GASTEROMYCETES 



475 



of these unusual genera is unknown and hence their systematic position 

 is still uncertain. 



Throughout the family we have found an increasing differentiation 

 of the peridium. 



The fructifications of Astraeus hygrometricus stellatus (A.Geaster hygro- 

 metricus) are hypogaeous and surrounded by mycelium. The peridium 

 consists of a thin, papery endoperidium and a stronger exoperidium, 

 of three layers, an outer, thin, looser, a thicker, corky, middle layer and 

 a horny collenchyma, inner layer (Fig. 302). In both outer layers, the 

 hyphae are irregularly intertwined. At maturity the whole gleba is 

 disorganized and there remain only the spores with the thick-walled 

 capillitium. The exoperidium dehisces stellately, the fructification rises 

 above the soil. Because of the hygroscopicity of the innermost radially 

 fibrous layer, the exoperidium curves inward in dry weather and expands 



FiG. 302. — Astraeus hygrometricus. 1. Mature fructification. 2. Section of young 

 fructification, i, i', i", endoperidia; a', a", exoperidia; g, gleba. (Natural size; after 

 Tavel and Bary.) 



in damp. The endoperidium ruptures irregularly or stellately at the 

 top and the spores are dispersed. 



In Geaster velutinus (Cunningham, 1927), the mycelial layer consisting 

 of a dense palisade, 1 to 2 mm. thick of coarse, deeply colored hyphae, is 

 first differentiated from the ground tissue. In the next stage the primor- 

 dia of the fleshy and fibrillose layers of the exoperidium, the gleba and the 

 sterile central tissue, the columella, are differentiated. The fibrillose 

 layer consists of periclinally arranged large and small hyphae with later 

 the addition of some hyphae similar to those of the mycelial layer. The 

 fleshy, or inner layer of the exoperidium begins as a layer of large, com- 

 pactly woven hyphae between the fibrillose layer and the gleba. The 

 cell walls gelify and the tissue gradually becomes pseudoparenchyma. 

 It is interrupted by the thickened base of the columella. 



Shortly after the differentiation of the fleshy layer begins, a dome of 

 cavities appears between this layer and the columella. The cavities 

 vary in size and shape, apparently being formed by the tearing apart of 

 the hyphae. These cavities become lined with large inflated cells, the 



