316 INTRODUCTION TO CEYPTOGAMIC BOTANY. 



genera. There are, first, the various species of rust and mil- 

 dew which are parasitic upon living plants, and which may be 

 considered as analogous to the brighter coloured moulds. These, 

 as we shall hereafter see, are separated by peculiar characters 

 from all other Fungi. They were long despised as worthy of 

 little scientific notice, though recognised as the cause of many 

 of the diseases to which our cereals are subject, and are now 

 found to be amongst the most interesting of Fungi, and, in 

 spite of the views of linger, Turpiu, and others, are decidedly 

 as distinct plants as the Pha3nogamous parasites. Secondly, 

 there are the analogues of the black moulds springing at once 

 from the former division by the curious genus Xenodochus, 

 which in general present little interest, except from the form 

 of their spores ; and, thirdly, there are the analogues of the 

 Sphcerice, if, indeed, far the greater portion are not mere second 

 forms of fructification of various Spheeriaceous genera. 



840. As regards the first, wherever Phsenogams occur, they 

 may be found accompanying them, without much restriction 

 as to chmate. Cereals are subject to the same parasites 

 wherever they grow. Barley, for instance, on the hot banks 

 of the Soane, is infested as at home with Ustilago segetum; 

 and Puccirda graminwrn extends as far south as New 

 Zealand. Cystopus candidus occurs both in the extreme 

 north and south, and species grow in Western Thibet, identical 

 with those of Europe. As there are few natural families of 

 plants which are exempt from these parasites, and species are 

 often confined to a particular natural order, different species 

 may be expected to occur in different parts of the world, even 

 taking species with a considerable degree of latitude. The 

 curious genus Ravenelia (Fig. 69, c, d) has at present been dis- 

 covered only in warmer districts ; but while under one form it 

 infests the different species of Desmodium in South Carolina, 

 under another it is abundant on Abras and Acacia in Behar. 

 GroTKirtium, which affects the leaves of certain plants in the 

 warmer parts of Europe, in South Carolina and Ohio appears 

 on Thesium, Quercus, and Coinptonia. Peridermium, which 

 is the pest of pines in Europe, under a different species is no 

 less destructive amongst the Himalayas ; and it occurs in 



