INJURIES INDUCED BY PLANTS 67 



plant or it remains enclosed by the epidermis, in which case 

 it appears as a black semi-transparent swelling. 



The spores of smut may retain their capacity for germination 

 for several years. On the recurrence of favourable conditions 

 they usually produce a stout germ-tube called the promycelium 

 (vorkeim\ which, after attaining a length equal to two or three 

 times the diameter of the spore, forms a number of smaller 

 spores, known as sporidia, at its apex or on its sides. 



Frequently the promycelium breaks up directly into a number 

 of sporidia. In the case of those species which produce whorls 

 of sporidia at the apex of the promycelium, a process of fusion 

 takes place between adjoining sporidia, and these afterwards 

 drop off in pairs. 



When a germinating smut-spore or sporidium comes into 

 contact with a suitable young host-plant, it sends its germ-tube 

 through the epidermis, and thus gets into the tissues of the 

 stem, where the mycelium grows upwards, chiefly intercellular, 

 without producing any apparent damage. It is only in the parts 

 of the plant where spores are formed that the tissues are 

 destroyed. 



Those smut-spores which fall to the ground before or during 

 harvest usually germinate at once, and perish in the absence 

 of suitable young host-plants.* This being the case, the disease 

 persists from year to year, for the most part, owing to the employ- 

 ment of seed to which smut-spores adhere externally. When the 

 corn is being threshed the detachment of the spores from 

 smutted plants offers ample opportunity for the contamination 

 of the seed-grain. Frequently, however, the spores are conveyed 

 to the field in manure which has been made with smutted 

 straw. 



On account of the germination of the smut-spores being 

 dependent in great measure on moisture in the air and soil, 

 the occurrence of the disease is favoured in a soil whose 

 physical condition either naturally or owing to the liberal 

 application of farm-yard manure enables it to retain large 

 quantities of water. 



It follows from what has been said that attention should first 



* [Brefeld has shown that the " sporidia " may reproduce by budding 

 saprophytically during long periods in the manured soil. ED.] 



F 2 



