FOOD 



165 



before it possesses any virulent properties to the grass 

 tribe. It germinates, producing a small filamentous out- 

 growth which in turn bears usually four very small spores 

 of a third kind. These are known as 

 sporidia (Fig. 27, H). They are trans- 

 ported by the wind to another plant, 

 usually the barberry, in the leaves of 

 which the sporidia of Puccinia cause a 

 filamentous growth, or mycelium, which 

 bears a third kind of spore known as an 

 (scidiospore ; this placed on wheat or 

 grass infects them with rust. 



The efi'ect of the attacks of rust on 

 the stems and leaves is to divert the 



nourishment in the plant, with the 



result that the grain becomes shrivelled, 



thin, and inferior, while the straw may 



be quite unmarketable owing to its 



colour. 



Ergot (Fig. 28) is a fangoid disease 



produced by Clariceps purpurea. It is 



popularly supposed to attack only rye, 



but maize, wheat, and grasses are also 



affected. 



Ergot is a horn-shaped, grooved body, 



known as a sclerotium ; it is of a pur- 

 plish-black colour, often half an inch in 



length, standing out prominently from 



the flowering head of the plant. When 



fully developed it drops out and Hes 



dormant in the ground from summer to W 



spring. The sclerotia then germinate, Kg-, ^28^. -Spi^^^^^^^^ 



and put out small stalks each with a 



rounded head containing sporangia with their spores 



(Fig. 29). These when carried into the air again infect the 



grass family, and so complete the cycle of their existence. 



* ' Diseases of Field and Garden Crops ' : W. G-. Smith. 



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