Fun­go­ma­nia III. Every­body Talks About the Weather...

...But Mush­rooms Do Some­thing About It

Fun­go­ma­nia con­tin­ues, this be­ing the third of three con­sec­u­tive fun­gal posts

by Elio

We fin­ish our Fun­go­ma­nia se­ries of this year with a ques­tion: Why when you feel the cap of a mush­room in the woods, does it feel cool and some­what moist? If you blamed evap­o­ra­tion, you would be right. But how come this takes place? It turns out that evap­o­ra­tion leads to the cre­ation of air cur­rents, and that is a good thing for the dis­per­sal of the mush­rooms' spores. Mush­rooms have an in­tense pre­oc­cu­pa­tion for spread­ing their spores afield, a way to en­sure the oc­cu­pancy of new and dis­tant sites.

Fig­ure 2. Mush­room spores read­ily dis­perse from pilei (caps) that are crowded close to­gether or close to the ground. (A) Agar­i­cus cal­i­for­ni­cus mush­rooms grow un­der a layer of plant lit­ter on the UCLA cam­pus. (B) Oys­ter mush­rooms (Pleu­ro­tus os­trea­tus) grow crowded to­gether in a stand. (C) Even when mush­rooms are iso­lated from ex­ter­nal air­flows, an asym­met­ric tongue of spores is de­posited far from the pileus. (Scale bar: 5 cm.) (D) Both cul­tured (left data) as well as wild-col­lected cir­cu­lar pilei (right data) pro­duced ex­tended asym­met­ric spore de­posits. The pa­ra­me­ter used to char­ac­ter­ize the asym­me­try of the spore de­posit is de­scribed in Ma­te­ri­als and Methods. (E) Air­flows car­ry­ing spores out from the gap can be di­rectly ob­served us­ing a laser light sheet and high-speed cam­era. Shown: Lentin­ula edo­des. (Scale bar: 1 cm.). (Source)

So in­tense is this con­cern that mush­rooms make spores in prodi­gious amounts. Some are known to pro­duce a bil­lion spores per day from a sin­gle spec­i­men. My­col­o­gists have long rel­ished the elu­ci­da­tion of how the spores clear the tis­sue on which they are made. This is a ne­ces­sity if the spores are to dis­perse in the en­vi­ron­ment. It has been known for some time that the spores of mush­rooms, such as the Por­to­bel­los of the store, make their spores on the sur­face of the gills, the lamel­lae seen un­der the cap when you turn such mush­rooms up­side down. The spores are made on sur­face cells called ba­sidia, hence the name for these mush­rooms, the Ba­sid­iomycetes. Of course, if this were all, you would end up with a heap of spores all in the same place. To avoid that, the fungi have evolved a clever way to pro­pel the spores away from the gill sur­face and into air cur­rents that carry them aloft. As­ton­ish­ing is that the ejec­tion takes place at enor­mous speed, ini­tially 25 me­ters per sec­ond, and at the nearly im­plau­si­ble pres­sure of 180,000 x g. We dis­cussed this mech­a­nism be­fore. This is an amaz­ing tale, but what hap­pens next? How to ex­plain why mush­rooms are suc­cess­ful at spore dis­per­sal even if they are in crowded con­di­tions, such as grow­ing in close prox­im­ity to their brethren. And what about those mush­rooms whose cap is close to the ground?

A study by sev­eral en­gi­neers and math­e­mati­cians (the se­nior au­thor is Mar­cus Roper) an­swers the ques­tion first posed above and ex­plains what good this is. They found that wa­ter evap­o­rat­ing from the caps of mush­rooms cre­ates up­ward air cur­rents ca­pa­ble of car­ry­ing fun­gal spores some 10 cm into the air. Mush­room caps dry out quickly, more so than plants. The process of evap­o­ra­tion cools the sur­round­ing air, which ren­ders the air denser and makes it spread down­wards. This is com­pen­sated by warm air flow­ing up­wards, which re­sults in ed­dies that can lift spores even over ob­sta­cles. For those in­ter­ested in the en­gi­neer­ing de­tails, spore dis­per­sal re­quires that the air flow be asym­met­ri­cal, which is in part ob­tained by the caps of the mush­room be­ing asym­met­ri­cal.

In con­clu­sion, mush­rooms make their own cli­mate. Clever, no?

Fig­ure 3. Cold air leav­ing the gap be­neath the cap is re­placed by pulling warm air to the cap. In the pres­ence of an ex­ter­nal wall, nu­mer­i­cal sim­u­la­tions (cold out­flow and warm in­flow can con­nect to form a closed con­vective eddy. Our sim­u­la­tions show that spores in the cold grav­ity cur­rent may be drawn up the wall by the closed eddy. Here, shaded curves show the stream­lines of the flow cre­ated by the cooled pileus. Solid lines show sim­u­lated spore tra­jec­to­ries. The ex­ter­nal wall is made up of two slanted sur­faces on the right-hand side of the im­age. (Source)