigleba - similar Russula, light and rigid as styrofoam. Pale olive interior when mature.
Lepiota (Amogaster) viridigleba - a trufflized Chlorophyllum with a pale green gleba and exterior staining orange.
Other phyla - Fungi outside of the Basidiomycota and Ascomycota are little known because few species produce fruiting bodies large enough to see, but there are a few interesting gastroid mushrooms in other phyla. They have gigantic spores (50-200 microns compared to 10 microns for an average basidiospore) that can practically be seen with only a hand lens. Anything weird you find with giant spores should be suspected of being in one of these groups. Rarely noticed, to be sure, but not actually that rare. It makes you think - thousands upon thousands of microscopically small species and only a handful evolve to produce large fruiting bodies. Often with an incomplete outer layer, like Gautieria.
Zygomycota - outer skin wears away in age like Gautieria, and the truffles (except for E. pisiformis) exude a milky latex like Lactarius truffles (Arcangeliella, Leucogaster and Leucophleps) but the textures and colours are different.
True Truffles are usually solid and marbled inside or hollow (or chambered with wrinkled tissue) except for the very thick skinned cottony Elaphomyces. Unlike every other mushroom on this page, they belong to the phylum Ascomycota, mostly in the Pezizomycetes (order Pezizales). Many of these are also probably much more common than is generally believed, but being underground they are simply not found very often.
The most famous truffles are the Italian white and black, Tuber magnatum and Tuber melanosporum, which can go from $500 to $1,000 a pound. Our truffles are not that highly prized yet, going for around $300 a pound, but I predict that someday soon when enough people have tasted a properly harvested fresh Oregon truffle, they will realize they are just about as good, and the price will go up. I would love to try a blind tasting of them.
One last thing I should say... for those of you who have only tried truffle oil on your food and are wondering what the big deal is, a real truffle has dozens (maybe hundreds) of different chemicals combining to make a wonderfully complex flavour like a very expensive red wine. Truffle oil will sometimes include only a few of the main ingredients making something that is not even close to the real experience. You have to grate fresh truffle on your food yourself to experience it correctly.
Also smallish, usually <5cm unless otherwise indicated.
First, the marbled species, most easily confused with Alpova/
Tuber gibbosum - the Oregon White Truffle. Highly prized at up to $300/lb. Matures winter-spring. As Rhizopogon is the classic false truffle, Tuber is the classic true truffle. Douglas fir.
Tuber oregonense - another prized Oregon white. Usually matures fall-winter. May have more reddish tones.
Tuber californicum - smaller, exterior and young interior more brown.
T. pacificum - mottled brown and white felty truffle, interior black & white contrasting veins
Tuber candidum - red, also not that tasty, prefers oak.
T. quercicola - similar red truffle with a rough, scaly exterior. Southern species.
There are many other Tubers.
Choiromyces alveolatus - very difficult to tell apart from the related Tuber, unreliably said to be more lumpy than spherical and more dense. Much like the Oregon white, pale orange-brown inside from paler spores.
C. meandriformis - larger, <10cm
Hydnobolites californicus - also much like Tuber with pale colours, the interior canals (veins) opening up at the furrows of the exterior, but this is very subtle. Pezizaceae. Also with paler spores, never darkening inside as much as Tuber.
Cazia flexiascus - also much like Tuber, spherical, purple-grey interior and usually pale exterior, less brown than the Oregon whites. Pezizaceae.
Leucangium carthusianum - the Oregon Black Truffle. Highly prized at up to $300/lb.
The marbling pattern is in pockets instead of wavy lines. No wonder they are tasty, they are in the Morchella family.
Kalapuya brunnea, the related prized Oregon Brown, rough exterior, same pocketed marbling pattern. Morchella family as well.
Barssia oregonensis - straight veins that don't meander as much as Tuber and a belly button-like depression. <2.5cm. Helvellaceae
Balsamia magnata - wartier with orange exterior, same straightened vein pattern. <2.5cm. Also Helvella family.
Pachyphloeus (Pachyphlodes) carneus grp - Pezizaceae family. Warty exterior thicker than other veined genera. Odd veins. Orange, <2.5cm.
P. citrinus grp - greenish tan.
P. austro-oregonensis - purplish-brown.
Fischerula subcaulis - a rare Morchellaceae species that looks like Tuber with a short columella, and may be mistaken for a false truffle!
Lastly the non-marbled species, that are either uniformly cottony/powdery, empty or chambered (folded material inside). Interestingly enough, Penicillium and Aspergillus, two famous molds, are anamorphs (asexual stage fungi) somewhat related to the Elaphomyces truffle.
Elaphomyces muricatus - "deer truffle". Thick, pimply rind,
E. asperulus (granulatus grp) - does not have the reticulation in the rind.
Sarcosphaera coronaria (crassa) - not a truffle but looks like one until it opens up. Large, hollow and opens like a crown, purple inside (white when young). Molecularly a Peziza. <10cm.
Genabea cerebriformis - paler, warty, empty or chambered, Pyronema family. <2.5cm
Genea arenaria - hairy warts, paler interior, with a tuft of mycelium at the base, oak, southern species.
Gilkeya compacta is a red hollow truffle, somewhat warty, also related to Pyronema. < 2.5cm
Geopora cooperi is a large truffle filled with folded material with large chambers between them, and with a fuzzy exterior. Also in the Pyronema cup family. 5cm+
Hydnotrya variiformis - Hydnotrya spp are chambered, smooth exterior truffles, often found inside rotten wood. This one is orange-tan. Related to Gyromitra false morels. <4cm.
Peziza ellipsospora - larger than Hydnotrya, <10cm, purple-brown without red tones, southern species. Also smooth. Under oaks.
Congratulations! You are now familiar with a diverse array of fungi from many different branches of the tree of life, many of which share the property of not being able to forcibly discharge their spores, but rely on animals to stay alive. For specialized literature on the hypogeous (underground) species, please see North American Truffles by the Trappes et. al., or for a detailed treatment of the "true" truffles, see Ascomycete Fungi of North America by Michael Beug et. al. There aren't as many good field guides to these interesting mushrooms as there are for the more "regular" types. A good online resource is http://www.natruffling.org.