THALLOPHYTA: FUNGI 151 



Following the formation of a binnoleate cell by the fusion of two uni- 

 nucleate cells, a short branch arises into which the two nuclei pass. A 

 hook-hke lateral outgrowth, pointing toward the base of the cell, then 

 appears at a point directly opposite the two nuclei (Fig. 123.4, B). After 

 both of these divide, one of the daughter nuclei passes into the hook and a 

 cross wall forms at its base, another wall continuing across the branch 

 (Fig. 123C, D). Thus two nuclei of opposite sex are in the terminal cell, 

 one nucleus being in the lower cell and one in the hook. The tip of the 

 hook now fuses with the lower cell to form a "clamp connection." The 

 nucleus in the hook passes into the lower cell, which thereby becomes 

 binucleate (Fig. 123£'). The terminal cell continues to grow and, at each 

 cell division, a new clamp connection is formed. 



A mycelium with clamp connections is characteristic of many Basidio- 

 mycetes, occurring in at least some members of all the orders except the 

 Uredinales. Clamp formation in the Basidiomycetes is thought to corre- 

 spond to hook formation in the Ascomycetes where, however, it is limited 

 to the ascogenous hyphae. It must be remembered that in both groups 

 there are many members without any such formations, the ascus or 

 basidium developing directly from the terminal cell of a hypha. Clamp 

 connections are not present on the mycelium of Agaricus campestris or its 

 cultivated variety. 



7. Gasteromycetales 



Like the Hymenomycetales, the Gasteromycetales are often broken 

 up into several smaller orders. Nearly all its members are saprophytic 

 on humus, but a few grow on decaying wood. There are about 1,000 

 species. The very complex basidiocarp entirely encloses the hymenium, 

 remaining closed or opening only after the spores are mature. The 

 basidiocarp is composed of an outer peridium and a central gleha, the 

 latter generally containing many chambers. In the lower forms the 

 chambers are filled with hyphae bearing terminal basidia; in the higher 

 forms the chambers are lined with a definite hymenium. The basidia are 

 one-celled and bear four terminal basidiospores, each at the end of a 

 sterigma. 



Families. The principal families of Gasteromycetales, distinguished 

 from one another by the character of the peridium and gleba, are as 



follows: 



1. Hymenogastraceae. This family is intermediate between the Hyme- 

 nomycetales and the Gasteromycetales. The peridium is simple, being 

 one-layered and rupturing irregularly. The glebal chambers are Uned 

 with basidia borne at the ends of lateral branches of the glebal hyphae. 

 Because the basidiocarps are subterranean, these forms are not commonly 

 seen. The chief genera are Hymenogaster and Rhizopogon. 



