THALLOPHYTA: FUNGI 



147 



whitish bloom. There is no formation of basidiocarps, the basidia arising 

 directly from the mycelium. In this and succeeding orders the basidium 

 is one-celled. The young basidium has two nuclei that fuse, two succes- 

 sive nuclear divisions follow, and four basidiospores are developed, each 

 at the end of a sterigma. 



6. Hymenomycetales 



This large order of approximately 15,000 species is usually split up 

 into several smaller orders, but here will be regarded as one homogeneous 

 group. Most of the members are saprophytic on humus, bark, decaying 

 wood, etc. Some are parasitic on trees, often 

 causing considerable damage. The Hymeno- 

 mycetales have complex basidiocarps with 

 basidia in a definite hymenial layer that be- 

 comes freely exposed. The basidia are one- 

 celled and bear four basidiospores, each at the 

 end of a slender sterigma. 



Families. The families of Hymenomyce- 

 tales are distinguished from one another on 

 the basis of the form of the basidiocarp and 

 the position of the hymenium. The principal 

 families are as follows: 



1. Thelephoraceae. These forms produce 

 simple basidiocarps appearing on tree trunks. 

 Some resemble leathery incrustations with the 

 hymenium on the smooth upper surface, while some are bracket-like 

 with the hymenium on the lower surface. Others have the hymenium 

 on the outside of a funnel-like basidiocarp. The representative genus 

 is Thelephora, with about 150 species. 



2. Clavariaceae. The coral fungi produce erect, fleshy basidiocarps 

 that are usually branched like coral, the hymenium covering the surface 

 of the branches (Fig. 120). They are commonly white or yellowish, but 

 sometimes are more brightly colored. In some forms the basidiocarps are 

 club-shaped and unbranched, with a complete hymenial covering. The 

 principal genus is Clavaria, wdth about 250 species. 



3. Hydnaceae. These are the tooth fungi, the hymenium being borne 

 on tooth-like or spine-like processes that generally point downward. 

 The simpler forms occur as rounded masses or thin sheets of indefinite 

 form. Some are more or less branched. Others have a stalk and an 

 umbrella-like pileus that bears teeth on its lower side. The main genus 

 is Hydnum, with about 150 species. 



4. Polyporaceae. The pore fungi bear a number of tubes or grooves 

 lined with a hvmenium. The basidiocarp may be crustaceous, the tubes 



Fiu. 120. A coral fungus 

 {Clavaria), natural size. 



