270 



GENERAL BOTANY 



trama. The hyphge of this central trama turn outward toward 

 the surface of the lamella and end in large, club-shaped cells 

 which form the hymenium, or spore-bearing layer (Fig. 148, d). 

 This hymenium covers the entire surface of the lamella like an 

 epidermis, and is composed of two kinds of club-shaped cells, 



namely, the fertile cells ; 

 called basidia, which form 

 the spores, and the sterile 

 cells, or paraphyses, which 

 separate the basidia at cer- 

 tain intervals. In a surface 

 view of a lamella the ends 

 of the sterile cells and of 

 the spore-bearing basidia 

 are seen. If such a view 

 is taken of a gill under 

 the low power of a com- 

 pound microscope, an ap- 

 pearance like that in 

 Fig. 148, <?, is shown. 

 The sterile cells now ap- 

 pear as large, circular, 

 light-colored cells, while 

 the spores borne by the 

 basidia are seen in groups 

 of four small, dark bodies. 

 This view also gives one 

 a good idea of the con- 

 tinuous epidermislike character of the hymenium. When the 

 hymenium begins to bear spores, each basidium buds out at 

 its large, free end into four slender protrusions of the cell wall, 

 called sterigmata, and each sterigma then swells up at the end 

 into a spherical spore cell, into which cytoplasm and a nucleus 

 flow from the main body of the basidium. The spores change in 

 color as they mature, and cause a change in the color of the gill 

 from white to black, broAvn, or pink in many species of mush- 

 rooms. When they are ripe they are shot off from the sterigmata 



FIG. 149. Two species of puffballs, the upper 

 one shedding spores 



