MUSHROOMS AND TOADSTOOLS 235 



discharged, and permits the spores to fall more easily past the neighbor- 

 ing gill surfaces. 



Development of the Fruit Bodies.— Kikmson^ has studied the develop- 

 ment of the mushroom {Agaricus {PsaUiota) campestris) (Fig. 94). — 

 The youngest stage is the homogeneous primordium 01 the carpophore 

 composed of slender, uniform, dense hyphae, intricately interwoven, and 

 surrounded by a thin layer of hyphse of a looser arrangement. This 

 layer is the universal veil which grows until the form of the fruit 

 appears when it is torn into white floccose patches on the pileus. In 

 the very young primordium then there is no evidence of a differentiation 

 into stem and pileus and at this stage stained longitudinal sections show 

 two small deeply stained internal areas near the upper end of the young 

 fruit body and some distance from the surface. The hyphae here are 

 richer in protoplasm and form an annular area within the fruit body. 

 This area now increases in extent and many hyphae grow from its 

 upper portion downward to form the primordial layer of the hymenium. 

 These downward growing hyphge are slender and terete and taper 

 pointed, which enables them to push between the surrounding hyphse. 

 Soon after these hymenial hyphse grow downward there is a cessation of 

 growth. Just below this area which results in the rupture and separa- 

 tion of the hyphse at this point in a corresponding internal annular area, 

 forming the well-known "gill cavity" which at first is very minute. 



With the formation of this annular primordium of the hymenium 

 the primordia of the stem, veil and pileus are differentiated. The 

 period of elongation of the parts after they have been organized follows 

 in succession. The marginal veil completes its period of elongation 

 first, then the stem, followed by the pileus, and finally, the hymenium 

 where in examples studied Atkinson secured two-spored basidia. 



A somewhat similar development takes place in Agaricus Rodmani, 

 a form which grows in grassy ground and paved gutters in cities from 

 May to July. The sequence of events in the growth of the fruit body is 

 given by Atkinson. ^ He finds that the primordium of the fruit body 

 is oval in form and homogeneous in structure, consisting of intricately 

 woven hyphse. The hymenophore primordium arises as an internal 



^Atkinson, George F.: The Development of Agaricus campestris. Botanical 

 Gazette, 42: 215-221, September, 1906. 



2 Atkinson, George F.: Morphology and Development of Agaricus Rodmani. 

 Proceedings American Philosophical Society, 191 5: 309-343, with 7 plates. 



