60 DISEASES OF CROPS. 



withered. The nodular growths (of a brownish-black 

 colour, although white at first) are composed of a compact, 

 hard mycelium. Each nodule is called a sclerotium, which 

 is capable of hibernating for a longer or shorter time 

 according to circumstances. When one of these bodies 

 grows, it gives rise to a perfect fungus bearing a cup-like 

 head containing spores (l^ig. 25 A). The spores after 

 germination reproduce the mycelium, which may either 

 produce spores or become a sclerotium. Fig. 25 A repre- 

 sents a sclerotium giving rise to hyphse ; each hypha 

 bearing a cup-like head (somewhat similar to the pileus 

 or cap of the mushroom) containing millions of spores. 



If a transverse section of the hymenium, pileus (?), or 

 cup-like head is examined under the microscope, it is seen 

 to consist of a number of perpendicular cells (Fig. 26 B). 

 Each cell contains an ascus (a kind of " sack ") filled with 

 sporidia i or spores (Pig. 25 C). The paraphysis ^ (shown 

 in Fig. 25, C 2) is an " organ " developed from the cells of 

 the lower part of the hymenium. At certain times the 

 asci open at the top, and the spores are liberated. It has 

 been estimated that each cup-like head contains from two 

 to three million spores. 



Prevention. — (1) Artificial manures are better suited 

 than farmyard manure for the cultivation of potatoes. 

 Mr. W. G. Smith, F.L.S., states that " it is bad in practice 

 to place potato sets in immediate contact with decaying 

 vegetable matter and farmyard manure ; such materials 

 always contain an immense number of disease germs, both 

 of animal and vegetable origin. The manures used for 

 potatoes should always be old and thoroughly decayed, 

 and it is perhaps best that the cut faces of the sets should 



' Sporidia are spores /rec in an ascus. ^ Its function is unknown. 



