BASIDIOMYCETES 245 



that have been investigated in the Ascomycetes. Since, how- 

 ever, a production of sexual organs has been observed, in one 

 case or another, prior to the formation of all the different kinds 

 of ascus-fruits encountered in this group, the view is generally 

 held that its members are to be regarded as being descended 

 from Fungi which exhibited a sexual process, now functionless, 

 at least in the vast majority of cases. In fact, in most Asco- 

 mycetes no traces of sexual organs are to be found. 



The young asci in all cases arise from binucleate cells, the 

 first step in their development consisting in the fusion of the 

 two nuclei, and this is regarded by many as giving the same 

 stimulus as a sexual fusion. The single nucleus thus produced 

 undergoes successive division into eight, whereupon membranes 

 are formed independently around each nucleus and the adjacent 

 cytoplasm, so that eight ascospores are cut out ; a small portion 

 of the cytoplasm remains, however, which is not incorporated 

 in the latter. 



The Basidiomycetes, which are likewise characterised by a 

 special mode of spore-formation in which a definite number (4) 

 of spores is usually constricted off from the mother-cell, are 

 altogether devoid of sexual organs. The group includes many 

 diverse Fungi, mostly saprophytes, familiar examples being the 

 Mushroom (Agaricus, Fig. 132), various Toadstools (e.g. Coprinus, 

 Boletus), Puff-balls (Ly coper don, Fig. 134, C), etc. The Smuts 

 (Ustilaginece) and Rusts (Uredinece) are usually also included in 

 the Basidiomycetes, but exhibit many peculiar features of their 

 own. 



The Rusts are of special importance as being the cause of 

 many serious diseases of crops, and of these the Rust of Wheat 

 (Puccinia graminis) is, unfortunately, all too common. Like 

 many other members of the group, it possesses a very complicated 

 life-history, whose phases, in this species, occur on two different 

 hosts. In summer the parasite attacks the leaves and stems of 

 various Grasses, and betrays its presence by the development of 

 orange-coloured streaks upon them. These are due to clusters 

 of unicellular thick-walled spores (summer- or uredo-spores) of an 

 orange colour and beset with numerous minute spines ; each spore 

 arises at the end of a projecting hypha (Fig. 131, b, .). After 

 detachment they may be blown by the wind on to another 



