FUNGI. INTRODUCTORY 397 



The attack may, however, be upon the stem or root, or even the 

 ovary. The effect is to produce swellings and malformations such 

 as those of the roots of Crucifers, called " Club-root " (Fig. 295), 

 or of the grain of " Ergot of Rye " (Fig. 293). 



Some Fungi lead a constantly parasitic life, as the Rusts (Uredineae) 

 do. Others are as constantly saprophytic, like the Saprolegniae. 

 Others again may be sometimes the one, sometimes the other : and 

 this may be so in the individual life. Thus Pythium, the " Damping 

 off Fungus," attacks the seedling-host and kills it, but continues to 

 live on the corpse (p. 400). It is first parasitic, then saprophytic. 

 But the converse has been shown in Sclerotinia sclerotiorum, where a 

 period of saprophytic nourishment is a necessary condition for its 

 success in perforating the living host. It has been regarded as a fungus 

 which is in course of " education " for passage from the saprophytic 

 to the parasitic life. It is thus impossible to lay down any general 

 rule of priority for parasitism or saprophytism : and it is only in 

 certain cases that the one habit or the other can be assigned to any 

 definite systematic group. 



The life of Fungi is very varied. No organisms show greater resource 

 in the acquisition of food. But their propagative methods are no 

 less effective. Originally sprung from aquatic organisms, some 

 show this clearly in their reproductive organs, which often involve 

 motility of zoospores in external water, as in many Phycomycetes. 

 But the more advanced types are commonly propagated by means 

 which are clearly related to life in the air. In most Eumycetes 

 minute bud-like bodies called conidia are borne in prodigious numbers, 

 and they are small enough to be carried as dry dusty bodies through 

 the air. The conidia of common Moulds are present everywhere 

 about the dwellings of man : so that any suitable medium is apt to 

 be invaded by them, provided the conditions of temperature and 

 moisture are favourable. This explains the apparently spontaneous 

 appearance of moulds on bread, leather, jam, etc., when kept in a 

 confined space. The spread of fungal diseases is usually by similar 

 means. One of the most surprising facts in this relation is the very 

 constant recurrence of certain Fungi on isolated and restricted media. 

 The horns of sheep cast away on a Scottish hill-side are commonly 

 invaded by a horn-destroying Fungus, Onygena. But any one such 

 horn may be isolated far from any visible source of infection. 

 This shows the ubiquity of fungal germs. It suggests also the other 

 side of the question, the vast number of germs that never find the 

 suitable nidus. 



