446 BOTANY OF THE LIVING PLANT 



In the Mushroom the germination of the spores presents a difficult 

 problem. It seems probable that one incident, and perhaps a neces- 

 sary one, is that they should pass through the alimentary tract 

 of some herbivorous animal : for the spores would naturally be 

 taken in with the grass they eat, while the Mushroom grows on old 

 pastures manured by their dung. Related fungi do not always 

 present such difficulties, and are readily raised from spores. In the 

 case of the Mushroom no other propagative bodies exist, nor have 

 any sexual organs been recognised. The fruit-bodies arise from 

 a mycelium with paired nuclei. The young basidium contains 

 two nuclei which later fuse. The fusion-nucleus undergoes two 

 divisions — involving reduction — and each of the four nuclei formed 

 becomes the nucleus of a basidiospore. The fact that in the Hy- 

 menomycetes there is a fusion of paired nuclei followed by reduction 

 shows that the basidiospores are of the nature of tetraspores. 



On the germination of the basidiospore a considerable mycelium of uni- 

 nucleate cells may be formed. Between different hyphae fusions or anastom- 

 oses are common, the attendant nuclear migrations restoring the binucleate 

 condition. Such binucleate hyphae, on further growth, may give rise to the 

 fructifications described above. Some of the Hymenomycetales are self- 

 compatible so that fusions may take place between hyphae of the same 

 mycelium and thus the fructifications may eventually be obtained from the 

 germination of a single spore. But, as pure culture experiments have shown, 

 many others are self-incompatible and hyphal fusions and nuclear transferences 

 between compatible strains are necessary for the development of fructifications. 



Basidio-Lichens. 



Certain Basidiomycetous Fungi take part in the composition of 

 Lichens, but this is much less frequent than in the Ascomycetes. 

 The most familiar example is the genus Cora, found not uncommonly 

 in the tropics, growing on the ground or on trees. Its form is not 

 unlike a Stereum, or Thelephora ; and, like them, its hymenium is 

 on the lower surface. The fact that the fungal constituent of some 

 Lichens can be referred to Basidiomycetous Fungi may be held as 

 a final proof of their being compound organisms (see pp. 427-428). 



Fungi as Subaerial Plants. 

 The Fungi, sprung perhaps from various Algal sources, show some 

 degree of adjustment to subaerial life parallel to that already seen in 

 Green Plants. But as the hypha is the basis of their construction, 

 their vegetative system gives less opportunity for adaptive change 

 than their propagative organs. 



