FUNGI. 57 



sporangial filament (Fig. 32, F, H). These small sporangia 

 have no columella. 



Other moulds are sometimes met with, parasitic upon the 

 larger species of Mucor. 



Related to the black moulds are the insect moulds (Ento- 

 mopthorece), which attack and destroy insects. The commonest 

 of these attacks the house flies in autumn, when the flies, thus 

 infested, may often be found sticking to window panes, and 

 surrounded by a whitish halo of the spores that have, been 

 thrown off by the fungus. 



ORDER II. WHITE BUSTS AND MILDEWS (Peronosporece) 



These are exclusively parasitic fungi, and grow within the 

 tissues of various flowering plants, sometimes entirely destroy- 

 ing them. 



As a type of this group we will select a very common one 

 (Oystopus bliti), that is always to be found in late summer and 

 autumn growing on pig weed (Amarantus). It forms whitish, 

 blister-like blotches about the size of a pin head on the leaves 

 and stems, being commonest on the under side of the leaves 

 (Fig. 33, A). In the earlier stages the leaf does not appear 

 much affected, but later becomes brown and withered about 

 the blotches caused by the fungus. 



If a thin vertical section of the leaf is made through one of these 

 blotches, and mounted as described for Mucor, the latter is found to be 

 composed of a mass of spores that have been produced below the epidermis 

 of the leaf, and have pushed it up by their growth. If the section is a 

 very thin one, we may be able to make out the structure of the fungus, 

 and then find it to be composed of irregular, tubular, much- branched fila- 

 ments, which, however, are not divided by cross-walls. These filaments 

 run through the intercellular spaces of the leaf, and send into the cells 

 little globular suckers, by means of which the fungus feeds. 



The spores already mentioned are formed at the ends of crowded fila- 

 ments, that push up, and finally rupture the epidermis (Fig. 33, B). They 



