[Chop. XLIV 



THE FUNGI 



558 



Some of the ascomycetes produce numerous conidia during the growing 

 season. 



The mycehum of the downy mildew parasitic on potatoes may over- 

 winter in the tuber and the next spring grow into the tissues of the new 

 potato plant developing from that tuber. Branched conidiophores grow 

 outward through the stomates, and conidia are formed from the terminal 

 branches. The conidia may be carried by wind or water to other parts of 

 the plant or to other plants. Motile spores develop within each conidium. 

 When the leaves are wet, the motile spores emerge from the conidia and 

 germinate almost immediately. The resulting hvphae penetrate the host, 

 and new internal mycelia develop. Sexual reproduction ma\' take place 

 when the fungus is growing in culture, but apparently does not occur 

 when the fungus is growing in the potato plant. ^ The reproductive struc- 

 tures of the downy mildew of tobacco are illustrated in Fig. 243. 



Cup fungi, morels, and truffles. The cup fungi are usually found grow- 

 ing on soil and on decaying wood. The vegetative bodv, or mvcelium. 



Fig. 244. Photomicrographs of reproductixe structures of a cup fungus (Peziza) : 

 A, section through the cup which is lined with a layer (hymenium) of asci and 

 intermingled sterile hyphae; B, a portion of the hymenium further magnified to 

 show asci and ascospores in more detail. 



is largely underground or near the surface. The part usually visible, the 

 fruiting body, is cup-shaped and lined with a layer of parallel asci and 

 sterile hyphae. Each ascus generally contains 8 ascospores (Fig. 244). 

 The morel, prized for its flavor, is related to the cup fungi, but has a 

 hollow reproductive body covered above with a layer of asci. The fungus 

 is not edible after the spores have matured and should be eaten only 

 when immature. 



^ It is of historical interest to recall that this so-called "potato blight" caused a serious 

 famine in Ireland in 1843-46 and led to tlie repeal of the Corn Laws. These laws had 

 imposed severe restrictions on the importation of wheat and other small grains. Pre\'ious 

 to this date the annual consumption of potatoes in Ireland was 25 bushels per person. 



