I9I5.] DEVELOPMENT OF AGARICUS RODMANI. 321 



earliest appearance. But as this interpretation was shown by Hoff- 

 man (i860, p. 402) to be wrong, deBary^^ (1866, p. 63; 1884, p. 58, 

 312; 1887, p. 55, 289) studied a number of other forms and agreed 

 with Hoffman that the earhest stage of the young pahsade hymeno- 

 phore was level, or smooth. 



IH. The Differentiation of Parts in the Primordial Ground 



Tissue. 



There are four principal parts of the fruit body which are dif- 

 ferentiated in the ground tissue of the basidiocarp primordium, the 

 hymenophore, pileus, stem and veil. The primary differentiation in 

 the ground tissue of Agaricus rodmani is the origin of the hymeno- 

 phore primordium. As described above this arises as an internal 

 annular zone of new growth, a little above the middle of the small 

 oval primordial basidiocarp. It consists of numerous hyphal 

 branches which extend downward and obliquely outward. These 

 new hyphse are nearly or quite parallel, are at first slender and taper 

 very gradually to the free end. This form assists them in making 

 their way through the mesh of the ground tissue. They are rich in 

 protoplasm, become compacted by increase in number and diameter, 

 and thus in sections, take on a deep color when stains are applied 

 (see Figs. 3-16). The origin of this internal hymenophore zone 

 differentiates at once the stem and pileus areas, or fundaments, but 

 the organization of the stem and pileus occurs later. 



In the early origin of the primordial hymenophore zone, Agaricus 

 rodmani agrees with Agaricus campestris'^^ as presented in a study 

 of the commercial varieties, alaska and bohemia. In that paper I 

 pointed out that we should not necessarily expect the first evidence 

 of differentiation to be the appearance of the hymenophore primor- 

 dium in plants not yet studied though it is probable that at least 

 some of the other species of Agaricus (PsaUiota) may show the 

 same peculiarity. This suggestion is justified by the situation in 

 Agaricus rodmani. The same situation exists in Armillaria mellea.'^'^ 



^s Atkinson, Geo. F., " The Development of Agaricus compestris," Bot. 

 Universal Veil ' in Agaricus," Myc. Centralb., 2, 13-19 pis. 1-3, 1914. 

 Gas., 42 : 241-264, pis. 7-12, 1906. 



1" Atkinson, Geo. F., " The Development of Armillaria mellea," Myc. 

 Centralb., 4: 113-121, pis. i, 2, 1914. 



