8l2 



ECOLOGY 



Various features of structure or habit facilitate spore dispersal. As 

 previously noted, many fungi bear spores externally, so that they are 

 readily blown away as soon as they are abstricted. In the fleshy fungi, 

 the spore-containing organs often are borne on conspicuous apogeo- 

 tropic stipes, which thus elevate the spores into a good position for wind 

 dispersal (figs. 1078, 2, 197). In many of these forms the spores are dis- 

 charged from the gills, after which they drop into positions where they 

 may be wafted off by air currents. In Coprinus (figs. 198, 199), which 

 has a cylindrical fruit body, the spores mature first in the lower part, 

 which then curves outward, and hence does not hinder the dispersal 

 of those which ripen later. Where spores are borne within sporangia 

 or similar organs, there often are no special features which facilitate 

 spore removal, it being necessary for the enveloping organs to rot away 



before the spores 

 can be dispersed. 

 In some cases there 

 is definite dehis- 

 cence, as in Geaster, 

 where the sporan- 

 gial wall (peridium) 

 has two layers, of 

 which the outer 

 splits into star- 

 shaped segments 

 (whence the com- 

 mon name, earth 

 star), while the in- 

 ner has an apical 

 opening (fig. 1124); in the related puff balls the outer layer breaks 

 irregularly. In Geaster the hyphae are arranged at right angles to the 

 surface in the inner (i.e. upper when open) part of the ray and parallel 

 to the surface in the outer part. Hence in moist weather the inner part 

 absorbs the more water and the rays open (fig. 1124), while they close in 

 dry weather (fig. 1123), since the inner part loses the more water. This 

 hygroscopic mechanism has been thought to facilitate spore dispersal ; 

 the dry closed structure is bowled along by the wind like a tumbleweed, 

 and the rain washes out spores from the opened structure. 



In a few fungi, spores are scattered by agents other than wind. In Pilobolus 

 (fig. 630) the columella of the sporangium ultimately bursts by reason of increasing 



1123 



FIGS. 1123, 1124. An earth star (Geaster hygrometricus); 



1123, a fructification, as seen in dry weather, the peridium 

 rays (r) being incurved about the spore-bearing portion; 



1124, a fructification, as seen in moist weather, the peridium 

 rays being expanded; note the aperture (a) through which 

 the spores escape. 



