322 ATKINSON— MORPHOLOGY AND [April 23. 



In the specimens of Agaricus arvensis^^ studied, the lagging behind 

 of the ground tissue below the zone where the hymenophore primor- 

 dium arises occurs before any differentiation of this zone is dis- 

 tinguishable, for a light area with a looser mesh occurs in an an- 

 nular zone which marks the distinction between the stem and pileus 

 areas. Or the lagging behind of the ground tissue may occur simul- 

 taneously with the appearance of the primordial hymenophore zone 

 and the outline of the pileus area. In a number of forms studied 

 by Fayod^" the primordium of the pileus is organized, in the apex 

 of the young homogeneous basidiocarp, as a new zone of growth, in 

 the form of an inverted bowl, shown by the darker staining of the 

 hyphae rich in protoplasm, forming a pileus producing layer 

 ("couche pileogene"). This method of differentiation he accepts 

 as a general law for the Agariceae, the only exception admitted by 

 him being the coriaceous forms of Lentinus. Agaricus rodmani, 

 the commercial varieties of Agaricus campestris {Columbia and 

 alaska) and Stropharia ambigua (Peck) Zeller,^"' also form excep- 

 tions to this rule. The primordium of the pileus in these forms 

 may be regarded as diffuse within the upper part of the young 

 basidiocarp, the differentiation and organization of the pileus mar- 

 gin beginning in conjunction with the organization of the primordial 

 hymenophore zone, though in Stropharia ambigua the inverted bowl- 

 shaped zone of new growth in the upper part of the pileus area is 

 soon organized.-'' Other forms recently investigated which conform 

 to the general law laid down by Fayod, are certain species of 

 Hypholoma ( Allen), -^ Hypholoma fascicularis and Clitocybe laccata 

 by Beer,-^ Lepiota'^^ clypeolaria and Amanitopsis vaginata.^'^ 



IS Atkinson, Geo. F., " The Development of Agaricus arvensis and A. 

 conitulus," Am. Jour. Bot, i, 3-22, pis. i, 2, 1914. "Homology of the 

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



1^ Fayod, V., " Prodrome d'une histoire naturelle des Agaricinees," Ann. 

 Sci. Nat. Bot., VIL, 9, 181-411, pis. 6, 7, i88g. 



20 Zeller, S. M., " The Development of Stropharia ambigua," Mycologia, 

 6, 139-145: pis. 124, 125, 1914. 



21 Allen, Caroline L., "The Development of some Species of Hypho- 

 loma," Ann. Myc, 4, 387-394, pis. 5-7, 1906. 



22 Beer, R., " Notes on the Development of the Carpophore in Some 

 Agarincacese," Ann. Bot., 252: 683-689, pi. 52, 1911. 



23 Atkinson, Geo. F., "The Development of Lepiota clypeolaria," Ann. 

 Myc, 12, 346-356, pis. 13-16, 1914. 



